Incredible Virus Discovery Offers Clues About the Origins of Complex Life

Researchers from The University of Texas at Austin (UT Austin) announce their findings in the journal Nature Microbiology as the first evidence of viruses infecting a class of bacteria that may contain the progenitors of all complex life. The amazing discovery gives fascinating hints about the beginnings of complex life and suggests fresh angles for researching the idea that viruses were crucial to the emergence of sophisticated life forms like humans and other animals.

All complex life forms, including people, starfish, and trees, which have cells with a nucleus and are referred to as eukaryotes, are thought to have developed when archaea and bacteria combined to create a hybrid creature. This theory is highly accepted. According to recent findings, the earliest eukaryotes are the direct offspring of the Asgard archaea. The most recent study, conducted by Brett Baker's lab and Ian Rambo, a former doctorate student at UT Austin, gives insight on how viruses may have also had an impact on this billions of year old history.

Comparison of all known virus genomes. Those viruses with similar genomes are grouped together including those that infect bacteria (on the left), eukaryotes (on the right and bottom center). The viruses that infect Asgard archaea are unique from those that have been described before. Credit: University of Texas at Austin

“This study is opening a door to better resolving the origin of eukaryotes and understanding the role of viruses in the ecology and evolution of Asgard archaea,” according to Rambo. “There is a hypothesis that viruses may have contributed to the emergence of complex cellular life.”

Rambo is alluding to the controversial theory of viral eukaryogenesis. It implies that viruses, in addition to bacteria and archaea, may have contributed a genetic component to the eukaryotic genesis. This most recent discovery does not end that argument, but it does provide some intriguing hints.

The recently identified viruses that infect presently thriving Asgard archaea do have certain characteristics with viruses that infect eukaryotes, such as the capacity to duplicate their own DNA and take use of their hosts' protein modification mechanisms. Since they differ from viruses that infect other archaea or sophisticated living forms, these recovered Asgard viruses exhibit traits of both viruses that infect eukaryotes and prokaryotes, which have cells without a nucleus.

“The most exciting thing is they are completely new types of viruses that are different from those that we’ve seen before in archaea and eukaryotes, infecting our microbial relatives," according to Baker, corresponding author of the paper and associate professor of integrative biology and marine science.

In deep-sea sediments and hot springs all over the world, the Asgard archaea have been found, but only one strain has been successfully cultivated in a lab. This archaea presumably developed more than 2 billion years ago, and its offspring are still alive today. Researchers gather their genetic material from the environment and put together their genomes to identify them. In this most recent work, the researchers searched the Asgard genomes for repetitive DNA sequences called CRISPR arrays, which contain tiny fragments of viral DNA that can be accurately linked to viruses that have previously infected these bacteria. 

They were able to recognize these sneaky viral invaders, which affect creatures important to the intricate creation narrative of eukaryotes, thanks to their genetic "fingerprints."

“We are now starting to understand the implication and role that viruses could have had in the eukaryogenesis puzzle,” stated Valerie De Anda, a research associate at UT Austin and a co-author of the study.

Reference: “Genomes of six viruses that infect Asgard archaea from deep-sea sediments” 27 June 2022, Nature Microbiology.

DOI: 10.1038/s41564-022-01150-8