Evolutionary relationships of iridoviruses and divergence of ascoviruses from invertebrate iridoviruses in the superfamily Megavirales.

The family Iridoviridae of the superfamily Megavirales currently consists of five genera. Three of these, Lymphocystivirus, Megalocytivirus and Ranavirus, are composed of species that infect vertebrates, and the other two, Chloriridovirus and Iridovirus, contain species that infect invertebrates. Until recently, the lack of genomic sequence data limited investigation of the evolutionary relationships between the invertebrate iridoviruses (IIVs) and vertebrate iridoviruses (VIVs), as well as the relationship of these viruses to those of the closely related family Ascoviridae, which only contains species that infect insects. To help clarify the phylogenetic relationships of these viruses, we recently published the annotated genome sequences of five additional IIV isolates. Here, using classical approaches of phylogeny via maximum likelihood, a Bayesian approach, and resolution of a core protein tree, we demonstrate that the invertebrate and vertebrate IV species constitute two lineages that diverged early during the evolution of the family Iridoviridae, before the emergence of the four IIV clades, previously referred to as Chloriridoviruses, Polyiridoviruses, Oligoiridoviruses and Crustaceoiridoviruses. In addition, we provide evidence that species of the family Ascoviridae have a more recent origin than most iridoviruses, emerging just before the differentiation between the Oligoiridoviruses and Crustaceoiridovirus clades. Our results also suggest that after emergence, based on their molecular clock, the ascoviruses evolved more quickly than their closest iridovirus relatives.