Researchers from The University of Texas at Austin write in Nature Microbiology that they have identified the first evidence of viruses infecting a population of microorganisms that may include the origins of all complex life.

The finding provides intriguing hints regarding the beginnings of complex life and promotes fresh areas of research into the theory that viruses were crucial to the development of sophisticated life forms like humans and other animals.

It is a well-established theory that all eukaryotes, or complex living forms with cells containing a nucleus, including humans, starfish, and trees, descended from a hybrid archaea-bacterial creature.

A recent study indicates that the ancestors of eukaryotes are direct offspring of the so-called Asgard archaea. Ian Rambo (a former UT Austin doctorate student) and other members of Brett Baker’s lab shed insight on how viruses may have also played a role in this multibillion-year-old history.

Rambo added, “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.”

It has been proposed that viruses played a role in the development of sophisticated cellular life.

Rambo is referring 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 provides some intriguing hints but does not resolve that controversy.

The newly found viruses that infect living Asgard archaea share certain characteristics with viruses that infect eukaryotes, such as the capacity to duplicate their own DNA and hijack the protein modification mechanisms of their hosts. The fact that these recovered Asgard viruses exhibit characteristics of viruses that infect eukaryotes and prokaryotes, which have cells without a nucleus, distinguishes them from other viruses that infect archaea or complex organisms.

According to Baker, associate professor of marine science and integrative biology and corresponding author of the study, “the most exciting thing is that 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.”

The Asgard archaea probably first appeared more than 2 billion years ago, and their descendants are still alive today. They have been found in deep-sea sediments and hot springs all over the world, but only one strain has been grown in the lab so far.

Scientists gather their genetic material from the environment and piece together their genomes to identify them.

In this most recent study, the researchers analyzed the Asgard genomes for repetitive DNA sequences known as CRISPR arrays, which contain little fragments of viral DNA that can be precisely matched to viruses that infected these bacteria in the past.

With these genetic “fingerprints,” they were able to find these sneaky viruses that infect organisms that play important roles in the complicated story of how eukaryotes came to be.

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

Image Credit: Brett Baker

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