“Mirusviruses: A New Phylum of DNA Viruses Shaped Like Herpesviruses and Linked to Giant Viruses”

Double-stranded DNA viruses come in two main flavors, classified according to their shapes. One contains large and giant DNA viruses that attack complex cells, but also includes much smaller viruses that infect bacteria. These viruses are shaped like footballs. The other flavor has tails and primarily infects bacteria and archaea, but also contains the herpesvirus family, which infects animals.

The disparate properties of these viruses have raised questions that plague virologists: Where do herpes viruses come from? And how are large and giant DNA viruses related to smaller viruses in their domain?

Tara Oceans is “an international, multidisciplinary project aimed at assessing the complexity of ocean life across comprehensive taxonomic and spatial scales”. Project researchers sail around the five oceans and two seas (the Red and the Mediterranean), collecting plankton to try to understand the ocean ecosystem. In new work reported in Nature, a team has extracted plankton from sunny oceans (that’s a technical term: only up to 200 meters below the surface, where light penetrates and photosynthesis occurs). They studied all planktonic DNA viruses by comparing a single characteristic gene among them.

This analysis revealed a new phylum of DNA viruses that are shaped like herpes viruses and infect eukaryotes, but share a key enzyme with large and giant viruses. Scientists have named the phylum mirusvirus, for the Latin word after deathmeaning surprising or strange.

They identified seven different clades of mirusviruses and found them all over the world. Three were found exclusively in the Arctic Ocean, while only one was specific to temperate waters. And even though they have only just been identified, it seems that mirusviruses are among the most abundant eukaryotic viruses in sunny oceans. They are also very active in the plankton they infect, indicating that they likely play a key role in the marine ecosystem.

The two DNA virus kingdoms linked by mirusviruses – one containing the giant viruses and the other containing the herpesviruses – are ancient lineages. So, the paths they all took to get to where they are today are still unclear. The similar shape of mirusviruses and herpesviruses suggests that these eukaryotic viruses share a common ancestor. The authors speculate that the shared enzyme was transferred between a giant virus and this common ancestor, although they don’t know which lineage got it first or how it was transferred. Later, herpes viruses lost the gene through reductive evolution. Then somewhere along the way they acquired the ability to infect animals. We are lucky.

Nature, 2023. DOI : 10.1038/s41586-023-05962-4