Iridoviruses were distinguished from chloriridoviruses by the color of the iridescence displayed by infected insects or concentrated virus stocks and by virion size. Recent phylogenetic analysis of 10 members of this subfamily indicate that a revision is needed and that phylogenetic placement, rather than color or virion size, is the most appropriate metric to differentiate members of these two genera (Figure 6.Iridoviridae).
Virions are icosahedral particles that may or may not be enveloped. Virions display a fringe of proteins surrounding the particle. Particle diameter is 120–130 nm in ultrathin section. Iridoviruses are assumed to contain approximately 1,472 capsomers arranged as 20 trisymmetrons and 12 pentasymmetrons. A detailed description of virion morphology is presented in the Iridoviridae family section using invertebrate iridescent virus 6 (IIV6), the type species of the genus Iridovirus, as the model.
Physicochemical and physical properties
Virions have a Mol Wt of approximately 1.28×109, a buoyant density of 1.30–1.33 g cm−3, and a sedimentation coefficient of 2,020–2,250S. IIV6 is sensitive to chloroform, SDS, sodium deoxycholate, ethanol, pH 3 and pH 11, but is not sensitive to trypsin, lipase, phospholipase A2 or EDTA. Sensitivity to ether and chloroform depends on the assay system employed.
The 212 kbp genome of IIV6 contains 215 putative ORFs. Dot plot comparison of the IIV6 (genus Iridovirus) and IIV3 (genus Chloriridovirus) genomes shows no co-linearity.
Although 215 ORFs have been identified in IIV6, it is unclear whether the large number of putative proteins is due to a more complex virion, or an increase in non-structural proteins that impact virulence, host evasion, and viral replicative events. SDS-PAGE has identified more than 30 virion-associated polypeptides ranging in size from 11–200 kDa whereas proteomic analysis suggests up to twice as many (Wong et al., 2011, Ince et al., 2010). The virion core contains a major component of 12.5 kDa and at least five additional proteins.
Genome organization and replication
Although IIV6 and IIV3 contain 68 ORFs in common, co-linearity is not seen indicating that, as with other iridovirids, gene order is plastic and genes within the same temporal class (i.e., immediate early, delayed early, and late) are not clustered.
Iridoviruses have been isolated from a wide range of arthropods, particularly insects in aquatic or damp habitats. For example IIV6 (also designated Chilo iridescent virus after the host from which it was first isolated, Chilo suppressalis, the rice stem borer) infects over 100 insect species. Patently infected animals and purified viral pellets display violet, blue or turquoise iridescence. Covert, non-lethal infections may be common in certain hosts. No evidence exists for transovarial transmission and where horizontal transmission has been demonstrated, it is usually by cannibalism or predation of infected invertebrate hosts. Following experimental infection, many members of the genus can replicate in a large number of insects. In nature, the host range appears to vary but there is evidence, for some viruses, of natural transmission across insect orders and even phyla. Invertebrate iridescent viruses have a global distribution. Interestingly, reptiles and amphibians fed iridovirus-infected insects appear to become infected (Papp et al., 2014).
Early work indicated that IIV6 was serologically unrelated to any other small iridescent virus, but other isolates were related to various extents (see below). Cross-reactivity is likely to reflect the degree of sequence identity within the major capsid protein and other structural proteins.
Derivation of names
Invertebrate iridescent viruses (i.e., IIVs) were designated in the order in which they were identified. They were also given common names based on the host from which they were isolated, i.e., IIV6, Chilo iridescent virus; IIV1, Tipula iridescent virus.
Species demarcation criteria
The major capsid protein of IIV1 shows 66.4% amino acid (aa) sequence identity to that of IIV6 and approximately 50% or lower aa sequence identity to iridovirids in other genera. Less than 1% DNA–DNA hybridization was detected by the dot-blot method between IIV1 and IIV6 genomic DNA (stringency: 26% mismatch). Restriction endonuclease profiles (HindII, EcoRI, SalI) showed a coefficient of similarity of <66% between IIV1 and IIV6. Moreover, these species did not share common antigens when tested by tube precipitation, infectivity neutralization, reversed single radial immunodiffusion or enzyme-linked immunosorbent assay. Given the current ease of sequence determination, future demarcation of viral species will likely rely more on genomic sequence analysis, host range, clinical features, etc., and less on restriction endonuclease profiles, hybridization data, and immunological cross-reactivity.