Bacilladnaviridae: refined taxonomy and new insights into the biology and evolution of diatom-infecting DNA viruses

Bacilladnaviruses are single-stranded DNA viruses that infect diatoms that, so far, have been primarily identified in marine organisms and environments. Using a viral metagenomics approach, we discovered 13 novel bacilladnaviruses originating from samples of mud-flat snail (<i>Amphibola crenata</i>; <i>n</i>=3 genomes) and benthic sediments (<i>n</i>=10 genomes) collected from the Avon-Heathcote Estuary in New Zealand. Comparative genomics and phylogenetic analysis of the new bacilladnavirus sequences in the context of the previously classified members of the family helped refine and further expand the <i>Bacilladnaviridae</i> taxonomy. Here, based on the replication-associated protein phylogeny and pairwise identities, we established 4 new genera - <i>Aberdnavirus</i>, <i>Keisodnavirus</i>, <i>Puahadnavirus</i> and <i>Seawadnavirus</i> - and 13 new species within the family. Comparison of the bacilladnavirus capsid protein sequences suggests that the positively charged N-terminal region (R-arm) is required for encapsidation of the larger genomes, whereas the smaller bacilladnavirus genomes can be packaged in the absence of the R-arm subdomain. Furthermore, analysis of the bacilladnavirus genomes revealed that members of three genera encode a highly derived variant of a phospholipase A1, which is predicted to be involved in the lysis of the infected diatoms and/or facilitates the entry of the virions into the host cells. Collectively, our results allow refining of the taxonomy of bacilladnaviruses and provide new insights into the biology and evolution of this understudied group of diatom viruses.