Characterization of repetitive DNA regions and methylated DNA in ascovirusgenomes

The accompanying phylogenetic study of large double-stranded DNA viruses ba sed on their delta DNA polymerase genes suggests that ascoviruses (family A scoviridae) and iridoviruses (family Iridoviridae) are closely related and may share a common ancestor. This relationship was unexpected because of ma rked differences between these viruses. Iridoviruses produce icosahedral vi rions and occur broadly among vertebrates and invertebrates, whereas ascovi ruses typically produce reniform or bacilliform virions and are restricted to insect hosts, primarily lepidopterans. Detailed comparisons of these two virus types are not possible because fundamental information on the proper ties of the virions and their genomes is lacking, especially for ascoviruse s. To facilitate further investigation of the putative evolutionary relatio nship between ascoviruses and iridoviruses, the genomes of representative v iruses from each family were compared with respect to physical configuratio n, presence of DNA repeats and degree of DNA methylation. Genomes from Spod optera frugiperda (SfAV1), Heliothis virescens (HvAV3) and Diadromus pulche llus (DpAV4) ascoviruses were all found to be circular and partially superh elical and to contain large interspersed repeats of 1-3 kbp. Mosquito (IV t ype 3), lepidopteran (IV type 6) and isopod (IV type 31) iridovirus genomes were all linear and lacked large regions of repetitive DNA, Ascovirus and iridovirus genomes were methylated and one, DpAV4, had the highest degree o f methylation of any reported animal DNA virus. The major differences in th e physical and biochemical characteristics of ascoviruses and iridoviruses reported here provide a foundation for further studies of their relatedness while making their possible close relationship and divergence during evolu tion of even greater interest.