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Iridoviridae

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Virus classification
Group:	Group I (dsDNA)
Family:	Iridoviridae
Genera
Megalocytivirus Ranavirus Iridovirus Chloriridovirus Lymphocystivirus

Iridoviridae is a family of viruses with an icosahedral virion structure and a double-stranded DNA genome. A broad range of cold-blooded vertebrates and invertebrates, usually associated with damp or aquatic environments, serve as natural hosts. Iridoviruses are globally distributed, vary in virulence, and can be of economic and ecologic significance . There are currently 11 known species in this family, divided among 5 genera . Invertebrate iridoviruses may be transmitted by endoparasitic wasps, parasitic nematodes or by cannibalism .

Nomenclature

The name is derived from Iris, the Greek goddess of the rainbow. This name was chosen because of the "rainbow-like" iridescence observed in heavily infected insects and pelleted samples of invertebrate iridoviruses . The common name for any member of the Iridoviridae family is "Iridovirus", after the genus Iridovirus.

Taxonomy

The virus taxonomy is based on the release of the International Committee on Taxonomy of Viruses . In this family, there are currently 5 genera containing 11 species.

Group: dsDNA

Order: Unassigned

• Family: Iridoviridae

  • Genus: Chloriridovirus

    • Invertebrate iridescent virus 3
  • Genus: Iridovirus

    • Invertebrate iridescent virus 1
    • Invertebrate iridescent virus 6
  • Genus: Lymphocystivirus

    • Lymphocystis disease virus 1
  • Genus: Megalocytivirus

    • Infectious spleen and kidney necrosis virus
  • Genus: Ranavirus

    • Ambystoma tigrinum virus
    • Bohle iridovirus
    • Epizootic haematopoietic necrosis virus
    • European catfish virus
    • Frog virus 3
    • Santee-Cooper ranavirus

Structure

The virions are icosahedral with triangulation number (T) = 189–217, 120–350 nm in diameter and made up of three components: an outer proteinaceous capsid, an intermediate lipid membrane, and a central core containing DNA-protein complexes. The capsid is comprised predominantly of capsomers made of the major capsid protein (MCP). The outer surface of the capsid is covered with rather short fibrils of unknown function. The lipid membrane contains transmembrane proteins of unknown function. Additionally, zip monomers, zip dimers, finger proteins and anchor proteins have been identified in the capsid and in the association with the lipid membrane. Zip and finger proteins are involved in linking capsomers, whereas anchor proteins connect the capsid with the lipid membrane .

Some of the viruses also have an outer envelope. The presence or absence of an envelope depends on whether they budded from the host cell membrane (enveloped viruses) or were arranged in paracrystalline arrays within the host cell cytoplasm and then were released by cell lysis (unenveloped viruses). Both types of virions are infectious, but the envelope increases the specific infectivity .

Genus	Structure	Symmetry
Lymphocystivirus	polyhedral	T=189-217
Megalocytivirus	polyhedral	T=189-217
Ranavirus	polyhedral	T=133 or 147
Iridovirus	polyhedral	T=147
Chloriridovirus	polyhedral	T=189-217

Physical properties

Iridoviridae virions are stable in water at 4°C over a longer period. Sensitivity to chloroform and ether depends on the assay employed and sensitivitiy to pH varies between iridoviruses. The virions are inactivated within 30 min at >55°C and by UV-irradiation. Some members are sensitive to desiccation .

Genomes

The virion core contains mostly a single linear dsDNA molecule. It contains terminal and redundant sequences and is circularly permuted. The genomes vary between 103 and 220 kilobases in size and have a G+C-content from about 28 to 55% . The genomes of vertebrate iridoviruses (Lymphocystivirus, Megalocytivirus, and Ranavirus) are highly methylated with about 25% of their cytosine residues methylated by a virally encoded DNA methyltransferase, but little to no methylation is found in genomes of invertebrate iridoviruses (Chloriridovirus, Iridovirus) .

Virion entry & Replication

Virus particles enter the cell by either receptor-mediated endocytosis (enveloped virions) or by uncoating at the plasma membrane (naked virions). The viral DNA is transported to the host cell nucleus, where it is transcribed by host RNA polymerase II modified by the virus . Meanwhile, host macromolecular synthesis is stopped.

The viral DNA polymerase produces progeny DNA from the parental genome, which is then transported to the cytoplasm and operated as template for replication in the cytoplasm. Large branched concatemers of viral DNA are synthesized and packaged into virions. The virion is then released either by budding from the cell membrane or via cell lysis .

Pathogenesis

Little is known about the pathogenesis of iridoviruses. The pathogenesis is, however, temperature dependent and iridoviruses are thus confined to poikilothermic hosts.

Host range

Members of the Iridoviridae are able to infect a broad host range. Iridoviruses of the genus Chloriridovirus and Iridovirus are restricted to invertebrates (e.g. lepidoptera and orthoptera), whereas Lymphocystivirus, Megalocytivirus and Ranavirus infect poikilothermic vertebrates such as fish, amphibians and reptile species .