New viruses with connections to both giant viruses and herpesviruses

Metagenomic data from the Tara oceans on plankton allowed the discovery of a large group of DNA viruses that are abundant from the equator to the poles: mirusviruses. These viruses play a role in regulating plankton by infecting a large number of single-celled organisms at any one time.

They are very complex with a surprising genomic makeup: the genes key to the formation of their viral particle, a feature of these DNA viruses, have a direct evolutionary link with herpesviruses. The latter is common in animals (half of the world’s population is infected with the herpes virus) but is not present in other forms of life, leaving the question of their evolutionary origin unanswered.

The discovery of mirusviruses indicates that the ancestors of herpesviruses infected marine single-celled organisms. The evolutionary history of virions holds other surprises. Despite the apparent link to herpesviruses, the majority of mirusvirus genes, including those involved in reproducing viral genomes, are similar to those of giant viruses, a completely distinct viral group that intrigues scientists with its startling characteristics. This evolutionary ‘chimera’ in mirusviruses is unique and can provide information about the evolution of DNA viruses.

“In 2019, our research team noticed an unusual evolutionary signal in the vast amounts of sequence data provided by the Tara Oceans Project. By tracking this signal, we discovered and subsequently characterized a large group of DNA viruses: mirusviruses. This discovery was published in nature It marks the beginning of a new adventure and a gateway for the scientific community to discover and study virus virions in any number of ecosystems,” explains Tom Delmont, an expert in microbial ecology at the National Center for Scientific Research and recent author of the study.

“Tara Oceans has changed our understanding of the plankton ecology. Our study proves that this incredible expedition also provides answers to fundamental evolutionary questions. Much remains to be discovered and understood about mirusviruses. They are not yet cultivated, and there are no images of their viral particle,” said Morgan Jaya, a virus evolution expert. In CEA and first author:

More on the Tara Ocean Expedition (2009-2013)

For nearly four years, the schooner Tara has sailed into every ocean basin on a unique mission: to obtain a global picture of plankton ecosystems around the world. This overlooked and invisible biodiversity is an important marker of the health of our planet and its climate system. Nearly 35,000 samples of viruses, bacteria, algae and plankton have been collected and analyzed and are still being analyzed.

The largest genetic sequencing effort ever undertaken on a marine ecosystem, the expedition has brought to light the majority of previously unknown microbial genes. In collaboration with the Tara Ocean Foundation, this campaign primarily involved teams from CNRS, CEA, and EMBL, who are members of the Tara Oceans Consortium and the GO-SEE (Global Ocean Systems Ecology and Evolution) research network.