ICTV Virus Taxonomy Profile: Iridoviridae

1Virions display icosahedral symmetry and contain an internal lipid membrane located between the DNA–protein core and the outer capsid (Table 1, Fig. 1). Depending upon the genus, the capsid may have numerous external fibrils. Mature infectious virions may remain non-enveloped or bud from the plasma membrane and acquire an envelope.

2The virus genome is a single molecule of double-stranded DNA that is circularly permuted and terminally redundant. The degree of redundancy varies from 5–50 % depending on the virus species. The genomes of all vertebrate iridoviruses, with one exception, are highly methylated due to a virus-encoded DNA cytosine methyltransferase. Twenty-six core proteins are common to all members of the family.

3Replication involves both nuclear and cytoplasmic compartments. In the nucleus, host RNA polymerase II directs the synthesis of immediate–early virus mRNAs. In addition a virus-encoded DNA polymerase synthesizes genome- to twice-genome-size copies of the incoming DNA. This DNA is transported to the cytoplasm, where it undergoes a second round of replication to form large concatemers. Late gene expression is catalysed by a virus-encoded transcriptase. Virion morphogenesis occurs within assembly sites, and completed particles form paracrystalline arrays or bud from the plasma membrane (Fig. 2).

4Phylogenetic analyses indicate that invertebrate and vertebrate iridovirus lineages diverged early during the evolution of the family. Ascoviruses appear to have emerged recently from the former lineage and share with iridoviruses a common ancestry with marseilleviruses.

5Ranaviruses are promiscuous pathogens capable of infecting three classes of ectothermic vertebrates (bony fish, amphibians and reptiles). Ranaviruses infect not only multiple species within a class, but some (e.g. Bohle iridovirus) are also capable of infecting hosts from different classes. Infections are systemic, involve multiple internal organs and may lead to high levels of morbidity and mortality among cultured, commercially important fish and amphibians, as well as endangered wild species.

6Members infect >50 species of marine and freshwater fish; systemic disease involves multiple internal organs.

7Lymphocystiviruses infect >100 species of marine and freshwater fish, leading to the formation of wart-like growths composed of clusters of individual, infected cells (some as large as 1 mm) primarily on the skin, but sometimes on internal organs. Morbidity may be high but mortality tends to be low.

8Members of these genera infect >100 insect and crustacean species. Patent infections involve massive levels of virus replication that result in infected larvae displaying marked iridescence, whereas covert infections may reduce the reproductive capacity of the host. The genera were previously distinguished based on virion size and the iridescent colour of infected larvae; phylogenetic analysis of complete genome sequences now gives a superior method of differentiation.

9Full ICTV Online (10th) Report: www.ictv.global/report/iridoviridae.