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The mating of yeast, also known as yeast sexual reproduction, is a biological process that promotes genetic diversity and adaptation in yeast species. Yeast species, such as Saccharomyces cerevisiae (baker's yeast), are single-celled eukaryotes that can exist as either haploid cells, which contain a single set of chromosomes, or diploid cells, which contain two sets of chromosomes. Haploid yeast cells come in two mating types, a and α, each producing specific pheromones to identify and interact with the opposite type, thus displaying simple sexual differentiation.^[a] A yeast cell's mating type is determined by a specific genetic locus known as MAT, which governs its mating behaviour. Haploid yeast can switch mating types through a form of genetic recombination, allowing them to change mating type as often as every cell cycle. When two haploid cells of opposite mating types encounter each other, they undergo a complex signaling process that leads to cell fusion and the formation of a diploid cell. Diploid cells can reproduce asexually, but under nutrient-limiting conditions, they undergo meiosis to produce new haploid spores.

The differences between a and α cells, driven by specific gene expression patterns regulated by the MAT locus, are crucial for the mating process. Additionally, the decision to mate involves a highly sensitive and complex signaling pathway that includes pheromone detection and response mechanisms. In nature, yeast mating often occurs between closely related cells, although mating type switching and pheromone signaling allow for occasional outcrossing to enhance genetic variation. Furthermore, certain yeast species, like Schizosaccharomyces pombe and Cryptococcus neoformans, have unique mating behaviours and regulatory mechanisms, demonstrating the diversity and adaptability of yeast reproductive strategies.

Mating types

Yeast cells can stably exist in either a diploid or a haploid form. Both haploid and diploid yeast cells reproduce by mitosis, in which daughter cells bud from mother cells. Haploid cells are capable of mating with other haploid cells of the opposite mating type (an a cell can only mate with an α cell and vice versa) to produce a stable diploid cell. Diploid cells, usually upon facing stressful conditions like nutrient depletion, can undergo meiosis to produce four haploid spores: two a spores and two α spores.^[1]^[2]

Differences between a and α cells

a cells produce a-factor, a mating pheromone which signals the presence of an a cell to neighbouring α cells.^[3] a cells respond to α-factor, the α cell mating pheromone, by growing a projection (known as a shmoo, due to its distinctive shape resembling the Al Capp cartoon character Shmoo) towards the source of α-factor.^[4] Similarly, α cells produce α-factor, and respond to a-factor by growing a projection towards the source of the pheromone.^[5] The selective response of haploid cells to the mating pheromones of the opposite mating type allows mating between a and α cells, but not between cells of the same mating type.^[6]

These phenotypic differences between a and α cells are due to a different set of genes being actively transcribed and repressed in cells of the two mating types. a cells activate genes which produce a-factor and produce a cell surface receptor (Ste2) which binds to α-factor and triggers signaling within the cell.^[7]^[8] a cells also repress the genes associated with being an α cell. Conversely, α cells activate genes which produce α-factor and produce a cell surface receptor (Ste3) which binds and responds to a-factor, and α cells repress the genes associated with being an a cell.^[9]

MAT locus

The different sets of transcriptional repression and activation, which characterize a and α cells, are caused by the presence of one of two alleles for a mating-type locus called MAT: MATa or MATα located on chromosome III.^[10] The MAT locus is usually divided into five regions (W, X, Y, Z1, and Z2) based on the sequences shared among the two mating types.^[11] The difference lie in the Y region (Ya and Yα), which contains most of the genes and promoters.^[7]

The MATa allele of MAT encodes a gene called a1, which directs the a-specific transcriptional program (such as expressing STE2 and repressing STE3) that defines an a haploid cell. The MATα allele of MAT encodes the α1 and α2 genes, which directs the α-specific transcriptional program (such as expressing STE3, repressing STE2, and producing prepro-α-factor) that defines an α haploid cell.^[7] S. cerevisiae has an a2 gene with no apparent function that shares much of its sequence with α2; however, other yeast species like Candida albicans do have a functional and distinct MATa2 gene.^[6]^[10]

Differences between haploid and diploid cells

Haploid cells are one of two mating types (a or α) and respond to the mating pheromone produced by haploid cells of the opposite mating type.^[4] Haploid cells cannot undergo meiosis.^[12] Diploid cells do not produce or respond to either mating pheromone and do not mate, but they can undergo meiosis to produce four haploid cells.^[13]

Like the differences between haploid a and α cells, different patterns of gene repression and activation are responsible for the phenotypic differences between haploid and diploid cells.^[14] In addition to the transcriptional patterns of a and α cells, haploid cells of both mating types share a haploid transcriptional pattern which activates haploid-specific genes (such as HO) and represses diploid-specific genes (such as IME1).^[15] Conversely, diploid cells activate diploid-specific genes and repress haploid-specific genes.^[16]

The different gene expression patterns of haploid and diploid cells are attributable to the MAT locus. Haploid cells only contain one copy of each of the 16 chromosomes and therefore only possess one MAT allele (either MATa or MATα), which determines their mating type.^[17] Diploid cells result from the mating of an a cell and an α cell, and they possess 32 chromosomes (in 16 pairs), including one chromosome bearing the MATa allele and another chromosome bearing the MATα allele.^[18] The combination of the information encoded by the MATa allele (the a1 gene) and the MATα allele (the α1 and α2 genes) triggers the diploid transcriptional program.^[19] Conversely, the presence of only one MAT allele, either MATa or MATα, triggers the haploid transcriptional program.^[20]^[7]

Through genetic engineering, a MATa allele can be added to a MATα haploid cell, causing it to behave like a diploid cell.^[21] The cell will not produce or respond to mating pheromones, and when starved, the cell will unsuccessfully attempt to undergo meiosis with fatal results.^[21] Similarly, deletion of one copy of the MAT locus in a diploid cell, leaving either a MATa or MATα allele, will cause a diploid cell to behave like a haploid cell of the associated mating type.^[22]^[23]

a-like faker cells

α cells with inactivated α1 and α2 genes at the MAT locus will exhibit the mating behavior of a cells. When an a-like faker (alf) cell mates with an α cell, they form a diploid cell lacking an active copy of the a1 gene. As a result, these diploid cells cannot form the a1-α2 protein complex needed to repress haploid-specific genes. This diploid cell will act like a haploid α cell, producing α pheromones to mate with an a haploid cell, resulting in aneuploidy.^[24]

Since α cells do not ordinarily mate with each other, the presence of a-like faker cells in a population of α cells can be detected in an a-like faker assay. This test exposes the MATα population, which lacks an active copy of the HIS3 gene, to a tester strain like YPH316 yeast, which lack a HIS1 gene, on YEPD agar. After transferring the pairs of yeast strains onto Sabouraud agar, only those that formed diploid cells by having a-like faker cells mate with the tester strain will be capable of synthesizing the amino acid histidine to survive. The extent of chromosome instability can be inferred from the proportion of surviving pairs since a-like faker cells naturally arise from damage to Chromosome III in yeast cells.^[25]

Decision to mate

Mating in yeast is stimulated by a cells' a-factor or α cells' α-factor pheromones binding the Ste3 receptor of α cells or Ste2 receptor of a cells, respectively, activating a heterotrimeric G protein.^[26]^[27]^[28] The dimeric portion of this G-protein recruits Ste5 and its MAPK cascade to the membrane, resulting in the phosphorylation of Fus3.^[29]

The switching mechanism arises as a result of competition between the Fus3 protein (a MAPK protein) and the phosphatase Ptc1.^[30] These proteins both attempt to control the four phosphorylation sites of Ste5, a scaffold protein, with Fus3 attempting to phosphorylate the phosphosites and Ptc1 attempting to dephosphorylate them.^[31]

Presence of α-factor induces recruitment of Ptc1 to Ste5 via a four-amino acid motif located within the Ste5 phosphosites.^[32] Ptc1 then dephosphorylates Ste5, resulting in the dissociation of the Fus3-Ste5 complex.^[33] Fus3 dissociates in a switch-like manner, dependent on the phosphorylation state of the four phosphosites.^[34] All four phosphosites must be dephosphorylated in order for Fus3 to dissociate.^[35]^[36] Fus3's ability to compete with Ptc1 decreases as Ptc1 is recruited, and thus the rate of dephosphorylation increases with the presence of pheromone.^[37]

Kss1, a homologue of Fus3, does not affect shmooing, and does not contribute to the switch-like mating decision.^[38]^[39]

In yeast, mating as well as the production of shmoos occur via an all-or-none, switch-like mechanism.^[40] This switch-like mechanism allows yeast cells to avoid making an unwise commitment to a highly demanding procedure.^[41] The decision to mate must balance being energy-conservative and fast enough to avoid losing the potential mate.^[42]

Yeast maintain an ultra-sensitivity to mating through:

Multi-site phosphorylation – Fus3 only dissociates from Ste5 and becomes fully active when all four of the phosphosites are dephosphorylated. Even one phosphorylated site will result in immunity to α-factor.^[43]
Two-stage binding – Fus3 and Ptc1 bind to separate docking sites on Ste5. Only after docking can they act on the phosphosites.^[44]
Steric hindrance – competition between Fus3 and Ptc1 to control the four phosphosites on Ste3

a and α yeast share the same mating response pathway, with the only difference being the type of receptor that each mating type possesses.^[45] Thus, the above description of an a-type yeast stimulated with α-factor resembles the mechanism of an α-type yeast stimulated with a-factor.^[46]^[47]

Mating type switching

Wild type haploid yeast are capable of switching mating type between a and α.^[48] Consequently, even if a single haploid cell of a given mating type founds a colony of yeast, mating type switching will cause cells of both a and α mating types to be present in the population.^[49]^[50] Combined with the strong drive for haploid cells to mate with cells of the opposite mating type and form diploids, mating type switching and consequent mating will cause the majority of cells in a colony to be diploid, regardless of whether a haploid or diploid cell founded the colony.^[51] The vast majority of yeast strains studied in laboratories have been altered such that they cannot perform mating type switching (by deletion of the HO gene; see below). This allows the stable propagation of haploid yeast, as haploid cells of the a mating type will remain a cells (and α cells will remain α cells), unable to form diploid cells unless artificially exposed to the other mating type.^[52]

HML and HMR: the silent mating cassettes

Haploid yeast switch mating type by replacing the information present at the MAT locus.^[53] For example, an a cell will switch to an α cell by replacing the MATa allele with the MATα allele.^[54] This replacement of one allele of MAT for the other is possible because yeast cells carry an additional silenced copy of both the MATa and MATα alleles: the HML (homothallic mating left) locus typically carries a silenced copy of the MATα allele, and the HMR (homothallic mating right) locus typically carries a silenced copy of the MATa allele.^[7] The silent HML and HMR loci are often referred to as the silent mating cassettes, as the information present there is 'read into' the active MAT locus.^[55]

These additional copies of the mating type information do not interfere with the function of whatever allele is present at the MAT locus because they are not expressed, so a haploid cell with the MATa allele present at the active MAT locus is still an a cell, despite also having a silenced copy of the MATα allele present at HML.^[56] Only the allele present at the active MAT locus is transcribed, and thus only the allele present at MAT will influence cell behaviour.^[6] Hidden mating type loci are epigenetically silenced by SIR proteins, which form a heterochromatin scaffold that prevents transcription from the silent mating cassettes.^[57]

Mechanics of the mating type switch

The process of mating type switching is a gene conversion event initiated by the HO gene.^[58] The HO gene is a tightly regulated haploid-specific gene that is only activated in haploid cells during the G₁ phase of the cell cycle.^[59] The protein encoded by the HO gene is a DNA endonuclease, which physically cleaves DNA, but only at the MAT locus (due to the DNA sequence specificity of the HO endonuclease).^[60]

Once HO cuts the DNA at MAT, exonucleases are attracted to the cut DNA ends and begin to degrade the DNA on both sides of the cut site.^[61] This DNA degradation by exonucleases eliminates the DNA which encoded the MAT allele; however, the resulting gap in the DNA is repaired by copying in the genetic information present at either HML or HMR, filling in a new allele of either the MATa or MATα gene. Thus, the silenced alleles of MATa and MATα present at HML and HMR serve as a source of genetic information to repair the HO-induced DNA damage at the active MAT locus.^[7]

Directionality of the mating type switch

The repair of the MAT locus after cutting by the HO endonuclease almost always results in a mating type switch.^[7]^[60] When an a cell cuts the MATa allele present at the MAT locus, the cut at MAT will almost always be repaired by copying the information present at HML.^[6] This results in MAT being repaired to the MATα allele, switching the mating type of the cell from a to α.^[62] Similarly, an α cell which has its MATα allele cut by the HO endonuclease will almost always repair the damage using the information present at HMR, copying the MATa gene to the MAT locus and switching the mating type of α cell to a.^[63]

This is the result of a recombination enhancer (RE) located on the left arm of chromosome III.^[64] Normally, a cells have Mcm1 bind to the RE to promote recombination using the HML region.^[65] Deletion of the RE causes a cells to instead repair using HMR, maintaining their status as a cells rather than switching mating types.^[66] In α cells, the α2 factor binds at the RE to repress recombination using the HML region.^[67] Thus, yeast have a predetermined tendency toward DNA repair of the MAT locus using the HMR region.^[68]

Mating and inbreeding

In 2006, evolutionary geneticist Leonid Kruglyak found that S. cerevisiae matings only involve out-crossing between different strains roughly once every 50,000 cell divisions. The vast majority of yeast mating instead involves members of the same strain because mating type switching allows a single ascus to produce both mating types from a single haploid cell.^[69] This suggests that yeast primarily maintain their capability to mate through recombinational DNA repair during meiosis, rather than natural selection for fitness among a population with high genetic variability.^[70]

Special cases

Fission yeast

Schizosaccharomyces pombe is a facultative sexual yeast that can undergo mating when nutrients are limited.^[71] Exposure of S. pombe to hydrogen peroxide, which causes oxidative stress to DNA, strongly induces mating, meiosis, and formation of meiotic spores.^[72] Thus, meiosis and meiotic recombination may be an adaptation for repairing DNA damage.^[73] The MAT locus' structure in S. pombe resembles S. cerevisiae. The mating-type switching system is similar but evolved independently.^[6]

Self-mating in Cryptococcus neoformans

Cryptococcus neoformans is a basidiomycetous fungus that grows as a budding yeast in culture and infected hosts. C. neoformans causes life-threatening meningoencephalitis in immunocompromised patients. It undergoes a filamentous transition during the sexual cycle to produce spores, the suspected infectious agent. The vast majority of environmental and clinical isolates of C. neoformans are of mating type α. Filaments ordinarily have haploid nuclei, but these can undergo a process of diploidization (perhaps by endoreduplication or stimulated nuclear fusion) to form diploid cells termed blastospores.^[74]

The diploid nuclei of blastospores can then undergo meiosis, including recombination, to form haploid basidiospores that can then be dispersed.^[74] This process is referred to as monokaryotic fruiting. This process depends on the gene dmc1, a conserved homologue of the bacterial RecA and eukaryotic RAD51 genes. Dmc1 mediates homologous chromosome pairing during meiosis and repair of double-strand breaks in DNA.^[75] Meiosis in C. neoformans may be performed to promote DNA repair in DNA-damaging environments, such as host-mediated responses to infection.^[74]

Notes

^ For the sake of clarity, this article bolds the Latin letter "a" and uses regular font weight for the Greek α. The usual convention is to print both in the same weight, but doing so would make the two letters hard to tell apart in italicized text.

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External links

[1] For the sake of clarity, this article bolds the Latin letter "a" and uses regular font weight for the Greek α. The usual convention is to print both in the same weight, but doing so would make the two letters hard to tell apart in italicized text.

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+{{Short description|Biological process of yeast}}
 {{cs1 config |name-list-style=vanc |display-authors=6}}
-[[File:Shmoos s cerevisiae.jpg|thumb|''[[Saccharomyces cerevisiae]]'' mating the '''a''' with shmoo responding to α-factor]]
+[[File:Shmoos s cerevisiae.jpg|thumb|''[[Saccharomyces cerevisiae]]'' mating with an '''a''' cell projecting a shmoo in response to α-factor]]
-The '''mating of yeast''', also known as '''yeast sexual reproduction''', is an important biological process integral to the [[biological life cycle|lifecycle]] of various yeast species, facilitating [[genetic diversity]] and [[adaptation]]. [[Yeast]]s such as ''[[Saccharomyces cerevisiae]]'' (bakers yeast) are simple [[microorganism|single-celled]] [[eukaryote]]s with both a [[diploid]] and [[haploid]] mode of existence. Mating of yeast only occurs between haploids, which can be either the '''a''' or α (alpha) [[mating type]] and thus display simple [[sexual differentiation]].<ref>For the sake of clarity, this article bolds the Latin letter "a" and uses regular [[font weight]] for the Greek α. The usual convention is to print both in the same weight, but doing so would make the two letters hard to tell apart in [[italicized]] text.</ref> Mating type is determined by a single [[locus (genetics)|locus]], ''MAT'', which in turn governs the sexual behaviour of both haploid and diploid cells. Through a form of [[genetic recombination]], haploid yeast can switch mating type as often as every [[cell cycle]].
+The '''mating of yeast''', also known as '''yeast sexual reproduction''', is a biological process that promotes [[genetic diversity]] and adaptation in [[yeast]] species. Yeast species, such as ''[[Saccharomyces cerevisiae]]'' (baker's yeast), are single-celled [[eukaryote]]s that can exist as either [[haploid]] cells, which contain a single set of [[chromosome]]s, or diploid cells, which contain two sets of chromosomes. Haploid yeast cells come in two [[mating type]]s, '''a''' and α, each producing specific [[pheromone]]s to identify and interact with the opposite type, thus displaying simple [[sexual differentiation]].{{efn|For the sake of clarity, this article bolds the Latin letter "a" and uses regular [[font weight]] for the Greek α. The usual convention is to print both in the same weight, but doing so would make the two letters hard to tell apart in [[italicized]] text.}} A yeast cell's mating type is determined by a specific [[genetic locus]] known as ''MAT'', which governs its mating behaviour. Haploid yeast can switch mating types through a form of [[genetic recombination]], allowing them to change mating type as often as every [[cell cycle]]. When two haploid cells of opposite mating types encounter each other, they undergo a complex [[cell signaling|signaling]] process that leads to cell fusion and the formation of a diploid cell. Diploid cells can [[asexual reproduction|reproduce asexually]], but under nutrient-limiting conditions, they undergo [[meiosis]] to produce new haploid spores.
+The differences between '''a''' and α cells, driven by specific [[gene expression]] patterns regulated by the ''MAT'' locus, are crucial for the mating process. Additionally, the decision to mate involves a highly sensitive and complex signaling pathway that includes pheromone detection and response mechanisms. In nature, yeast mating often occurs between closely related cells, although mating type switching and pheromone signaling allow for occasional [[outcrossing]] to enhance [[genetic variation]]. Furthermore, certain yeast species, like ''[[Schizosaccharomyces pombe]]'' and ''[[Cryptococcus neoformans]]'', have unique mating behaviours and regulatory mechanisms, demonstrating the diversity and adaptability of yeast reproductive strategies.
 == Mating types ==
-Yeasts can stably exist as either a diploid or a haploid. Both haploid and diploid yeast cells reproduce by [[mitosis]], with daughter cells budding from mother cells. Haploid cells are capable of mating with other haploid cells of the opposite mating type (an '''a''' cell can only mate with an α cell, and vice versa) to produce a stable diploid cell. Diploid cells, usually upon facing stressful conditions such as nutrient depletion, can undergo [[meiosis]] to produce four haploid [[spore]]s: two a spores and two α spores.
+Yeast cells can stably exist in either a diploid or a haploid form. Both haploid and diploid yeast cells reproduce by [[mitosis]], in which daughter cells bud from mother cells. Haploid cells are capable of mating with other haploid cells of the opposite mating type (an '''a''' cell can only mate with an α cell and vice versa) to produce a stable diploid cell. Diploid cells, usually upon facing stressful conditions like nutrient depletion, can undergo [[meiosis]] to produce four haploid [[spore]]s: two '''a''' spores and two α spores.<ref name="Bergman_2001">{{cite book | vauthors = Bergman LW | title = Two-Hybrid Systems | chapter = Growth and maintenance of yeast | series = Methods in Molecular Biology (Clifton, N.J.) | volume = 177 | pages = 9–14 | date = 2001 | pmid = 11530618 | doi = 10.1385/1-59259-210-4:009 | isbn = 1-59259-210-4 }}</ref><ref name="Börner_2023">{{cite journal | vauthors = Börner GV, Hochwagen A, MacQueen AJ | title = Meiosis in budding yeast | journal = Genetics | volume = 225 | issue = 2 | pages = | date = October 2023 | pmid = 37616582 | pmc = 10550323 | doi = 10.1093/genetics/iyad125 }}</ref>
 ===Differences between a and α cells===
-[[File:Yeast mating scheme.svg|thumb|Two haploid yeast of opposite mating types secrete pheromones, grow projections and mate.]]
+[[File:Yeast mating scheme.svg|thumb|Two haploid yeast of opposite mating types secrete pheromones, grow projections, and mate]]
-'''a''' cells produce ''''a'''-factor', a mating [[pheromone]] which signals the presence of an '''a''' cell to neighbouring α cells.<ref>{{cite journal | vauthors = Michaelis S, Barrowman J | title = Biogenesis of the Saccharomyces cerevisiae pheromone a-factor, from yeast mating to human disease | journal = Microbiology and Molecular Biology Reviews | volume = 76 | issue = 3 | pages = 626–651 | date = September 2012 | pmid = 22933563 | pmc = 3429625 | doi = 10.1128/MMBR.00010-12 }}</ref> '''a''' cells respond to α-factor, the α cell mating pheromone, by growing a projection (known as a shmoo, due to its distinctive shape resembling the [[Al Capp]] cartoon character [[Shmoo]]) towards the source of α-factor.<ref name="Merlini_2013">{{cite journal | vauthors = Merlini L, Dudin O, Martin SG | title = Mate and fuse: how yeast cells do it | journal = Open Biology | volume = 3 | issue = 3 | pages = 130008 | date = March 2013 | pmid = 23466674 | pmc = 3718343 | doi = 10.1098/rsob.130008 }}</ref> Similarly, α cells produce α-factor, and respond to '''a'''-factor by growing a projection towards the source of the pheromone.<ref>{{cite journal | vauthors = Chen W, Nie Q, Yi TM, Chou CS | title = Modelling of Yeast Mating Reveals Robustness Strategies for Cell-Cell Interactions | journal = PLOS Computational Biology | volume = 12 | issue = 7 | pages = e1004988 | date = July 2016 | pmid = 27404800 | pmc = 4942089 | doi = 10.1371/journal.pcbi.1004988 | bibcode = 2016PLSCB..12E4988C | doi-access = free | veditors = Edelstein-Keshet L }}</ref> The response of haploid cells only to the mating pheromones of the opposite mating type allows mating between '''a''' and α cells, but not between cells of the same mating type.<ref name="Hanson_2017">{{cite journal | vauthors = Hanson SJ, Wolfe KH | title = An Evolutionary Perspective on Yeast Mating-Type Switching | journal = Genetics | volume = 206 | issue = 1 | pages = 9–32 | date = May 2017 | pmid = 28476860 | pmc = 5419495 | doi = 10.1534/genetics.117.202036 }}</ref>
-These [[phenotype|phenotypic]] differences between '''a''' and α cells are due to a different set of [[gene]]s being actively [[transcription (biology)|transcribed]] and repressed in cells of the two mating types. '''a''' cells activate genes which produce '''a'''-factor and produce a [[cell surface receptor]] (Ste2) which binds to α-factor and triggers [[cell signaling|signaling]] within the cell.<ref name="Haber_2012">{{cite journal | vauthors = Haber JE | title = Mating-type genes and MAT switching in Saccharomyces cerevisiae | journal = Genetics | volume = 191 | issue = 1 | pages = 33–64 | date = May 2012 | pmid = 22555442 | pmc = 3338269 | doi = 10.1534/genetics.111.134577 }}</ref><ref>{{cite journal | vauthors = Gastaldi S, Zamboni M, Bolasco G, Di Segni G, Tocchini-Valentini GP | title = Analysis of random PCR-originated mutants of the yeast Ste2 and Ste3 receptors | journal = MicrobiologyOpen | volume = 5 | issue = 4 | pages = 670–686 | date = August 2016 | pmid = 27150158 | pmc = 4985600 | doi = 10.1002/mbo3.361 }}</ref> '''a''' cells also repress the genes associated with being an α cell. Similarly, α cells activate genes which produce α-factor and produce a cell surface receptor (Ste3) which binds and responds to '''a'''-factor, and α cells repress the genes associated with being an '''a''' cell.<ref>{{Cite book | vauthors =  Brenner S, Miller JH  |url=https://www.sciencedirect.com/referencework/9780122270802/encyclopedia-of-genetics |title=Encyclopedia of Genetics |publisher=[[Academic Press]] |year=2001 |isbn=978-0-12-227080-2 |pages=275–278 |language=en |chapter=Cassette Model |doi=10.1006/rwgn.2001.0162 |access-date=2024-05-09 |chapter-url=https://www.sciencedirect.com/science/article/abs/pii/B0122270800001622}}</ref>
+'''a''' cells produce '''a'''-factor, a mating [[pheromone]] which signals the presence of an '''a''' cell to neighbouring α cells.<ref>{{cite journal | vauthors = Michaelis S, Barrowman J | title = Biogenesis of the Saccharomyces cerevisiae pheromone a-factor, from yeast mating to human disease | journal = Microbiology and Molecular Biology Reviews | volume = 76 | issue = 3 | pages = 626–651 | date = September 2012 | pmid = 22933563 | pmc = 3429625 | doi = 10.1128/MMBR.00010-12 }}</ref> '''a''' cells respond to α-factor, the α cell mating pheromone, by growing a projection (known as a shmoo, due to its distinctive shape resembling the [[Al Capp]] cartoon character [[Shmoo]]) towards the source of α-factor.<ref name="Merlini_2013">{{cite journal | vauthors = Merlini L, Dudin O, Martin SG | title = Mate and fuse: how yeast cells do it | journal = Open Biology | volume = 3 | issue = 3 | pages = 130008 | date = March 2013 | pmid = 23466674 | pmc = 3718343 | doi = 10.1098/rsob.130008 }}</ref> Similarly, α cells produce α-factor, and respond to '''a'''-factor by growing a projection towards the source of the pheromone.<ref>{{cite journal | vauthors = Chen W, Nie Q, Yi TM, Chou CS | title = Modelling of Yeast Mating Reveals Robustness Strategies for Cell-Cell Interactions | journal = PLOS Computational Biology | volume = 12 | issue = 7 | pages = e1004988 | date = July 2016 | pmid = 27404800 | pmc = 4942089 | doi = 10.1371/journal.pcbi.1004988 | bibcode = 2016PLSCB..12E4988C | doi-access = free | veditors = Edelstein-Keshet L }}</ref> The selective response of haploid cells to the mating pheromones of the opposite mating type allows mating between '''a''' and α cells, but not between cells of the same mating type.<ref name="Hanson_2017">{{cite journal | vauthors = Hanson SJ, Wolfe KH | title = An Evolutionary Perspective on Yeast Mating-Type Switching | journal = Genetics | volume = 206 | issue = 1 | pages = 9–32 | date = May 2017 | pmid = 28476860 | pmc = 5419495 | doi = 10.1534/genetics.117.202036 }}</ref>
+These [[phenotype|phenotypic]] differences between '''a''' and α cells are due to a different set of [[gene]]s being actively [[transcription (biology)|transcribed]] and repressed in cells of the two mating types. '''a''' cells activate genes which produce '''a'''-factor and produce a [[cell surface receptor]] (Ste2) which binds to α-factor and triggers [[cell signaling|signaling]] within the cell.<ref name="Haber_2012">{{cite journal | vauthors = Haber JE | title = Mating-type genes and MAT switching in Saccharomyces cerevisiae | journal = Genetics | volume = 191 | issue = 1 | pages = 33–64 | date = May 2012 | pmid = 22555442 | pmc = 3338269 | doi = 10.1534/genetics.111.134577 }}</ref><ref>{{cite journal | vauthors = Gastaldi S, Zamboni M, Bolasco G, Di Segni G, Tocchini-Valentini GP | title = Analysis of random PCR-originated mutants of the yeast Ste2 and Ste3 receptors | journal = MicrobiologyOpen | volume = 5 | issue = 4 | pages = 670–686 | date = August 2016 | pmid = 27150158 | pmc = 4985600 | doi = 10.1002/mbo3.361 }}</ref> '''a''' cells also repress the genes associated with being an α cell. Conversely, α cells activate genes which produce α-factor and produce a cell surface receptor (Ste3) which binds and responds to '''a'''-factor, and α cells repress the genes associated with being an '''a''' cell.<ref>{{Cite book | vauthors =  Brenner S, Miller JH  |url=https://www.sciencedirect.com/referencework/9780122270802/encyclopedia-of-genetics |title=Encyclopedia of Genetics |publisher=[[Academic Press]] |year=2001 |isbn=978-0-12-227080-2 |pages=275–278 |language=en |chapter=Cassette Model |doi=10.1006/rwgn.2001.0162 |access-date=2024-05-09 |chapter-url=https://www.sciencedirect.com/science/article/abs/pii/B0122270800001622}}</ref>
-=== The ''MAT'' locus ===
-The different sets of transcriptional repression and activation which characterize '''a''' and α cells are caused by the presence of one of two [[allele]]s of a [[mating-type locus]] called ''MAT'': ''MAT'''a''''' or ''MATα'' located on chromosome III.<ref name = "Tsong_2003">{{cite journal | vauthors = Tsong AE, Miller MG, Raisner RM, Johnson AD | title = Evolution of a combinatorial transcriptional circuit: a case study in yeasts | journal = Cell | volume = 115 | issue = 4 | pages = 389–399 | date = November 2003 | pmid = 14622594 | doi = 10.1016/S0092-8674(03)00885-7 }}</ref> The MAT locus is usually divided into five regions (W, X, Y, Z1, and Z2) based on the sequences shared among the two mating types.<ref>{{cite journal | vauthors = Lee CS, Haber JE | title = Mating-type Gene Switching in Saccharomyces cerevisiae | journal = Microbiology Spectrum | volume = 3 | issue = 2 | date = April 2015 | pages = MDNA3–0013–2014 | pmid = 26104712 | doi = 10.1128/microbiolspec.MDNA3-0013-2014 | veditors = Gellert M, Craig N }}</ref> The difference lies in the Y region (Y'''a''' and Yα), which contains most of the genes and promoters.<ref name="Haber_2012" />
+=== ''MAT'' locus ===
-The ''MAT'''a''''' allele of ''MAT'' encodes a gene called '''a'''1, which in haploids direct the transcription of the '''a'''-specific transcriptional program (such as expressing ''STE2'' and repressing ''STE3'') that defines an '''a''' cell. The ''MATα'' allele of ''MAT'' encodes the α1 and α2 genes, which in haploids direct the transcription of the α-specific transcriptional program (such as expressing ''STE3'', repressing ''STE2,'' producing [[Prepro-alpha-factor|prepro-α-factor]]) which causes the cell to be an α cell.<ref name="Haber_2012" /> ''S. cerevisiae'' has an '''a'''2 gene with no apparent function that shares much of its sequence with α2; however, other yeasts like ''[[Candida albicans]]'' do have a functional and distinct MAT'''a'''2 gene.<ref name = "Tsong_2003" /><ref name="Hanson_2017" />
+The different sets of transcriptional repression and activation, which characterize '''a''' and α cells, are caused by the presence of one of two [[allele]]s for a [[mating-type locus]] called ''MAT'': ''MAT'''a''''' or ''MATα'' located on chromosome III.<ref name = "Tsong_2003">{{cite journal | vauthors = Tsong AE, Miller MG, Raisner RM, Johnson AD | title = Evolution of a combinatorial transcriptional circuit: a case study in yeasts | journal = Cell | volume = 115 | issue = 4 | pages = 389–399 | date = November 2003 | pmid = 14622594 | doi = 10.1016/S0092-8674(03)00885-7 | doi-access = free }}</ref> The ''MAT'' locus is usually divided into five regions (W, X, Y, Z1, and Z2) based on the sequences shared among the two mating types.<ref>{{cite journal | vauthors = Lee CS, Haber JE | title = Mating-type Gene Switching in Saccharomyces cerevisiae | journal = Microbiology Spectrum | volume = 3 | issue = 2 | date = April 2015 | pages = MDNA3–0013–2014 | pmid = 26104712 | doi = 10.1128/microbiolspec.MDNA3-0013-2014 | veditors = Gellert M, Craig N }}</ref> The difference lie in the Y region (Y'''a''' and Yα), which contains most of the genes and promoters.<ref name="Haber_2012" />
+The ''MAT'''a''''' allele of ''MAT'' encodes a gene called '''a'''1, which directs the '''a'''-specific transcriptional program (such as expressing ''STE2'' and repressing ''STE3'') that defines an '''a''' haploid cell. The ''MATα'' allele of ''MAT'' encodes the α1 and α2 genes, which directs the α-specific transcriptional program (such as expressing ''STE3'', repressing ''STE2,'' and producing [[Prepro-alpha-factor|prepro-α-factor]]) that defines an α haploid cell.<ref name="Haber_2012" /> ''S. cerevisiae'' has an '''''a'''2'' gene with no apparent function that shares much of its sequence with α2; however, other yeast species like ''[[Candida albicans]]'' do have a functional and distinct ''MAT'''a'''2'' gene.<ref name="Hanson_2017" /><ref name = "Tsong_2003" />
 ===Differences between haploid and diploid cells===
+[[File:Yeast Mating Types.png|thumb|upright=1.5|Regulation of mating in haploid and diploid yeast cells using the ''MAT'' locus]]
-[[Haploid]] cells are one of two mating types ('''a''' or α), and respond to the mating pheromone produced by haploid cells of the opposite mating type, and can mate with cells of the opposite mating type.<ref name="Merlini_2013" /> Haploid cells cannot undergo [[meiosis]].<ref>{{cite journal | vauthors = Wagstaff JE, Klapholz S, Esposito RE | title = Meiosis in haploid yeast | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 79 | issue = 9 | pages = 2986–2990 | date = May 1982 | pmid = 7045878 | doi = 10.1073/pnas.79.9.2986 | doi-access = free | pmc = 346333 }}</ref> [[Diploid]] cells do not produce or respond to either mating pheromone and do not mate, but can undergo [[meiosis]] to produce four haploid cells.<ref>{{cite journal | vauthors = Yeager R, Bushkin GG, Singer E, Fu R, Cooperman B, McMurray M | title = Post-Transcriptional Control of Mating-Type Gene Expression during Gametogenesis in <i>Saccharomyces cerevisiae</i> | journal = Biomolecules | volume = 11 | issue = 8 | pages = 1223 | date = August 2021 | pmid = 34439889 | pmc = 8394074 | doi = 10.3390/biom11081223 | doi-access = free }}</ref>
+[[Haploid]] cells are one of two mating types ('''a''' or α) and respond to the mating pheromone produced by haploid cells of the opposite mating type.<ref name="Merlini_2013" /> Haploid cells cannot undergo [[meiosis]].<ref>{{cite journal | vauthors = Wagstaff JE, Klapholz S, Esposito RE | title = Meiosis in haploid yeast | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 79 | issue = 9 | pages = 2986–2990 | date = May 1982 | pmid = 7045878 | doi = 10.1073/pnas.79.9.2986 | doi-access = free | pmc = 346333 | bibcode = 1982PNAS...79.2986W }}</ref> [[Diploid]] cells do not produce or respond to either mating pheromone and do not mate, but they can undergo [[meiosis]] to produce four haploid cells.<ref>{{cite journal | vauthors = Yeager R, Bushkin GG, Singer E, Fu R, Cooperman B, McMurray M | title = Post-Transcriptional Control of Mating-Type Gene Expression during Gametogenesis in ''Saccharomyces cerevisiae'' | journal = Biomolecules | volume = 11 | issue = 8 | pages = 1223 | date = August 2021 | pmid = 34439889 | pmc = 8394074 | doi = 10.3390/biom11081223 | doi-access = free }}</ref>
-Like the differences between haploid '''a''' and α cells, different patterns of gene repression and activation are responsible for the [[phenotype|phenotypic]] differences between haploid and diploid cells.<ref>{{cite journal | vauthors = Solieri L, Cassanelli S, Huff F, Barroso L, Branduardi P, Louis EJ, Morrissey JP | title = Insights on life cycle and cell identity regulatory circuits for unlocking genetic improvement in Zygosaccharomyces and Kluyveromyces yeasts | journal = FEMS Yeast Research | volume = 21 | issue = 8 | date = December 2021 | pmid = 34791177 | pmc = 8673824 | doi = 10.1093/femsyr/foab058 }}</ref> In addition to the specific '''a''' and α transcriptional patterns, haploid cells of both mating types share a haploid transcriptional pattern which activates haploid-specific genes (such as ''HO'') and represses diploid-specific genes (such as ''IME1'').<ref>{{cite journal | vauthors = Nagaraj VH, O'Flanagan RA, Bruning AR, Mathias JR, Vershon AK, Sengupta AM | title = Combined analysis of expression data and transcription factor binding sites in the yeast genome | journal = BMC Genomics | volume = 5 | issue = 1 | pages = 59 | date = August 2004 | pmid = 15331021 | pmc = 517709 | doi = 10.1186/1471-2164-5-59 | doi-access = free }}</ref> Similarly, diploid cells activate diploid-specific genes and repress haploid-specific genes.<ref>{{cite journal | vauthors = Voth WP, Stillman DJ | title = Changes in developmental state: demolish the old to construct the new | journal = Genes & Development | volume = 17 | issue = 18 | pages = 2201–2204 | date = September 2003 | pmid = 12975315 | doi = 10.1101/gad.1142103 }}</ref>
+Like the differences between haploid '''a''' and α cells, different patterns of gene repression and activation are responsible for the [[phenotype|phenotypic]] differences between haploid and diploid cells.<ref>{{cite journal | vauthors = Solieri L, Cassanelli S, Huff F, Barroso L, Branduardi P, Louis EJ, Morrissey JP | title = Insights on life cycle and cell identity regulatory circuits for unlocking genetic improvement in Zygosaccharomyces and Kluyveromyces yeasts | journal = FEMS Yeast Research | volume = 21 | issue = 8 | date = December 2021 | pmid = 34791177 | pmc = 8673824 | doi = 10.1093/femsyr/foab058 }}</ref> In addition to the transcriptional patterns of '''a''' and α cells, haploid cells of both mating types share a haploid transcriptional pattern which activates haploid-specific genes (such as ''HO'') and represses diploid-specific genes (such as ''IME1'').<ref>{{cite journal | vauthors = Nagaraj VH, O'Flanagan RA, Bruning AR, Mathias JR, Vershon AK, Sengupta AM | title = Combined analysis of expression data and transcription factor binding sites in the yeast genome | journal = BMC Genomics | volume = 5 | issue = 1 | pages = 59 | date = August 2004 | pmid = 15331021 | pmc = 517709 | doi = 10.1186/1471-2164-5-59 | doi-access = free }}</ref> Conversely, diploid cells activate diploid-specific genes and repress haploid-specific genes.<ref>{{cite journal | vauthors = Voth WP, Stillman DJ | title = Changes in developmental state: demolish the old to construct the new | journal = Genes & Development | volume = 17 | issue = 18 | pages = 2201–2204 | date = September 2003 | pmid = 12975315 | doi = 10.1101/gad.1142103 | doi-access = free }}</ref>
-The different gene expression patterns of haploids and diploids are again due to the ''MAT'' locus. Haploid cells only contain one copy of each of the 16 [[chromosome]]s and thus can only possess one allele of ''MAT'' (either ''MAT'''a''''' or ''MATα''), which determines their mating type.<ref>{{Cite book | vauthors = Watkinson SC, Boddy L, Money NP  |url=https://www.sciencedirect.com/topics/immunology-and-microbiology/fungal-mating-type-gene |title=The Fungi |publisher=[[Academic Press]] |year=2016 |isbn=978-0-12-382034-1 |edition=3rd |pages=99–139 |language=en |chapter=Chapter 4 - Genetics – Variation, Sexuality, and Evolution |doi=10.1016/B978-0-12-382034-1.00004-9 |chapter-url=https://www.sciencedirect.com/science/article/abs/pii/B9780123820341000049}}</ref> Diploid cells result from the mating of an '''a''' cell and an α cell, and thus possess 32 chromosomes (in 16 pairs), including one chromosome bearing the ''MAT'''a''''' allele and another chromosome bearing the ''MATα'' allele.<ref>{{cite journal | vauthors = Bizzarri M, Giudici P, Cassanelli S, Solieri L | title = Chimeric Sex-Determining Chromosomal Regions and Dysregulation of Cell-Type Identity in a Sterile Zygosaccharomyces Allodiploid Yeast | journal = PLOS ONE | volume = 11 | issue = 4 | pages = e0152558 | date = 2016-04-11 | pmid = 27065237 | pmc = 4827841 | doi = 10.1371/journal.pone.0152558 | bibcode = 2016PLoSO..1152558B | doi-access = free | veditors = Fairhead C }}</ref> The combination of the information encoded by the ''MAT'''a''''' allele (the '''a'''1 gene) and the ''MATα'' allele (the α1 and α2 genes) triggers the diploid transcriptional program.<ref>{{cite journal | vauthors = Zill OA, Rine J | title = Interspecies variation reveals a conserved repressor of alpha-specific genes in Saccharomyces yeasts | journal = Genes & Development | volume = 22 | issue = 12 | pages = 1704–1716 | date = June 2008 | pmid = 18559484 | pmc = 2428066 | doi = 10.1101/gad.1640008 }}</ref> Similarly, the presence of only a single allele of ''MAT'', whether it is ''MAT'''a''''' or ''MATα'', triggers the haploid transcriptional program.
+The different gene expression patterns of haploid and diploid cells are attributable to the ''MAT'' locus. Haploid cells only contain one copy of each of the 16 [[chromosome]]s and therefore only possess one ''MAT'' allele (either ''MAT'''a''''' or ''MATα''), which determines their mating type.<ref>{{Cite book | vauthors = Watkinson SC, Boddy L, Money NP  |url=https://www.sciencedirect.com/topics/immunology-and-microbiology/fungal-mating-type-gene |title=The Fungi |publisher=[[Academic Press]] |year=2016 |isbn=978-0-12-382034-1 |edition=3rd |pages=99–139 |language=en |chapter=Chapter 4 - Genetics – Variation, Sexuality, and Evolution |doi=10.1016/B978-0-12-382034-1.00004-9 |chapter-url=https://www.sciencedirect.com/science/article/abs/pii/B9780123820341000049}}</ref> Diploid cells result from the mating of an '''a''' cell and an α cell, and they possess 32 chromosomes (in 16 pairs), including one chromosome bearing the ''MAT'''a''''' allele and another chromosome bearing the ''MATα'' allele.<ref>{{cite journal | vauthors = Bizzarri M, Giudici P, Cassanelli S, Solieri L | title = Chimeric Sex-Determining Chromosomal Regions and Dysregulation of Cell-Type Identity in a Sterile Zygosaccharomyces Allodiploid Yeast | journal = PLOS ONE | volume = 11 | issue = 4 | pages = e0152558 | date = 2016-04-11 | pmid = 27065237 | pmc = 4827841 | doi = 10.1371/journal.pone.0152558 | bibcode = 2016PLoSO..1152558B | doi-access = free | veditors = Fairhead C }}</ref> The combination of the information encoded by the ''MAT'''a''''' allele (the '''a'''1 gene) and the ''MATα'' allele (the α1 and α2 genes) triggers the diploid transcriptional program.<ref>{{cite journal | vauthors = Zill OA, Rine J | title = Interspecies variation reveals a conserved repressor of alpha-specific genes in Saccharomyces yeasts | journal = Genes & Development | volume = 22 | issue = 12 | pages = 1704–1716 | date = June 2008 | pmid = 18559484 | pmc = 2428066 | doi = 10.1101/gad.1640008 }}</ref> Conversely, the presence of only one ''MAT'' allele, either ''MAT'''a''''' or ''MATα'', triggers the haploid transcriptional program.<ref name="Leupold_1980">{{cite journal | vauthors = Leupold U | title = Transposable mating-type genes in yeasts | journal = Nature | volume = 283 | issue = 5750 | pages = 811–812 | date = February 1980 | pmid = 6987523 | doi = 10.1038/283811a0 | bibcode = 1980Natur.283..811L }}</ref><ref name="Haber_2012" />
-The alleles present at the ''MAT'' locus are [[necessary and sufficient conditions|sufficient]] to program the mating behaviour of the cell. For example, using [[genetic engineering|genetic manipulations]], a ''MAT'''a''''' allele can be added to a ''MATα'' haploid cell.<ref>{{Cite book | vauthors = Lennarz WJ, Lane MD |url=https://www.sciencedirect.com/referencework/9780124437104/encyclopedia-of-biological-chemistry |title=Encyclopedia of Biological Chemistry |publisher=[[Academic Press]] |year=2004 |isbn=978-0-12-443710-4 |pages=200–203 |language=en |chapter=Transcriptional Silencing |doi=10.1016/B0-12-443710-9/00723-7 |access-date=2024-05-09 |chapter-url=https://www.sciencedirect.com/science/article/abs/pii/B0124437109007237}}</ref> Despite having a haploid complement of chromosomes, the cell now has both the ''MAT'''a''''' and ''MATα'' alleles, and will behave like a diploid cell: it will not produce or respond to mating pheromones, and when starved will attempt to undergo meiosis, with fatal results.<ref>{{cite journal | vauthors = Lengeler KB, Davidson RC, D'souza C, Harashima T, Shen WC, Wang P, Pan X, Waugh M, Heitman J | title = Signal transduction cascades regulating fungal development and virulence | journal = Microbiology and Molecular Biology Reviews | volume = 64 | issue = 4 | pages = 746–785 | date = December 2000 | pmid = 11104818 | pmc = 99013 | doi = 10.1128/MMBR.64.4.746-785.2000 }}</ref> Similarly, deletion of one copy of the ''MAT'' locus in a diploid cell, leaving only a single ''MAT'''a''''' or ''MATα'' allele, will cause a cell with a diploid complement of chromosomes to behave like a haploid cell.<ref>{{cite journal | vauthors = Lee K, Neigeborn L, Kaufman RJ | title = The unfolded protein response is required for haploid tolerance in yeast | journal = The Journal of Biological Chemistry | volume = 278 | issue = 14 | pages = 11818–11827 | date = April 2003 | pmid = 12560331 | doi = 10.1074/jbc.M210475200 | doi-access = free }}</ref><ref>{{Cite book | vauthors = Conn PM |title=Handbook of Models for Human Aging |publisher=[[Academic Press]] |year=2006 |isbn=978-0-12-369391-4 |language=en |chapter=17 - Telomeres and Aging in the Yeast Model System |pages=191–205 |doi=10.1016/B978-0-12-369391-4.X5000-0 |chapter-url=https://www.sciencedirect.com/science/article/abs/pii/B9780123693914500187}}</ref>
+Through [[genetic engineering]], a ''MAT'''a''''' allele can be added to a ''MATα'' haploid cell, causing it to behave like a diploid cell.<ref name=":0">{{Cite journal |last1=Maekawa |first1=Hiromi |last2=Kaneko |first2=Yoshinobu |date=2014-11-20 |editor-last=Heitman |editor-first=Joseph |title=Inversion of the Chromosomal Region between Two Mating Type Loci Switches the Mating Type in Hansenula polymorpha |journal=PLOS Genetics |language=en |volume=10 |issue=11 |pages=e1004796 |doi=10.1371/journal.pgen.1004796 |doi-access=free |issn=1553-7404 |pmc=4238957 |pmid=25412462}}</ref> The cell will not produce or respond to mating pheromones, and when starved, the cell will unsuccessfully attempt to undergo meiosis with fatal results.<ref name=":0" /> Similarly, deletion of one copy of the ''MAT'' locus in a diploid cell, leaving either a ''MAT'''a''''' or ''MATα'' allele, will cause a diploid cell to behave like a haploid cell of the associated mating type.<ref>{{cite journal | vauthors = Lee K, Neigeborn L, Kaufman RJ | title = The unfolded protein response is required for haploid tolerance in yeast | journal = The Journal of Biological Chemistry | volume = 278 | issue = 14 | pages = 11818–11827 | date = April 2003 | pmid = 12560331 | doi = 10.1074/jbc.M210475200 | doi-access = free }}</ref><ref>{{Cite book | vauthors = Conn PM |title=Handbook of Models for Human Aging |publisher=[[Academic Press]] |year=2006 |isbn=978-0-12-369391-4 |language=en |chapter=17 - Telomeres and Aging in the Yeast Model System |pages=191–205 |doi=10.1016/B978-0-12-369391-4.X5000-0 |chapter-url=https://www.sciencedirect.com/science/article/abs/pii/B9780123693914500187}}</ref>
+=== a-like faker cells ===
+α cells with inactivated ''α1'' and ''α2'' genes at the ''MAT'' locus will exhibit the mating behavior of '''a''' cells. When an '''a'''-like faker (alf) cell mates with an α cell, they form a diploid cell lacking an active copy of the '''''a'''1'' gene. As a result, these diploid cells cannot form the '''a'''1-α2 protein complex needed to repress haploid-specific genes. This diploid cell will act like a haploid α cell, producing α pheromones to mate with an '''a''' haploid cell, resulting in [[aneuploidy]].<ref>{{Cite journal |last1=Fraser |first1=James A. |last2=Heitman |first2=Joseph |date=23 February 2004 |title=Evolution of Fungal Sex Chromosomes |url=https://onlinelibrary.wiley.com/doi/10.1046/j.1365-2958.2003.03874.x |journal=[[Molecular Microbiology (journal)|Molecular Microbiology]] |language=en |volume=51 |issue=2 |pages=299–306 |doi=10.1046/j.1365-2958.2003.03874.x |pmid=14756773 |issn=0950-382X}}</ref>
+Since α cells do not ordinarily mate with each other, the presence of '''a'''-like faker cells in a population of α cells can be detected in an '''a'''-like faker assay. This test exposes the ''MATα'' population, which lacks an active copy of the ''[[HIS3]]'' gene, to a tester strain like YPH316 yeast, which lack a ''HIS1'' gene, on [[YEPD]] [[agar]]. After transferring the pairs of yeast strains onto [[Sabouraud agar]], only those that formed diploid cells by having '''a'''-like faker cells mate with the tester strain will be capable of synthesizing the [[amino acid]] [[histidine]] to survive. The extent of [[chromosome instability]] can be inferred from the proportion of surviving pairs since '''a'''-like faker cells naturally arise from damage to Chromosome III in yeast cells.<ref>{{Cite book |last1=Novoa |first1=Carolina A. |title=Genome Instability |last2=Ang |first2=J. Sidney |last3=Stirling |first3=Peter C. |date=17 October 2017 |publisher=[[Springer Nature]] |isbn=978-1-4939-7305-7 |editor-last=Muzi-Falconi |editor-first=Marco |pages=1–9 |chapter=The A-Like Faker Assay for Measuring Yeast Chromosome III Stability |series=Methods in Molecular Biology |volume=1672 |doi=10.1007/978-1-4939-7306-4_1 |pmid=29043612 |editor-last2=Brown |editor-first2=Grant W. |chapter-url=https://link.springer.com/protocol/10.1007/978-1-4939-7306-4_1}}</ref>
 ==Decision to mate==
-Mating in yeast is stimulated by the presence of a pheromone which binds to either the Ste2 receptor (in a-cells) or the Ste3 receptor (in α-cells).<ref>{{cite journal | vauthors = Versele M, Lemaire K, Thevelein JM | title = Sex and sugar in yeast: two distinct GPCR systems | journal = EMBO Reports | volume = 2 | issue = 7 | pages = 574–579 | date = July 2001 | pmid = 11463740 | pmc = 1083946 | doi = 10.1093/embo-reports/kve132 }}</ref><ref>{{cite journal | vauthors = Emmerstorfer-Augustin A, Augustin CM, Shams S, Thorner J | title = Tracking yeast pheromone receptor Ste2 endocytosis using fluorogen-activating protein tagging | journal = Molecular Biology of the Cell | volume = 29 | issue = 22 | pages = 2720–2736 | date = November 2018 | pmid = 30207829 | pmc = 6249837 | doi = 10.1091/mbc.E18-07-0424 | veditors = Glick BS }}</ref> The binding of this pheromone then leads to the activation of a [[heterotrimeric G protein]].<ref>{{cite journal | vauthors = Navarro-Olmos R, Kawasaki L, Domínguez-Ramírez L, Ongay-Larios L, Pérez-Molina R, Coria R | title = The beta subunit of the heterotrimeric G protein triggers the Kluyveromyces lactis pheromone response pathway in the absence of the gamma subunit | journal = Molecular Biology of the Cell | volume = 21 | issue = 3 | pages = 489–498 | date = February 2010 | pmid = 20016006 | pmc = 2814793 | doi = 10.1091/mbc.e09-06-0472 | veditors = Boone C }}</ref> The dimeric portion of this G-protein recruits Ste5 (and its related [[mitogen-activated protein kinase|MAPK]] cascade components) to the membrane, and ultimately results in the phosphorylation of [[Fus3]].<ref>{{cite journal | vauthors = Winters MJ, Pryciak PM | title = MAPK modulation of yeast pheromone signaling output and the role of phosphorylation sites in the scaffold protein Ste5 | journal = Molecular Biology of the Cell | volume = 30 | issue = 8 | pages = 1037–1049 | date = April 2019 | pmid = 30726174 | pmc = 6589907 | doi = 10.1091/mbc.E18-12-0793 | veditors = Boone C }}</ref>
+Mating in yeast is stimulated by '''a''' cells' '''a'''-factor or α cells' α-factor pheromones binding the Ste3 receptor of α cells or Ste2 receptor of '''a''' cells, respectively, activating a [[heterotrimeric G protein]].<ref>{{cite journal | vauthors = Versele M, Lemaire K, Thevelein JM | title = Sex and sugar in yeast: two distinct GPCR systems | journal = EMBO Reports | volume = 2 | issue = 7 | pages = 574–579 | date = July 2001 | pmid = 11463740 | pmc = 1083946 | doi = 10.1093/embo-reports/kve132 }}</ref><ref>{{cite journal | vauthors = Emmerstorfer-Augustin A, Augustin CM, Shams S, Thorner J | title = Tracking yeast pheromone receptor Ste2 endocytosis using fluorogen-activating protein tagging | journal = Molecular Biology of the Cell | volume = 29 | issue = 22 | pages = 2720–2736 | date = November 2018 | pmid = 30207829 | pmc = 6249837 | doi = 10.1091/mbc.E18-07-0424 | veditors = Glick BS }}</ref><ref>{{cite journal | vauthors = Navarro-Olmos R, Kawasaki L, Domínguez-Ramírez L, Ongay-Larios L, Pérez-Molina R, Coria R | title = The beta subunit of the heterotrimeric G protein triggers the Kluyveromyces lactis pheromone response pathway in the absence of the gamma subunit | journal = Molecular Biology of the Cell | volume = 21 | issue = 3 | pages = 489–498 | date = February 2010 | pmid = 20016006 | pmc = 2814793 | doi = 10.1091/mbc.e09-06-0472 | veditors = Boone C }}</ref> The dimeric portion of this G-protein recruits Ste5 and its [[mitogen-activated protein kinase|MAPK]] [[Signal transduction|cascade]] to the [[Cell membrane|membrane]], resulting in the phosphorylation of [[Fus3]].<ref>{{cite journal | vauthors = Winters MJ, Pryciak PM | title = MAPK modulation of yeast pheromone signaling output and the role of phosphorylation sites in the scaffold protein Ste5 | journal = Molecular Biology of the Cell | volume = 30 | issue = 8 | pages = 1037–1049 | date = April 2019 | pmid = 30726174 | pmc = 6589907 | doi = 10.1091/mbc.E18-12-0793 | veditors = Boone C }}</ref>
-The switching mechanism arises as a result of competition between the Fus3 protein (a MAPK protein) and the [[phosphatase]] [[Ptc1]].<ref>{{cite journal | vauthors = Malleshaiah MK, Shahrezaei V, Swain PS, Michnick SW | title = The scaffold protein Ste5 directly controls a switch-like mating decision in yeast | journal = Nature | volume = 465 | issue = 7294 | pages = 101–105 | date = May 2010 | pmid = 20400943 | doi = 10.1038/nature08946 | bibcode = 2010Natur.465..101M }}</ref> These proteins both attempt to control the 4 phosphorylation sites of [[Ste5]], a [[scaffold protein]] with Fus3 attempting to phosphorylate the phosphosites, and Ptc1 attempting to dephosphorylate them.<ref>{{cite journal | vauthors = Choudhury S, Baradaran-Mashinchi P, Torres MP | title = Negative Feedback Phosphorylation of Gγ Subunit Ste18 and the Ste5 Scaffold Synergistically Regulates MAPK Activation in Yeast | journal = Cell Reports | volume = 23 | issue = 5 | pages = 1504–1515 | date = May 2018 | pmid = 29719261 | pmc = 5987779 | doi = 10.1016/j.celrep.2018.03.135 }}</ref>
+The switching mechanism arises as a result of competition between the Fus3 protein (a MAPK protein) and the [[phosphatase]] [[Ptc1]].<ref>{{cite journal | vauthors = Malleshaiah MK, Shahrezaei V, Swain PS, Michnick SW | title = The scaffold protein Ste5 directly controls a switch-like mating decision in yeast | journal = Nature | volume = 465 | issue = 7294 | pages = 101–105 | date = May 2010 | pmid = 20400943 | doi = 10.1038/nature08946 | bibcode = 2010Natur.465..101M }}</ref> These proteins both attempt to control the four phosphorylation sites of [[Ste5]], a [[scaffold protein]], with Fus3 attempting to phosphorylate the phosphosites and Ptc1 attempting to dephosphorylate them.<ref>{{cite journal | vauthors = Choudhury S, Baradaran-Mashinchi P, Torres MP | title = Negative Feedback Phosphorylation of Gγ Subunit Ste18 and the Ste5 Scaffold Synergistically Regulates MAPK Activation in Yeast | journal = Cell Reports | volume = 23 | issue = 5 | pages = 1504–1515 | date = May 2018 | pmid = 29719261 | pmc = 5987779 | doi = 10.1016/j.celrep.2018.03.135 }}</ref>
-Presence of α-factor induces recruitment of Ptc1 to Ste5 via a 4 amino acid motif located within the Ste5 phosphosites.<ref>{{cite journal | vauthors = Chen RE, Thorner J | title = Function and regulation in MAPK signaling pathways: lessons learned from the yeast Saccharomyces cerevisiae | journal = Biochimica et Biophysica Acta (BBA) - Molecular Cell Research | volume = 1773 | issue = 8 | pages = 1311–1340 | date = August 2007 | pmid = 17604854 | pmc = 2031910 | doi = 10.1016/j.bbamcr.2007.05.003 }}</ref> Ptc1 then dephosphorylates Ste5, ultimately resulting in the dissociation of the Fus3-Ste5 complex.<ref>{{cite journal | vauthors = Ariño J, Casamayor A, González A | title = Type 2C protein phosphatases in fungi | journal = Eukaryotic Cell | volume = 10 | issue = 1 | pages = 21–33 | date = January 2011 | pmid = 21076010 | pmc = 3019798 | doi = 10.1128/EC.00249-10 }}</ref> Fus3 dissociates in a switch-like manner, dependent on the phosphorylation state of the 4 phosphosites.<ref>{{cite journal | vauthors = Nagiec MJ, McCarter PC, Kelley JB, Dixit G, Elston TC, Dohlman HG | title = Signal inhibition by a dynamically regulated pool of monophosphorylated MAPK | journal = Molecular Biology of the Cell | volume = 26 | issue = 18 | pages = 3359–3371 | date = September 2015 | pmid = 26179917 | pmc = 4569323 | doi = 10.1091/mbc.e15-01-0037 | veditors = Boone C }}</ref> All 4 phosphosites must be dephosphorylated in order for Fus3 to dissociate.<ref>{{cite journal | vauthors = Gartner A, Nasmyth K, Ammerer G | title = Signal transduction in Saccharomyces cerevisiae requires tyrosine and threonine phosphorylation of FUS3 and KSS1 | journal = Genes & Development | volume = 6 | issue = 7 | pages = 1280–1292 | date = July 1992 | pmid = 1628831 | doi = 10.1101/gad.6.7.1280 }}</ref><ref>{{cite journal | vauthors = Martins BM, Swain PS | title = Ultrasensitivity in phosphorylation-dephosphorylation cycles with little substrate | journal = PLOS Computational Biology | volume = 9 | issue = 8 | pages = e1003175 | date = 2013-08-08 | pmid = 23950701 | pmc = 3738489 | doi = 10.1371/journal.pcbi.1003175 | doi-access = free | bibcode = 2013PLSCB...9E3175M | veditors = Mac Gabhann F }}</ref> Fus3's ability to compete with Ptc1 decreases as Ptc1 is recruited, and thus the rate of dephosphorylation increases with the presence of pheromone.<ref>{{cite journal | vauthors = Ariño J, Velázquez D, Casamayor A | title = Ser/Thr protein phosphatases in fungi: structure, regulation and function | journal = Microbial Cell | volume = 6 | issue = 5 | pages = 217–256 | date = April 2019 | pmid = 31114794 | pmc = 6506691 | doi = 10.15698/mic2019.05.677 }}</ref>
+Presence of α-factor induces recruitment of Ptc1 to Ste5 via a four-amino acid motif located within the Ste5 phosphosites.<ref>{{cite journal | vauthors = Chen RE, Thorner J | title = Function and regulation in MAPK signaling pathways: lessons learned from the yeast Saccharomyces cerevisiae | journal = Biochimica et Biophysica Acta (BBA) - Molecular Cell Research | volume = 1773 | issue = 8 | pages = 1311–1340 | date = August 2007 | pmid = 17604854 | pmc = 2031910 | doi = 10.1016/j.bbamcr.2007.05.003 }}</ref> Ptc1 then dephosphorylates Ste5, resulting in the dissociation of the Fus3-Ste5 complex.<ref>{{cite journal | vauthors = Ariño J, Casamayor A, González A | title = Type 2C protein phosphatases in fungi | journal = Eukaryotic Cell | volume = 10 | issue = 1 | pages = 21–33 | date = January 2011 | pmid = 21076010 | pmc = 3019798 | doi = 10.1128/EC.00249-10 }}</ref> Fus3 dissociates in a switch-like manner, dependent on the phosphorylation state of the four phosphosites.<ref>{{cite journal | vauthors = Nagiec MJ, McCarter PC, Kelley JB, Dixit G, Elston TC, Dohlman HG | title = Signal inhibition by a dynamically regulated pool of monophosphorylated MAPK | journal = Molecular Biology of the Cell | volume = 26 | issue = 18 | pages = 3359–3371 | date = September 2015 | pmid = 26179917 | pmc = 4569323 | doi = 10.1091/mbc.e15-01-0037 | veditors = Boone C }}</ref> All four phosphosites must be dephosphorylated in order for Fus3 to dissociate.<ref>{{cite journal | vauthors = Gartner A, Nasmyth K, Ammerer G | title = Signal transduction in Saccharomyces cerevisiae requires tyrosine and threonine phosphorylation of FUS3 and KSS1 | journal = Genes & Development | volume = 6 | issue = 7 | pages = 1280–1292 | date = July 1992 | pmid = 1628831 | doi = 10.1101/gad.6.7.1280 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Martins BM, Swain PS | title = Ultrasensitivity in phosphorylation-dephosphorylation cycles with little substrate | journal = PLOS Computational Biology | volume = 9 | issue = 8 | pages = e1003175 | date = 2013-08-08 | pmid = 23950701 | pmc = 3738489 | doi = 10.1371/journal.pcbi.1003175 | doi-access = free | bibcode = 2013PLSCB...9E3175M | veditors = Mac Gabhann F }}</ref> Fus3's ability to compete with Ptc1 decreases as Ptc1 is recruited, and thus the rate of dephosphorylation increases with the presence of pheromone.<ref>{{cite journal | vauthors = Ariño J, Velázquez D, Casamayor A | title = Ser/Thr protein phosphatases in fungi: structure, regulation and function | journal = Microbial Cell | volume = 6 | issue = 5 | pages = 217–256 | date = April 2019 | pmid = 31114794 | pmc = 6506691 | doi = 10.15698/mic2019.05.677 }}</ref>
 Kss1, a homologue of Fus3, does not affect shmooing, and does not contribute to the switch-like mating decision.<ref>{{cite journal | vauthors = Li Y, Roberts J, AkhavanAghdam Z, Hao N | title = Mitogen-activated protein kinase (MAPK) dynamics determine cell fate in the yeast mating response | journal = The Journal of Biological Chemistry | volume = 292 | issue = 50 | pages = 20354–20361 | date = December 2017 | pmid = 29123025 | pmc = 5733576 | doi = 10.1074/jbc.AC117.000548 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Errede B, Vered L, Ford E, Pena MI, Elston TC | title = Pheromone-induced morphogenesis and gradient tracking are dependent on the MAPK Fus3 binding to Gα | journal = Molecular Biology of the Cell | volume = 26 | issue = 18 | pages = 3343–3358 | date = September 2015 | pmid = 26179918 | pmc = 4569322 | doi = 10.1091/mbc.e15-03-0176 | veditors = Boone C }}</ref>
-In yeast, mating as well as the production of shmoos occur via an all-or-none, switch-like mechanism.<ref>{{cite journal | vauthors = Wang X, Tian W, Banh BT, Statler BM, Liang J, Stone DE | title = Mating yeast cells use an intrinsic polarity site to assemble a pheromone-gradient tracking machine | journal = The Journal of Cell Biology | volume = 218 | issue = 11 | pages = 3730–3752 | date = November 2019 | pmid = 31570500 | pmc = 6829655 | doi = 10.1083/jcb.201901155 }}</ref> This switch-like mechanism allows yeast cells to avoid making an unwise commitment to a highly demanding procedure.<ref>{{cite journal | vauthors = Marini G, Nüske E, Leng W, Alberti S, Pigino G | title = Reorganization of budding yeast cytoplasm upon energy depletion | journal = Molecular Biology of the Cell | volume = 31 | issue = 12 | pages = 1232–1245 | date = June 2020 | pmid = 32293990 | pmc = 7353153 | doi = 10.1091/mbc.E20-02-0125 | veditors = Bloom K }}</ref> However, not only does the mating decision need to be conservative (in order to avoid wasting energy), but it must also be fast to avoid losing the potential mate.<ref>{{Cite journal | vauthors = Malek H, Long T |date=2019-01-01 |title=Adventures in Time and Space: What Shapes Behavioural Decisions in Drosophila melanogaster? |url=https://scholars.wlu.ca/etd/2127 |journal=Theses and Dissertations (Comprehensive)}}</ref>
+In yeast, mating as well as the production of shmoos occur via an all-or-none, switch-like mechanism.<ref>{{cite journal | vauthors = Wang X, Tian W, Banh BT, Statler BM, Liang J, Stone DE | title = Mating yeast cells use an intrinsic polarity site to assemble a pheromone-gradient tracking machine | journal = The Journal of Cell Biology | volume = 218 | issue = 11 | pages = 3730–3752 | date = November 2019 | pmid = 31570500 | pmc = 6829655 | doi = 10.1083/jcb.201901155 }}</ref> This switch-like mechanism allows yeast cells to avoid making an unwise commitment to a highly demanding procedure.<ref>{{cite journal | vauthors = Marini G, Nüske E, Leng W, Alberti S, Pigino G | title = Reorganization of budding yeast cytoplasm upon energy depletion | journal = Molecular Biology of the Cell | volume = 31 | issue = 12 | pages = 1232–1245 | date = June 2020 | pmid = 32293990 | pmc = 7353153 | doi = 10.1091/mbc.E20-02-0125 | veditors = Bloom K }}</ref> The decision to mate must balance being energy-conservative and fast enough to avoid losing the potential mate.<ref>{{Cite journal |last1=Morano |first1=Kevin A |last2=Grant |first2=Chris M |last3=Moye-Rowley |first3=W Scott |date=2012-04-01 |title=The Response to Heat Shock and Oxidative Stress in Saccharomyces cerevisiae |journal=Genetics |language=en |volume=190 |issue=4 |pages=1157–1195 |doi=10.1534/genetics.111.128033 |issn=1943-2631 |pmc=3316637 |pmid=22209905}}</ref>
+Yeast maintain an ultra-sensitivity to mating through:
-The decision to mate is extremely sensitive. There are 3 ways in which this ultrasensitivity is maintained:
-#Multi-site phosphorylation – Fus3 only dissociates from Ste5 and becomes fully active when all 4 of the phosphosites are dephosphorylated. Even one phosphorylated site will result in immunity to α-factor.<ref>{{cite journal | vauthors = Liu X, Bardwell L, Nie Q | title = A combination of multisite phosphorylation and substrate sequestration produces switchlike responses | journal = Biophysical Journal | volume = 98 | issue = 8 | pages = 1396–1407 | date = April 2010 | pmid = 20409458 | pmc = 2856190 | doi = 10.1016/j.bpj.2009.12.4307 | bibcode = 2010BpJ....98.1396L }}</ref>
+#Multi-site phosphorylation – Fus3 only dissociates from Ste5 and becomes fully active when all four of the phosphosites are dephosphorylated. Even one phosphorylated site will result in immunity to α-factor.<ref>{{cite journal | vauthors = Liu X, Bardwell L, Nie Q | title = A combination of multisite phosphorylation and substrate sequestration produces switchlike responses | journal = Biophysical Journal | volume = 98 | issue = 8 | pages = 1396–1407 | date = April 2010 | pmid = 20409458 | pmc = 2856190 | doi = 10.1016/j.bpj.2009.12.4307 | bibcode = 2010BpJ....98.1396L }}</ref>
-#Two-stage binding – Fus3 and Ptc1 bind to separate docking sites on Ste5. Only after docking can they bind to, and act on, the phosphosites.<ref>{{cite journal | vauthors = Bhaduri S, Pryciak PM | title = Cyclin-specific docking motifs promote phosphorylation of yeast signaling proteins by G1/S Cdk complexes | journal = Current Biology | volume = 21 | issue = 19 | pages = 1615–1623 | date = October 2011 | pmid = 21945277 | pmc = 3196376 | doi = 10.1016/j.cub.2011.08.033 | bibcode = 2011CBio...21.1615B }}</ref>
+#Two-stage binding – Fus3 and Ptc1 bind to separate docking sites on Ste5. Only after docking can they act on the phosphosites.<ref>{{cite journal | vauthors = Bhaduri S, Pryciak PM | title = Cyclin-specific docking motifs promote phosphorylation of yeast signaling proteins by G1/S Cdk complexes | journal = Current Biology | volume = 21 | issue = 19 | pages = 1615–1623 | date = October 2011 | pmid = 21945277 | pmc = 3196376 | doi = 10.1016/j.cub.2011.08.033 | bibcode = 2011CBio...21.1615B }}</ref>
-#Steric hindrance – competition between Fus3 and Ptc1 to control the 4 phosphosites on Ste3
+#Steric hindrance – competition between Fus3 and Ptc1 to control the four phosphosites on Ste3
-a and α yeast share the same mating response pathway, with the only difference being the type of receptor each mating type possesses.<ref>{{cite journal | vauthors = Gonçalves-Sá J, Murray A | title = Asymmetry in sexual pheromones is not required for ascomycete mating | journal = Current Biology | volume = 21 | issue = 16 | pages = 1337–1346 | date = August 2011 | pmid = 21835624 | pmc = 3159855 | doi = 10.1016/j.cub.2011.06.054 | bibcode = 2011CBio...21.1337G }}</ref> Thus the above description, given for a-type yeast stimulated with α-factor, works equally well for α-type yeast stimulated with a-factor.<ref>{{cite journal | vauthors = Banderas A, Koltai M, Anders A, Sourjik V | title = Sensory input attenuation allows predictive sexual response in yeast | journal = Nature Communications | volume = 7 | issue = 1 | pages = 12590 | date = August 2016 | pmid = 27557894 | pmc = 5007329 | doi = 10.1038/ncomms12590 | bibcode = 2016NatCo...712590B }}</ref><ref>{{cite journal | vauthors = Muller N, Piel M, Calvez V, Voituriez R, Gonçalves-Sá J, Guo CL, Jiang X, Murray A, Meunier N | title = A Predictive Model for Yeast Cell Polarization in Pheromone Gradients | journal = PLOS Computational Biology | volume = 12 | issue = 4 | pages = e1004795 | date = April 2016 | pmid = 27077831 | pmc = 4831791 | doi = 10.1371/journal.pcbi.1004795 | doi-access = free | bibcode = 2016PLSCB..12E4795M | veditors = Rao CV }}</ref>
+'''a''' and α yeast share the same mating response pathway, with the only difference being the type of receptor that each mating type possesses.<ref>{{cite journal | vauthors = Gonçalves-Sá J, Murray A | title = Asymmetry in sexual pheromones is not required for ascomycete mating | journal = Current Biology | volume = 21 | issue = 16 | pages = 1337–1346 | date = August 2011 | pmid = 21835624 | pmc = 3159855 | doi = 10.1016/j.cub.2011.06.054 | bibcode = 2011CBio...21.1337G }}</ref> Thus, the above description of an '''a'''-type yeast stimulated with α-factor resembles the mechanism of an α-type yeast stimulated with a-factor.<ref>{{cite journal | vauthors = Banderas A, Koltai M, Anders A, Sourjik V | title = Sensory input attenuation allows predictive sexual response in yeast | journal = Nature Communications | volume = 7 | issue = 1 | pages = 12590 | date = August 2016 | pmid = 27557894 | pmc = 5007329 | doi = 10.1038/ncomms12590 | bibcode = 2016NatCo...712590B }}</ref><ref>{{cite journal | vauthors = Muller N, Piel M, Calvez V, Voituriez R, Gonçalves-Sá J, Guo CL, Jiang X, Murray A, Meunier N | title = A Predictive Model for Yeast Cell Polarization in Pheromone Gradients | journal = PLOS Computational Biology | volume = 12 | issue = 4 | pages = e1004795 | date = April 2016 | pmid = 27077831 | pmc = 4831791 | doi = 10.1371/journal.pcbi.1004795 | doi-access = free | bibcode = 2016PLSCB..12E4795M | veditors = Rao CV }}</ref>
 ==Mating type switching==
 [[File:Yeast mating reasoning.svg|thumb|A haploid yeast dividing and undergoing a mating type switch, allowing mating and diploid formation.]]
-Wild type haploid yeast are capable of switching mating type between '''a''' and α.<ref>{{Cite book | vauthors =  Brenner S, Miller JH  |title=Encyclopedia of Genetics |publisher=[[Academic Press]] |year=2001 |isbn=978-0-12-227080-2 |pages=798–800 |language=en |chapter=Gene Rearrangement in Eukaryotic Organisms |doi=10.1006/rwgn.2001.0518 |chapter-url=https://www.sciencedirect.com/science/article/abs/pii/B0122270800005188}}</ref> Consequently, even if a single haploid cell of a given mating type founds a [[Clonal colony|colony]] of yeast, mating type switching will cause cells of both '''a''' and α mating types to be present in the population.<ref>{{cite journal | vauthors = Coughlan AY, Lombardi L, Braun-Galleani S, Martos AA, Galeote V, Bigey F, Dequin S, Byrne KP, Wolfe KH | title = The yeast mating-type switching endonuclease HO is a domesticated member of an unorthodox homing genetic element family | journal = eLife | volume = 9 | date = April 2020 | pmid = 32338594 | pmc = 7282813 | doi = 10.7554/eLife.55336 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Laney JD, Hochstrasser M | title = Ubiquitin-dependent degradation of the yeast Mat(alpha)2 repressor enables a switch in developmental state | journal = Genes & Development | volume = 17 | issue = 18 | pages = 2259–2270 | date = September 2003 | pmid = 12952895 | pmc = 196463 | doi = 10.1101/gad.1115703 }}</ref> Combined with the strong drive for haploid cells to mate with cells of the opposite mating type and form diploids, mating type switching and consequent mating will cause the majority of cells in a colony to be diploid, regardless of whether a haploid or diploid cell founded the colony.<ref>{{cite journal | vauthors = Wang Y, Lo WC, Chou CS | title = A modeling study of budding yeast colony formation and its relationship to budding pattern and aging | journal = PLOS Computational Biology | volume = 13 | issue = 11 | pages = e1005843 | date = November 2017 | pmid = 29121651 | pmc = 5697893 | doi = 10.1371/journal.pcbi.1005843 | doi-access = free | bibcode = 2017PLSCB..13E5843W | veditors = Komarova NL }}</ref> The vast majority of yeast [[strain (biology)|strains]] studied in [[laboratory|laboratories]] have been altered such that they cannot perform mating type switching (by deletion of the ''HO'' gene;<ref>{{Cite book|title=Handbook of nucleic acid purification|date=2009|publisher=CRC Press| vauthors = Liu D |isbn=9781420070972|location=Boca Raton|oclc=614294429|page=174}}</ref> see below); this allows the stable propagation of haploid yeast, as haploid cells of the '''a''' mating type will remain '''a''' cells (and α cells will remain α cells), and will not form diploids.
+Wild type haploid yeast are capable of switching mating type between '''a''' and α.<ref>{{Cite book | vauthors =  Brenner S, Miller JH  |title=Encyclopedia of Genetics |publisher=[[Academic Press]] |year=2001 |isbn=978-0-12-227080-2 |pages=798–800 |language=en |chapter=Gene Rearrangement in Eukaryotic Organisms |doi=10.1006/rwgn.2001.0518 |chapter-url=https://www.sciencedirect.com/science/article/abs/pii/B0122270800005188}}</ref> Consequently, even if a single haploid cell of a given mating type founds a [[Clonal colony|colony]] of yeast, mating type switching will cause cells of both '''a''' and α mating types to be present in the population.<ref>{{cite journal | vauthors = Coughlan AY, Lombardi L, Braun-Galleani S, Martos AA, Galeote V, Bigey F, Dequin S, Byrne KP, Wolfe KH | title = The yeast mating-type switching endonuclease HO is a domesticated member of an unorthodox homing genetic element family | journal = eLife | volume = 9 | date = April 2020 | pmid = 32338594 | pmc = 7282813 | doi = 10.7554/eLife.55336 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Laney JD, Hochstrasser M | title = Ubiquitin-dependent degradation of the yeast Mat(alpha)2 repressor enables a switch in developmental state | journal = Genes & Development | volume = 17 | issue = 18 | pages = 2259–2270 | date = September 2003 | pmid = 12952895 | pmc = 196463 | doi = 10.1101/gad.1115703 }}</ref> Combined with the strong drive for haploid cells to mate with cells of the opposite mating type and form diploids, mating type switching and consequent mating will cause the majority of cells in a colony to be diploid, regardless of whether a haploid or diploid cell founded the colony.<ref>{{cite journal | vauthors = Wang Y, Lo WC, Chou CS | title = A modeling study of budding yeast colony formation and its relationship to budding pattern and aging | journal = PLOS Computational Biology | volume = 13 | issue = 11 | pages = e1005843 | date = November 2017 | pmid = 29121651 | pmc = 5697893 | doi = 10.1371/journal.pcbi.1005843 | doi-access = free | bibcode = 2017PLSCB..13E5843W | veditors = Komarova NL }}</ref> The vast majority of yeast [[strain (biology)|strains]] studied in [[laboratory|laboratories]] have been altered such that they cannot perform mating type switching (by deletion of the ''HO'' gene; see below). This allows the stable propagation of haploid yeast, as haploid cells of the '''a''' mating type will remain '''a''' cells (and α cells will remain α cells), unable to form diploid cells unless artificially exposed to the other mating type.<ref>{{Cite book|title=Handbook of nucleic acid purification|date=2009|publisher=CRC Press| vauthors = Liu D |isbn=9781420070972|location=Boca Raton|oclc=614294429|page=174}}</ref>
-[[File:Yeast Mating Type Chromasome Diagram EN.svg|thumb|Location of the silent ''HML'' and ''HMR'' loci and the active ''MAT'' locus on yeast chromosome III.]]
+[[File:Yeast Mating Type Chromasome Diagram EN.svg|thumb|Location of the silent ''HML'' and ''HMR'' loci and the active ''MAT'' locus on yeast chromosome III]]
 ===''HML'' and ''HMR'': the silent mating cassettes===
-Haploid yeast switch mating type by replacing the information present at the ''MAT'' locus.<ref>{{cite journal | vauthors = Shore D | title = Genetic recombination: sex-change operations in yeast | journal = Current Biology | volume = 7 | issue = 1 | pages = R24–R27 | date = January 1997 | pmid = 9072164 | doi = 10.1016/S0960-9822(06)00012-1 | bibcode = 1997CBio....7..R24S }}</ref> For example, an '''a''' cell will switch to an α cell by replacing the ''MAT'''a''''' allele with the ''MATα'' allele.<ref>{{cite journal | vauthors = Klar AJ | title = The yeast mating-type switching mechanism: a memoir | journal = Genetics | volume = 186 | issue = 2 | pages = 443–449 | date = October 2010 | pmid = 20940334 | pmc = 2942867 | doi = 10.1534/genetics.110.122531 }}</ref> This replacement of one allele of ''MAT'' for the other is possible because yeast cells carry an additional [[gene silencing|silenced]] copy of both the ''MAT'''a''''' and ''MATα'' alleles: the ''HML'' ('''h'''omothallic '''m'''ating '''l'''eft) locus typically carries a silenced copy of the ''MATα'' allele, and the ''HMR'' ('''h'''omothallic '''m'''ating '''r'''ight) locus typically carries a silenced copy of the ''MAT'''a''''' allele.<ref name="Haber_2012" /> The silent ''HML'' and ''HMR'' loci are often referred to as the silent mating cassettes, as the information present there is 'read into' the active ''MAT'' locus.<ref>{{cite journal | vauthors = Weber JM, Ehrenhofer-Murray AE | title = Design of a minimal silencer for the silent mating-type locus HML of Saccharomyces cerevisiae | journal = Nucleic Acids Research | volume = 38 | issue = 22 | pages = 7991–8000 | date = December 2010 | pmid = 20699276 | pmc = 3001064 | doi = 10.1093/nar/gkq689 }}</ref>
-These additional copies of the mating type information do not interfere with the function of whatever allele is present at the ''MAT'' locus because they are not expressed, so a haploid cell with the ''MAT'''a''''' allele present at the active ''MAT'' locus is still an '''a''' cell, despite also having a (silenced) copy of the ''MATα'' allele present at ''HML''.<ref>{{cite journal | vauthors = Maroc L, Zhou-Li Y, Boisnard S, Fairhead C | title = A single Ho-induced double-strand break at the MAT locus is lethal in Candida glabrata | journal = PLOS Genetics | volume = 16 | issue = 10 | pages = e1008627 | date = October 2020 | pmid = 33057400 | pmc = 7591073 | doi = 10.1371/journal.pgen.1008627 | doi-access = free | veditors = Heitman J }}</ref> Only the allele present at the active ''MAT'' locus is transcribed, and thus only the allele present at ''MAT'' will influence cell behaviour.<ref name="Hanson_2017" /> Hidden mating type loci are epigenetically silenced by [[SIR protein]]s, which form a [[heterochromatin]] scaffold that prevents transcription from the silent mating cassettes.<ref>{{cite journal | vauthors = Faure G, Jézéquel K, Roisné-Hamelin F, Bitard-Feildel T, Lamiable A, Marcand S, Callebaut I | title = Discovery and Evolution of New Domains in Yeast Heterochromatin Factor Sir4 and Its Partner Esc1 | journal = Genome Biology and Evolution | volume = 11 | issue = 2 | pages = 572–585 | date = February 2019 | pmid = 30668669 | pmc = 6394760 | doi = 10.1093/gbe/evz010 | veditors = Wolfe K }}</ref>
+Haploid yeast switch mating type by replacing the information present at the ''MAT'' locus.<ref>{{cite journal | vauthors = Shore D | title = Genetic recombination: sex-change operations in yeast | journal = Current Biology | volume = 7 | issue = 1 | pages = R24–R27 | date = January 1997 | pmid = 9072164 | doi = 10.1016/S0960-9822(06)00012-1 | bibcode = 1997CBio....7..R24S | doi-access = free }}</ref> For example, an '''a''' cell will switch to an α cell by replacing the ''MAT'''a''''' allele with the ''MATα'' allele.<ref>{{cite journal | vauthors = Klar AJ | title = The yeast mating-type switching mechanism: a memoir | journal = Genetics | volume = 186 | issue = 2 | pages = 443–449 | date = October 2010 | pmid = 20940334 | pmc = 2942867 | doi = 10.1534/genetics.110.122531 }}</ref> This replacement of one allele of ''MAT'' for the other is possible because yeast cells carry an additional [[gene silencing|silenced]] copy of both the ''MAT'''a''''' and ''MATα'' alleles: the ''HML'' ('''h'''omothallic '''m'''ating '''l'''eft) locus typically carries a silenced copy of the ''MATα'' allele, and the ''HMR'' ('''h'''omothallic '''m'''ating '''r'''ight) locus typically carries a silenced copy of the ''MAT'''a''''' allele.<ref name="Haber_2012" /> The silent ''HML'' and ''HMR'' loci are often referred to as the silent mating cassettes, as the information present there is 'read into' the active ''MAT'' locus.<ref>{{cite journal | vauthors = Weber JM, Ehrenhofer-Murray AE | title = Design of a minimal silencer for the silent mating-type locus HML of Saccharomyces cerevisiae | journal = Nucleic Acids Research | volume = 38 | issue = 22 | pages = 7991–8000 | date = December 2010 | pmid = 20699276 | pmc = 3001064 | doi = 10.1093/nar/gkq689 }}</ref>
+These additional copies of the mating type information do not interfere with the function of whatever allele is present at the ''MAT'' locus because they are not expressed, so a haploid cell with the ''MAT'''a''''' allele present at the active ''MAT'' locus is still an '''a''' cell, despite also having a silenced copy of the ''MATα'' allele present at ''HML''.<ref>{{cite journal | vauthors = Maroc L, Zhou-Li Y, Boisnard S, Fairhead C | title = A single Ho-induced double-strand break at the MAT locus is lethal in Candida glabrata | journal = PLOS Genetics | volume = 16 | issue = 10 | pages = e1008627 | date = October 2020 | pmid = 33057400 | pmc = 7591073 | doi = 10.1371/journal.pgen.1008627 | doi-access = free | veditors = Heitman J }}</ref> Only the allele present at the active ''MAT'' locus is transcribed, and thus only the allele present at ''MAT'' will influence cell behaviour.<ref name="Hanson_2017" /> Hidden mating type loci are epigenetically silenced by [[SIR protein]]s, which form a [[heterochromatin]] scaffold that prevents transcription from the silent mating cassettes.<ref>{{cite journal | vauthors = Faure G, Jézéquel K, Roisné-Hamelin F, Bitard-Feildel T, Lamiable A, Marcand S, Callebaut I | title = Discovery and Evolution of New Domains in Yeast Heterochromatin Factor Sir4 and Its Partner Esc1 | journal = Genome Biology and Evolution | volume = 11 | issue = 2 | pages = 572–585 | date = February 2019 | pmid = 30668669 | pmc = 6394760 | doi = 10.1093/gbe/evz010 | veditors = Wolfe K }}</ref>
 ===Mechanics of the mating type switch===
 [[File:Yeast Mating Type Promoter Structure.png|thumb|right|Yeast mating type promoter structure]]
 The process of mating type switching is a [[gene conversion]] event initiated by the ''HO'' gene.<ref>{{cite journal | vauthors = Thon G, Maki T, Haber JE, Iwasaki H | title = Mating-type switching by homology-directed recombinational repair: a matter of choice | journal = Current Genetics | volume = 65 | issue = 2 | pages = 351–362 | date = April 2019 | pmid = 30382337 | pmc = 6420890 | doi = 10.1007/s00294-018-0900-2 }}</ref> The ''HO'' gene is a tightly regulated haploid-specific gene that is only activated in haploid cells during the [[G1 phase|G<sub>1</sub> phase]] of the [[cell cycle]].<ref>{{cite journal | vauthors = Krebs JE, Kuo MH, Allis CD, Peterson CL | title = Cell cycle-regulated histone acetylation required for expression of the yeast HO gene | journal = Genes & Development | volume = 13 | issue = 11 | pages = 1412–1421 | date = June 1999 | pmid = 10364158 | pmc = 316758 | doi = 10.1101/gad.13.11.1412 }}</ref> The [[protein]] encoded by the ''HO'' gene is a [[endonuclease|DNA endonuclease]], which physically cleaves DNA, but only at the ''MAT'' locus (due to the DNA sequence specificity of the HO endonuclease).<ref name = "Butler_2004">{{cite journal | vauthors = Butler G, Kenny C, Fagan A, Kurischko C, Gaillardin C, Wolfe KH | title = Evolution of the MAT locus and its Ho endonuclease in yeast species | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 101 | issue = 6 | pages = 1632–1637 | date = February 2004 | pmid = 14745027 | pmc = 341799 | doi = 10.1073/pnas.0304170101 | doi-access = free | bibcode = 2004PNAS..101.1632B }}</ref>
@@ Line 64: / Line 81: @@
 ===Directionality of the mating type switch===
 The repair of the ''MAT'' locus after cutting by the HO endonuclease almost always results in a mating type switch.<ref name="Haber_2012" /><ref name = "Butler_2004" /> When an '''a''' cell cuts the ''MAT'''a''''' allele present at the ''MAT'' locus, the cut at ''MAT'' will almost always be repaired by copying the information present at ''HML''.<ref name="Hanson_2017" /> This results in ''MAT'' being repaired to the ''MATα'' allele, switching the mating type of the cell from '''a''' to α.<ref>{{cite journal | vauthors = Belton JM, Lajoie BR, Audibert S, Cantaloube S, Lassadi I, Goiffon I, Baù D, Marti-Renom MA, Bystricky K, Dekker J | title = The Conformation of Yeast Chromosome III Is Mating Type Dependent and Controlled by the Recombination Enhancer | journal = Cell Reports | volume = 13 | issue = 9 | pages = 1855–1867 | date = December 2015 | pmid = 26655901 | pmc = 4681004 | doi = 10.1016/j.celrep.2015.10.063 }}</ref> Similarly, an α cell which has its ''MATα'' allele cut by the HO endonuclease will almost always repair the damage using the information present at ''HMR'', copying the ''MAT'''a''''' gene to the ''MAT'' locus and switching the mating type of α cell to '''a'''.<ref>{{cite journal | vauthors = Kaplun L, Ivantsiv Y, Bakhrat A, Raveh D | title = DNA damage response-mediated degradation of Ho endonuclease via the ubiquitin system involves its nuclear export | journal = The Journal of Biological Chemistry | volume = 278 | issue = 49 | pages = 48727–48734 | date = December 2003 | pmid = 14506225 | doi = 10.1074/jbc.M308671200 | doi-access = free }}</ref>
-This is the result of the action of a [[recombination enhancer]] (RE) located on the left arm of chromosome III.<ref>{{cite journal | vauthors = Houston P, Simon PJ, Broach JR | title = The Saccharomyces cerevisiae recombination enhancer biases recombination during interchromosomal mating-type switching but not in interchromosomal homologous recombination | journal = Genetics | volume = 166 | issue = 3 | pages = 1187–1197 | date = March 2004 | pmid = 15082540 | pmc = 1470794 | doi = 10.1534/genetics.166.3.1187 }}</ref> Deletion of this region causes '''a''' cells to incorrectly repair using HMR.<ref>{{cite journal | vauthors = Ruan C, Workman JL, Simpson RT | title = The DNA repair protein yKu80 regulates the function of recombination enhancer during yeast mating type switching | journal = Molecular and Cellular Biology | volume = 25 | issue = 19 | pages = 8476–8485 | date = October 2005 | pmid = 16166630 | pmc = 1265738 | doi = 10.1128/MCB.25.19.8476-8485.2005 }}</ref> In '''a''' cells, [[Mcm1]] binds to the RE and promotes recombination of the HML region<ref>{{cite journal | vauthors = Wu C, Weiss K, Yang C, Harris MA, Tye BK, Newlon CS, Simpson RT, Haber JE | title = Mcm1 regulates donor preference controlled by the recombination enhancer in Saccharomyces mating-type switching | journal = Genes & Development | volume = 12 | issue = 11 | pages = 1726–1737 | date = June 1998 | pmid = 9620858 | pmc = 316872 | doi = 10.1101/gad.12.11.1726 }}</ref>. In α cells, the α2 factor binds at the RE and establishes a repressive domain over RE such that recombination is unlikely to occur.<ref>{{cite journal | vauthors = Dodson AE, Rine J | title = Donor Preference Meets Heterochromatin: Moonlighting Activities of a Recombinational Enhancer in Saccharomyces cerevisiae | journal = Genetics | volume = 204 | issue = 3 | pages = 1065–1074 | date = November 2016 | pmid = 27655944 | pmc = 5105842 | doi = 10.1534/genetics.116.194696 }}</ref> An innate bias means that the default behaviour is repair from HMR. The exact mechanisms of these interactions are still under investigation.<ref>{{cite journal | vauthors = Catalani E, Fanelli G, Silvestri F, Cherubini A, Del Quondam S, Bongiorni S, Taddei AR, Ceci M, De Palma C, Perrotta C, Rinalducci S, Prantera G, Cervia D | title = Nutraceutical Strategy to Counteract Eye Neurodegeneration and Oxidative Stress in <i>Drosophila melanogaster</i> Fed with High-Sugar Diet | journal = Antioxidants | volume = 10 | issue = 8 | pages = 1197 | date = July 2021 | pmid = 34439445 | pmc = 8388935 | doi = 10.3390/antiox10081197 | doi-access = free }}</ref>
+This is the result of a [[recombination enhancer]] (RE) located on the left arm of chromosome III.<ref>{{cite journal | vauthors = Houston P, Simon PJ, Broach JR | title = The Saccharomyces cerevisiae recombination enhancer biases recombination during interchromosomal mating-type switching but not in interchromosomal homologous recombination | journal = Genetics | volume = 166 | issue = 3 | pages = 1187–1197 | date = March 2004 | pmid = 15082540 | pmc = 1470794 | doi = 10.1534/genetics.166.3.1187 }}</ref> Normally, '''a''' cells have [[Mcm1]] bind to the RE to promote recombination using the HML region.<ref>{{cite journal | vauthors = Wu C, Weiss K, Yang C, Harris MA, Tye BK, Newlon CS, Simpson RT, Haber JE | title = Mcm1 regulates donor preference controlled by the recombination enhancer in Saccharomyces mating-type switching | journal = Genes & Development | volume = 12 | issue = 11 | pages = 1726–1737 | date = June 1998 | pmid = 9620858 | pmc = 316872 | doi = 10.1101/gad.12.11.1726 }}</ref> Deletion of the RE causes '''a''' cells to instead repair using HMR, maintaining their status as '''a''' cells rather than switching mating types.<ref>{{cite journal | vauthors = Ruan C, Workman JL, Simpson RT | title = The DNA repair protein yKu80 regulates the function of recombination enhancer during yeast mating type switching | journal = Molecular and Cellular Biology | volume = 25 | issue = 19 | pages = 8476–8485 | date = October 2005 | pmid = 16166630 | pmc = 1265738 | doi = 10.1128/MCB.25.19.8476-8485.2005 }}</ref> In α cells, the α2 factor binds at the RE to repress recombination using the HML region.<ref>{{cite journal | vauthors = Dodson AE, Rine J | title = Donor Preference Meets Heterochromatin: Moonlighting Activities of a Recombinational Enhancer in Saccharomyces cerevisiae | journal = Genetics | volume = 204 | issue = 3 | pages = 1065–1074 | date = November 2016 | pmid = 27655944 | pmc = 5105842 | doi = 10.1534/genetics.116.194696 }}</ref> Thus, yeast have a predetermined tendency toward DNA repair of the ''MAT'' locus using the HMR region.<ref>{{Cite journal |last1=Sussel |first1=L |last2=Vannier |first2=D |last3=Shore |first3=D |date=1995-11-01 |title=Suppressors of defective silencing in yeast: effects on transcriptional repression at the HMR locus, cell growth and telomere structure. |url=https://academic.oup.com/genetics/article/141/3/873/6013526 |journal=Genetics |language=en |volume=141 |issue=3 |pages=873–888 |doi=10.1093/genetics/141.3.873 |issn=1943-2631 |pmc=1206851 |pmid=8582633}}</ref>
 ==Mating and inbreeding==
-Ruderfer et al.<ref name=Ruderfer>{{cite journal | vauthors = Ruderfer DM, Pratt SC, Seidel HS, Kruglyak L | title = Population genomic analysis of outcrossing and recombination in yeast | journal = Nature Genetics | volume = 38 | issue = 9 | pages = 1077–1081 | date = September 2006 | pmid = 16892060 | doi = 10.1038/ng1859 | s2cid = 783720 }}</ref> analyzed the ancestry of natural ''S. cerevisiae'' strains and concluded that matings involving out-crossing occur only about once every 50,000 cell divisions. Thus it appears that, in nature, mating is most often between closely related yeast cells. Mating occurs when haploid cells of opposite mating type ''MAT'''a''''' and ''MATα'' come into contact. Ruderfer et al.<ref name=Ruderfer/> pointed out that such contacts are frequent between closely related yeast cells for two reasons. The first is that cells of opposite mating type are present together in the same ascus, the sac that contains the cells directly produced by a single meiosis, and these cells can mate with each other. The second reason is that haploid cells of one mating type, upon cell division, often produce cells of the opposite mating type with which they can mate (see section "Mating type switching", above). The relative rarity in nature of meiotic events that result from out-crossing appears to be inconsistent with the idea that production of genetic variation is the primary selective force maintaining mating capability in this organism. However this finding is consistent with the alternative idea that the primary selective force maintaining mating capability is enhanced recombinational repair of DNA damage during meiosis,<ref>{{cite book | vauthors = Birdsell JA, Wills C | date = 2003 | chapter = The evolutionary origin and maintenance of sexual recombination: A review of contemporary models. | series = Evolutionary Biology Series | title = Evolutionary Biology | volume = 33 | pages = 27–137 | veditors =  MacIntyre RJ, Clegg MT | publisher = Springer | isbn = 978-0306472619 }}</ref> since this benefit is realized during each meiosis subsequent to a mating, whether or not out-crossing occurs.
+In 2006, evolutionary geneticist [[Leonid Kruglyak]] found that ''S. cerevisiae'' matings only involve out-crossing between different strains roughly once every 50,000 cell divisions. The vast majority of yeast mating instead involves members of the same strain because mating type switching allows a single [[ascus]] to produce both mating types from a single haploid cell.<ref name="Ruderfer_2006">{{cite journal | vauthors = Ruderfer DM, Pratt SC, Seidel HS, Kruglyak L | title = Population genomic analysis of outcrossing and recombination in yeast | journal = Nature Genetics | volume = 38 | issue = 9 | pages = 1077–1081 | date = September 2006 | pmid = 16892060 | doi = 10.1038/ng1859 | s2cid = 783720 }}</ref> This suggests that yeast primarily maintain their capability to mate through recombinational DNA repair during meiosis, rather than [[natural selection]] for [[Fitness (biology)|fitness]] among a population with high [[genetic variability]].<ref>{{cite book | vauthors = Birdsell JA, Wills C | date = 2003 | chapter = The evolutionary origin and maintenance of sexual recombination: A review of contemporary models. | series = Evolutionary Biology Series | title = Evolutionary Biology | volume = 33 | pages = 27–137 | veditors =  MacIntyre RJ, Clegg MT | publisher = Springer | isbn = 978-0306472619 }}</ref>
 == Special cases ==
 ===Fission yeast===
-''[[Schizosaccharomyces pombe]]'' is a facultative sexual yeast that can undergo mating when nutrients are limiting.<ref>{{cite journal | vauthors = Davey J | title = Fusion of a fission yeast | journal = Yeast | volume = 14 | issue = 16 | pages = 1529–1566 | date = December 1998 | pmid = 9885154 | doi = 10.1002/(SICI)1097-0061(199812)14:16<1529::AID-YEA357>3.0.CO;2-0 | s2cid = 44652765 | doi-access = free }}</ref> Exposure of ''S. pombe'' to hydrogen peroxide, an agent that causes oxidative stress leading to oxidative DNA damage, strongly induces mating, meiosis, and formation of meiotic spores.<ref>{{cite journal | vauthors = Bernstein C, Johns V | title = Sexual reproduction as a response to H2O2 damage in Schizosaccharomyces pombe | journal = Journal of Bacteriology | volume = 171 | issue = 4 | pages = 1893–1897 | date = April 1989 | pmid = 2703462 | pmc = 209837 | doi = 10.1128/jb.171.4.1893-1897.1989 }}</ref> This finding suggests that meiosis, and particularly meiotic recombination, may be an adaptation for repairing DNA damage.<ref>{{cite journal | vauthors = Staleva L, Hall A, Orlow SJ | title = Oxidative stress activates FUS1 and RLM1 transcription in the yeast Saccharomyces cerevisiae in an oxidant-dependent Manner | journal = Molecular Biology of the Cell | volume = 15 | issue = 12 | pages = 5574–5582 | date = December 2004 | pmid = 15385622 | pmc = 532035 | doi = 10.1091/mbc.e04-02-0142 }}</ref> The overall structure of the ''MAT'' locus is similar to that in ''S. cerevisiae''. The mating-type switching system is similar, but has evolved independently.<ref name="Hanson_2017" />
+''[[Schizosaccharomyces pombe]]'' is a facultative sexual yeast that can undergo mating when nutrients are limited.<ref>{{cite journal | vauthors = Davey J | title = Fusion of a fission yeast | journal = Yeast | volume = 14 | issue = 16 | pages = 1529–1566 | date = December 1998 | pmid = 9885154 | doi = 10.1002/(SICI)1097-0061(199812)14:16<1529::AID-YEA357>3.0.CO;2-0 | s2cid = 44652765 | doi-access = free }}</ref> Exposure of ''S. pombe'' to [[hydrogen peroxide]], which causes [[oxidative stress]] to [[DNA]], strongly induces mating, meiosis, and formation of meiotic spores.<ref>{{cite journal | vauthors = Bernstein C, Johns V | title = Sexual reproduction as a response to H2O2 damage in Schizosaccharomyces pombe | journal = Journal of Bacteriology | volume = 171 | issue = 4 | pages = 1893–1897 | date = April 1989 | pmid = 2703462 | pmc = 209837 | doi = 10.1128/jb.171.4.1893-1897.1989 }}</ref> Thus, meiosis and meiotic recombination may be an adaptation for repairing DNA damage.<ref>{{cite journal | vauthors = Staleva L, Hall A, Orlow SJ | title = Oxidative stress activates FUS1 and RLM1 transcription in the yeast Saccharomyces cerevisiae in an oxidant-dependent Manner | journal = Molecular Biology of the Cell | volume = 15 | issue = 12 | pages = 5574–5582 | date = December 2004 | pmid = 15385622 | pmc = 532035 | doi = 10.1091/mbc.e04-02-0142 }}</ref> The ''MAT'' locus' structure in ''S. pombe'' resembles ''S. cerevisiae''. The mating-type switching system is similar but evolved independently.<ref name="Hanson_2017" />
 ===Self-mating in ''Cryptococcus neoformans''===
-''[[Cryptococcus neoformans]]'' is a basidiomycetous fungus that grows as a budding yeast in culture and in an infected host. ''C. neoformans'' causes life-threatening meningoencephalitis in immune compromised patients. It undergoes a filamentous transition during the sexual cycle to produce spores, the suspected infectious agent. The vast majority of environmental and clinical isolates of ''C. neoformans'' are mating type α. Filaments ordinarily have haploid nuclei, but these can undergo a process of diploidization (perhaps by endoduplication or stimulated nuclear fusion) to form diploid cells termed [[blastospore]]s.<ref name=Lin>{{cite journal | vauthors = Lin X, Hull CM, Heitman J | title = Sexual reproduction between partners of the same mating type in Cryptococcus neoformans | journal = Nature | volume = 434 | issue = 7036 | pages = 1017–1021 | date = April 2005 | pmid = 15846346 | doi = 10.1038/nature03448 | s2cid = 3195603 | bibcode = 2005Natur.434.1017L }}</ref> The diploid nuclei of blastospores can then undergo meiosis, including recombination, to form haploid basidiospores that can then be dispersed.<ref name=Lin/> This process is referred to as monokaryotic fruiting. Required for this process is a gene designated ''dmc1'', a conserved homologue of genes [[RecA]] in bacteria, and [[RAD51]] in eukaryotes. ''Dmc1'' mediates homologous chromosome pairing during meiosis and repair of [[double-strand break]]s in DNA (see [[Meiosis]]; also Michod et al.<ref>{{cite journal | vauthors = Michod RE, Bernstein H, Nedelcu AM | title = Adaptive value of sex in microbial pathogens | journal = Infection, Genetics and Evolution | volume = 8 | issue = 3 | pages = 267–285 | date = May 2008 | pmid = 18295550 | doi = 10.1016/j.meegid.2008.01.002 | bibcode = 2008InfGE...8..267M | url = http://www.hummingbirds.arizona.edu/Faculty/Michod/Downloads/IGE%20review%20sex.pdf}}</ref>). Lin et al. suggested that one benefit of meiosis in ''C. neoformans'' could be to promote DNA repair in a DNA damaging environment that could include the defensive responses of the infected host.<ref name=Lin/>
+''[[Cryptococcus neoformans]]'' is a [[Basidiomycota|basidiomycetous]] fungus that grows as a budding yeast in culture and infected hosts. ''C. neoformans'' causes life-threatening [[meningoencephalitis]] in immunocompromised patients. It undergoes a filamentous transition during the sexual cycle to produce spores, the suspected infectious agent. The vast majority of environmental and clinical isolates of ''C. neoformans'' are of mating type α. Filaments ordinarily have haploid nuclei, but these can undergo a process of diploidization (perhaps by [[endoreduplication]] or stimulated nuclear fusion) to form diploid cells termed [[blastospore]]s.<ref name="Lin_2005">{{cite journal | vauthors = Lin X, Hull CM, Heitman J | title = Sexual reproduction between partners of the same mating type in Cryptococcus neoformans | journal = Nature | volume = 434 | issue = 7036 | pages = 1017–1021 | date = April 2005 | pmid = 15846346 | doi = 10.1038/nature03448 | s2cid = 3195603 | bibcode = 2005Natur.434.1017L }}</ref>
+The diploid nuclei of blastospores can then undergo meiosis, including recombination, to form haploid [[basidiospore]]s that can then be dispersed.<ref name="Lin_2005" /> This process is referred to as monokaryotic fruiting. This process depends on the gene ''dmc1'', a conserved homologue of the bacterial [[RecA]] and eukaryotic [[RAD51]] genes. ''Dmc1'' mediates homologous chromosome pairing during meiosis and repair of [[double-strand break]]s in DNA.<ref>{{cite journal | vauthors = Michod RE, Bernstein H, Nedelcu AM | title = Adaptive value of sex in microbial pathogens | journal = Infection, Genetics and Evolution | volume = 8 | issue = 3 | pages = 267–285 | date = May 2008 | pmid = 18295550 | doi = 10.1016/j.meegid.2008.01.002 | bibcode = 2008InfGE...8..267M | url = http://www.hummingbirds.arizona.edu/Faculty/Michod/Downloads/IGE%20review%20sex.pdf}}</ref> Meiosis in ''C. neoformans'' may be performed to promote DNA repair in DNA-damaging environments, such as host-mediated responses to infection.<ref name="Lin_2005" />
+== Notes ==
+{{notelist}}
 == References ==
@@ Line 83: / Line 109: @@
 == Further reading ==
 {{refbegin}}
-* {{cite book | vauthors = Scott MP, Matsudaira P, Lodish H, Darnell J, Zipursky L, Kaiser CA, Berk A, Krieger M |title=Molecular Cell Biology | edition = Fifth |publisher=WH Freeman and Col, NY |year=2004 |isbn=978-0-7167-4366-8 |url-access=registration |url=https://archive.org/details/molecularcellbio00harv }}
+* {{cite book | vauthors = Scott MP, Matsudaira P, Lodish H, Darnell J, Zipursky L, Kaiser CA, Berk A, Krieger M |title=Molecular Cell Biology | edition = Fifth |publisher=WH Freeman and Col, NY |year=2004 |isbn=978-0-7167-4366-8 |url-access=registration |url=https://archive.org/details/molecularcellbio00harv |ref=none}}
 * {{cite web |url=https://www.yeastgenome.org/locus/fus3 |title=Fus3 |author=<!--Staff writer(s); no by-line.--> |website=Saccharomyces Genome Database |publisher=SGD Project |access-date=21 March 2014}}
 {{refend}}
 == External links ==
-*[https://www.sciencedaily.com/releases/2008/03/080317094851.htm Fungi Can Tell Us About The Origin Of Sex Chromosomes]: study shows that there are great similarities between the parts of DNA that determine the sex of plants and animals and the parts of DNA that determine mating types in certain fungi. Accessed 5 April 2008.
 *[https://www.ibiology.org/cell-biology/yeast-life-cycle/ Andrew Murray's Seminar: Yeast Sex]
+*[https://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1000030 The Mating-Type Chromosome in the Filamentous Ascomycete ''Neurospora tetrasperma'' Represents a Model for Early Evolution of Sex Chromosomes]
 [[Category:Mating]]