Iridoviridae

Iridoviridae is a family of viruses with double-stranded DNA genomes.[1] Amphibians, fish, and invertebrates such as arthropods serve as natural hosts. There are currently 19 species in this family, divided among two subfamilies and six genera.[1][2]

Iridoviridae
TEM micrograph showing a cluster of Iridovirus virions
Virus classification
(unranked): Virus
Realm: Varidnaviria
Kingdom: Bamfordvirae
Phylum: Nucleocytoviricota
Class: Megaviricetes
Order: Pimascovirales
Family: Iridoviridae
Subfamilies and genera

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. It may refer to any member of the Iridoviridae family or a particular genus within Iridoviridae.

Taxonomy

The following subfamilies and genera are recognized (-virinae denotes subfamily and -virus denotes genus):

Structure

The virions are icosahedral with triangulation number (T) = 189–217, 120–350 nm in diameter and made up of three domains: an outer proteinaceous capsid, an intermediate lipid membrane, and a central core containing DNA-protein complexes. Some of the viruses also have an outer envelope. The presence or absence of an envelope depends on whether they budded from the cell membrane (enveloped viruses) or were arranged in paracrystalline arrays within the host cell cytoplasm and then were released by cell lysis (unenveloped viruses).

The linear genome varies between 150 and 303 kilobases in length. It contains terminal and redundant sequences and is circularly permuted.

Members of this family differ in their degree of genome methylation. The genera Chloriridovirus and Iridovirus lack a highly methylated genome. Members of Lymphocystivirus, Megalocytivirus, and Ranavirus have genomes with about 25% of their cytosine residues methylated by a virally encoded DNA methyltransferase.

GenusStructureSymmetryGenomic arrangementGenomic segmentation
LymphocystivirusPolyhedralT=189–217LinearMonopartite
MegalocytivirusPolyhedralT=189–217LinearMonopartite
RanavirusPolyhedralT=133 or 147LinearMonopartite
IridovirusPolyhedralT=147LinearMonopartite
ChloriridovirusPolyhedralT=189–217LinearMonopartite

Gene expression

Similar to the herpes viruses, transcription occurs in three stages: immediate-early, delayed-early, and late. Positive induction and negative feedback mechanisms exist in each stage, mediated by products of the other stages.

Replication

Virus particles enter the cell and uncoating occurs. 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 ceases.

Parental DNA produces a genome which is then the template for replication in the cytoplasm. Large concatemers of viral DNA are formed by recombination in the cytoplasm. Packaging of the new genomes into virions occurs in the cytoplasm and the virus is released either by budding from the cell membrane or cell lysis.

GenusHost detailsTissue tropismEntry detailsRelease detailsReplication siteAssembly siteTransmission
LymphocystivirusFishNoneCell receptor endocytosisLysis; buddingNucleusCytoplasmUnknown
MegalocytivirusFishNoneCell receptor endocytosisLysis; buddingNucleusCytoplasmUnknown
RanavirusFrogs; snakesNoneCell receptor endocytosisLysis; buddingNucleusCytoplasmContact
IridovirusInsectsNoneCell receptor endocytosisLysis; buddingNucleusCytoplasmContact
ChloriridovirusDiptera with aquatic larval stage, mainly mosquitoesNoneCell receptor endocytosisBuddingNucleusCytoplasmUnknown

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 family infect mainly invertebrates, but also some vertebrate species such as fish, amphibians and reptiles.

References
  1. "Iridoviridae". ICTV Online (10th) Report.
  2. "Viral Zone". ExPASy. Retrieved 15 June 2015.

MicrobiologyBytes: Iridoviruses, archived from the original on February 24, 2007, retrieved 2007-03-06 Viral Bioinformatics Resource Center & Viral Bioinformatics – Canada, University of Victoria, archived from the original on August 17, 2007, retrieved 2007-03-06

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