The first discovery of viruses infecting a group of microbes, which may include the ancestors of all complex life, has been found, researchers at the University of Texas at Austin report in Nature Microbiology. The discovery provides tantalizing clues to the origin of complex life and suggests new directions for exploring the hypothesis that viruses were essential to the evolution of humans and other complex life forms.
There is a well-founded hypothesis that all complex life forms such as humans, starfish and trees — which contain cells with a nucleus and are called eukaryotes — arose when archaea and bacteria fused into a hybrid organism. Recent research suggests that the first eukaryotes are direct descendants of the so-called Asgard archaea. The latest research, by Ian Rambo (a former doctoral student at UT Austin) and other members of Brett Baker’s lab, sheds light on how viruses may also have played a role in this billion-year-old history.
“This study opens a door to a better solution to the origin of eukaryotes and a better understanding of the role of viruses in the ecology and evolution of Asgard archaea,” Rambo said. “There is a hypothesis that viruses may have contributed to the emergence of complex cellular life.”
Rambo refers to a hotly debated hypothesis called viral eukaryogenesis. It suggests that, in addition to bacteria and archaea, viruses may have contributed a genetic component to the development of eukaryotes. This latest discovery doesn’t resolve that debate, but it does offer some interesting clues.
The newly discovered viruses currently infecting living Asgard archaea have some features similar to viruses that infect eukaryotes, including the ability to copy their own DNA and hijack protein modification systems from their hosts. The fact that these repaired Asgard viruses exhibit characteristics of both viruses that infect eukaryotes and prokaryotes, which have cells without a nucleus, makes them unique in that they are not exactly the same as those that infect other archaea or complex life forms.
“The most exciting thing is that it’s completely new types of viruses, different from the viruses we’ve seen before in archaea and eukaryotes, that infect our microbial relatives,” said Baker, associate professor of marine science and integrative biology and corresponding author of the study. .
The Asgard archaea, which probably evolved more than 2 billion years ago and whose descendants are still alive, have been discovered in deep-sea sediments and hot springs around the world, but so far only one species has been successfully bred in the lab. To identify them, scientists collect their genetic material from the environment and then puzzle their genomes together. In this latest study, the researchers scanned the Asgard genomes for repeating regions of DNA known as CRISPR arrays, which contain small pieces of viral DNA that can be precisely linked to viruses that previously infected these microbes. These genetic “fingerprints” enabled them to identify these covert viral invaders that infect organisms with a key role in the complex origin story of eukaryotes.
“We are now beginning to understand the implication and role that viruses may have had in the eukaryogenesis puzzle,” said Valerie De Anda, a research associate at UT Austin and co-author of the study.
The study’s other co-authors are Pedro Leão, a postdoctoral researcher at UT Austin, and Marguerite Langwig, formerly a master’s student at UT Austin and currently a PhD student at the University of Wisconsin-Madison. This work was supported by the Moore and Simons Foundations.
- Ian M. Rambo, Marguerite V. Langwig, Pedro Leão, Valerie De Anda, Brett J. Baker. Taken from six viruses that infect Asgard archaea from deep-sea sediments. Nature Microbiology, 2022; DOI: 10.1038/s41564-022-01150-8
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