Scientists at the University of Texas at Austin have discovered for the first time traces of viruses that infect the Asgard archaea, a group of prokaryotic microorganisms that may have given rise to multicellular organisms in ancient times. These viruses may have been the ancestors of modern viruses that infect eukaryotes. The discovery is described in an article published in the journal Nature Microbiology.
Previous research has shown that the first eukaryotes are likely direct descendants of the Asgard archaea. These microorganisms originated about two billion years ago and still live in deep sea sediments and hot springs around the world, but they are difficult to cultivate in the laboratory. According to one hypothesis, viruses that infect archaea played a key role in the emergence of eukaryotes and multicellular life.
Scientists used the Alvin submersible to collect sediment and microbial samples from a depth of two kilometers in the Gulf of California. They analyzed the isolated archaeal DNA looking for repetitive regions known as CRISPR containing small fragments of viral DNA.
The authors of the work were able to characterize six relatively large (up to 117 thousand base pairs) double-stranded DNA (dsDNA) genomes of viruses that infect two types of Asgard archaea, Lokiarchaeota and Helarchaeota. These genomes contain features of both prokaryotic and eukaryotic viruses. They found 1-5 percent of the genes associated with the eukaryotic giant viruses Nucleocytoviricota.