Historically, the larger virion size (180 nm compared to 120–130 nm) and the yellow-green iridescence of patently-infected larvae and purified viral pellets were used to distinguish members of the genus Chloriridovirus from those within the genus Iridovirus. Currently chloriridoviruses are distinguished from iridoviruses by phylogenetic analysis.
Particle diameter is up to 180 nm in ultrathin section. The trimers and pentamers of invertebrate iridescent virus 3 (IIV3) are larger than the corresponding structures of members of the genus Iridovirus, with probably 14 capsomers to each edge of the trimer. Particle size has historically been used to define viruses that are members of this genus, but the validity of that characteristic is uncertain. A fringe of hair-like fibrils surrounds virus particles (Figure 2B. Iridoviridae).
Physicochemical and physical properties
IIV3 has a Mol Wt of approximately 2.49-2.75 ×109, a buoyant density of approximately 1.354 g cm−3 in CsCl, and a sedimentation coefficient of 4440–4460S.
The genome of IIV3 is 191,132 bp with a G+C content of 48%. Of the 126 predicted genes, 68 are common between invertebrate iridescent virus 6 (IIV6, genus Iridovirus) and IIV3. Thirty-three IIV3 genes lack homologs in other iridovirids. There is little conservation of gene order among IIV3, IIV6, and invertebrate iridescent virus 9 (IIV9), e.g., between IIV3 and IIV9 only five clusters of three or more genes were detected (Wong et al., 2011).
Proteomic analysis of IIV9, a virus recently classified within this genus, suggests that up to 64 viral proteins are present within the virion. Putative genes found in IIV3 and IIV6 but not present in vertebrate iridovirids, include a DNA topoisomerase II, an NAD-dependent DNA ligase, SF1 helicase, IAP and a BRO protein. The genome of the type species, IIV3, is 191 kbp and encodes a minimum of 126 ORFs. Other viruses within the genus contain 148 – 191 ORFs.
Genome organization and replication
Only very limited colinearity has been observed between IIV3 and the genome of any other IIVs sequenced to date. The genes of IIV3, like those of other members of the family, do not appear to be grouped by temporal class. Moreover, they lack introns, are closely-spaced, and are not present on overlapping strands of the viral genome. Because suitable in vitro replication systems are lacking, little is known about the viral replication strategy. However, as with other members of the family, overall replication strategy is thought to be similar to that of frog virus 3.
Chloriridovirus-like infections were reported from Diptera with aquatic larval stages, mainly mosquitoes. There is evidence for transovarial transmission in mosquitoes infected by IIV3. Horizontal transmission is achieved by cannibalism or predation of infected mosquitoes. Patently infected larvae and purified pellets of virus display yellow-green iridescence although orange and red infections are known. IIV3 appears to have a narrow host range.
IIV3 is serologically distinct from members of other genera.
Derivation of names
Invertebrate iridescent viruses (i.e., IIVs) were designated in the order in which they were identified. However, some were designated based on the host species from which they were isolated, e.g., Anopheles minimus iridovirus.
Species demarcation criteria
Members of the genus show 50% or greater sequence identity within the major capsid protein gene. Criteria to distinguish species within the genus are under development. Isolates displaying 90% or greater nucleotide identity within the major capsid protein gene, or within a set of core genes, would likely be considered as members of the same viral species.