Phylogenetic position of the Diadromus pulchellus ascovirus DNA polymeraseamong viruses with large double-stranded DNA genomes

The Ascoviridae is a family of large double-stranded (ds) DNA insect viruse s that contains four species, the Spodoptera frugiperda (SfAV1), Trichoplus ia ni (TnAV2), Heliothis virescens (HvAV3) and Diadromus pulchellus (DpAV4) ascoviruses. These are unique among insect viruses in that the primary mea ns of transmission among their lepidopteran hosts is generally by being vec tored mechanically by hymenopteran parasitoids. Ascoviruses are similar in virion structure, but their relationships with their parasitoid vectors var y from being opportunistic to obligate. Little is known, however, about the relatedness of these viruses to one another or to other large dsDNA viruse s. We therefore cloned and sequenced the delta DNA polymerase gene of DpAV4 , characterized it and compared it to 59 eukaryotic and viral delta and eps ilon DNA polymerases. Phylogenetic analyses based on these genes revealed t hat the ascoviruses DpAV4 and SfAV1 formed a group of virus species distinc t from, but closely related to, species of the family Iridoviridae. Detaile d analyses of the relatedness of ascovirus species based on conserved delta DNA polymerase motifs showed two groups within the family Ascoviridae, one containing DpAV4 and the other containing SfAV1, TnAV2 and HvAV3, which wa s consistent with their host-vector relationships. Despite significant diff erences in capsid symmetry between ascoviruses and iridoviruses, these resu lts suggest that these viruses may have originated from a common ancestral virus.