Asgard archaea are globally distributed microorganisms related to eukaryotes – complex organisms including animals, plants, fungi, and amoebas. However, the viruses that infect these organisms were unknown until now. In new research, biologists have characterized six relatively large double-stranded DNA viruses that infect two groups of Asgard archaea: Lokiarchaeota and Helarchaeota. This discovery offers tantalizing clues to the origins of complex life and suggests new directions for exploring the hypothesis that viruses were essential for the evolution of complex life.
“Our study opens the door to a better resolution of the origin of eukaryotes and to understanding the role of viruses in the ecology and evolution of archaea in Asgard,” said Dr Ian Rambo, a researcher at the Department of marine sciences from the University of Texas at Austin.
“There is a hypothesis that viruses may have contributed to the emergence of complex cellular life.”
“We are referring to a very controversial hypothesis called viral eukaryogenesis. This suggests that in addition to bacteria and archaea, viruses may have contributed a genetic component to eukaryotic development.
“This latest finding doesn’t settle that debate, but it does offer some interesting clues.”
Archaea of Asgard, which likely evolved over 2 billion years ago, have been found in deep sea sediments and hot springs around the world.
To identify them, scientists take their genetic material from the environment and then reconstitute their genomes.
In the new study, Dr. Rambo and his colleagues analyzed sediments associated with hydrothermal vents from Guaymas Basin2000 m water depth, Gulf of California.
They scanned the genomes of the archaea of Asgard to look for repeating regions of DNA known as CRISPR chips, which contain small pieces of viral DNA that can be precisely matched to viruses that previously infected these microbes.
These DNA fingerprints allowed the team to identify six viral invaders that currently infect the archaea Asgard, Lokiarchaeota, and Helarchaeota.
Genomic analysis of these Asgard viruses revealed that they contain features of both prokaryotic and eukaryotic viruses.
“The most exciting thing is that these are completely new types of viruses, different from those we’ve seen before in archaea and eukaryotes, infecting our microbial relatives,” said researcher Dr Brett Baker. at the Department of Marine Science and the Department of Integrative Biology at the University of Texas at Austin.
“We are now beginning to understand the involvement and role that viruses might have had in the eukaryogenesis puzzle,” added Dr. Valerie De Anda, a researcher in the Department of Marine Science at the University of Texas at Austin.
The paper was published in the journal Natural microbiology.
IM Rambo et al. 2022. Genomes of six viruses that infect Asgard archaea from deep sea sediments. Nat Microbiol 7, 953-961; doi: 10.1038/s41564-022-01150-8