Iridoviridae

Taxonomy

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

• Alphairdovirinae
  • Lymphocystivirus
  • Megalocytivirus
  • Ranavirus
• Betairidovirinae
  • Bivalveiridovirus
  • Chloriridovirus
  • Daphniairidovirus
  • Decapodiridovirus
  • Iridovirus

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.

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.

DNA repair

Members of the Iridoviridae appear to have the capability to repair DNA damages.

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.