An international research team has discovered a new major group of DNA viruses, dubbed "mirusvirus." Their breakthrough was made possible by the identification of an evolutive signal far from all others in the marine DNA sequencing big data generated by Genoscope's Genomics Metabolics mixed research unit from the samples collected during the Tara Oceans expeditions. A very complex virus furthermore abundant at ocean surfaces where it infects a part of marine plankton, mirusvirus's genetic composition suggests a multi-rooted origin for it.

The majority of its genes, including those necessary for DNA replication and transcription, appear related to the giant viruses. These latter show high diversity in the world's oceans and have been fascinating the scientific community for decades due to their surprising properties. However, all the key genes involved in the formation of the virus particle and those defining the taxonomic identity of DNA viruses reveal a link with the herpesvirus family, which is widespread in animals but absent from other forms of life.

These results, published in Nature, are more than surprising because the herpesviruses and the giant viruses are completely different groups from an evolutionary perspective. Thus the mix observed in mirusvirus raises questions on the possibility of the integration of its genes in giant viruses, the existence of a common ancestor for giant viruses and herpesviruses, or the exchange of viral elements.

The discovery of mirusvirus brings novel opportunities for the field of ecology and the elucidation of DNA virus evolution pathways, in the seas and beyond. France's scientific community and particularly Genoscope will continue to forward the study of these viruses in the years to come. Mirusvirus surely has many more stories to tell.​