Taxonomic update for the giant algal chloroviruses of the family Phycodnaviridae.

Chloroviruses were first described in the late 1970s, and this opened a new area in virology. These giant DNA viruses, capable of infecting unicellular eukaryotic algae, are ubiquitous in inland waters and have been isolated from various locations worldwide, revealing extensive and complex genetic diversity. These viruses belong to the genus Chlorovirus, family Phycodnaviridae, order Algavirales. Currently, only six species are officially recognized by the International Committee on Taxonomy of Viruses (ICTV), but recent data from our group suggest the existence of many others. In this work, we present the lines of evidence supporting the creation of subgenera within the genus Chlorovirus, as well as robust criteria for the classification of more than 100 viral isolates. We submitted an official taxonomic proposal to the ICTV for the creation of the subgenera “Alphachlorovirus”, “Betachlorovirus”, and “Gammachlorovirus” to classify the viruses that have historically been called NC64A-virus, Pbi-virus, and SAG-virus, respectively. Each subgenus, supported by both phylogenomic and comparative genomic analysis, comprises a total of 20 species to date. We hope that the criteria presented here can serve as a basis for taxonomic advances in other groups of giant algal viruses. The taxonomic proposal is still under consideration by the ICTV.