Evidence for the evolution of ascoviruses from iridoviruses

Ascoviruses (family Ascoviridae) are large, enveloped, double-stranded (ds)DNA viruses that attack lepidopteran larvae and pupae, and are unusual in that they are transmitted by parasitic wasps during oviposition. Previous comparisons of DNA polymerase sequences from vertebrate and invertebrate viruses suggested that ascoviruses are closely related to iridoviruses. This relationship was unexpected because these viruses differ markedly in virion symmetry, genome configuration and cellular pathology. Here we present evidence based on sequence comparisons and phylogenetic analyses of a greater range of ascovirus proteins and their homologues in other large dsDNA viruses that ascoviruses evolved from iridoviruses. Consensus trees for the major capsid protein, DNA polymerase, thymidine kinase and ATPase III from representative ascoviruses, algal viruses (family Phycodnaviridae), vertebrate and invertebrate iridoviruses (family Iridoviridae) and African swine fever virus (ASFV; family Asfarviridae) showed that ascovirus proteins clustered most closely with those of the lepidopteran iridovirus Chilo iridescent virus (CIV) (Invertebrate iridescent virus 6). Moreover, analysis of the presence or absence of homologues of an additional 50 proteins encoded in the genome of Spodoptera frugiperda ascovirus (SfAV-1 a) showed that about 40 % occurred in CIV, with lower percentages encoded by the genomes of, respectively, vertebrate iridoviruses, phycodnaviruses and ASFV. The occurrence of three of these genes in SfAV-1a but not CIV was indicative of the evolutionary differentiation of ascoviruses from invertebrate iridoviruses.