New viruses named after Norse gods could have fueled the rise of complex life

Scientists have discovered the “fingerprints” of mysterious viruses hidden in an ancient group of microbes that may have helped fuel the rise of all complex life on Earth: from fungi to plants to humans.

These microbes – known as Asgard archaea after the home of the gods in Norse mythology – hide in icy sediments at the bottom of the ocean and in boiling hot springs, and existed on Earth before the first eukaryotic cells, which carry their DNA inside a nucleus. By infecting the archaea of ​​Asgard, virus may have influenced how these life forms first appeared, and may even have given rise to some of the earliest precursors to the nucleus, some scientists hypothesize (opens in a new tab). But until now, no virus infecting Asgard had been discovered.

Now, in a trio of studies published Monday (June 27) in the journal Nature Microbiology, scientists have identified a slew of viruses that can infect ancient archaea.

“These are the first studies of archaeal viruses from Asgard; nothing was known before,” said Susanne Erdmann, group leader of the Archaean Virology Research Group at the Max Planck Institute for Marine Microbiology in Bremen. , in Germany, who did not participate in the studies. . In the future, this line of research could reveal if and how viruses were involved in the emergence of eukaryotic cells on Earth, Erdmann told Live Science in an email.

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Dusting for viral “fingerprints”

In the new research, scientists searched for evidence of viral infection embedded in the DNA of Asgard’s archaea. This embedded evidence comes in the form of short snippets of viral DNA, called “CRISPR spacers.”

Most people who hear the term CRISPR think of famous gene editing tool that allows scientists to easily manipulate genetic sequences, said Ian Rambo, a former doctoral candidate at the University of Texas at the Austin Marine Science Institute and first author of one of the Natural microbiology (opens in a new tab) studies. However, this gene-editing tool was originally adapted from the natural defense mechanisms of bacteria and archaea, he told Live Science.

The acronym “CRISPR” stands for “Clusters of Regularly Spaced Short Palindromic Repeats” and refers to a region of DNA composed of short, repeated sequences with “spacers” sandwiched between each repeat. Bacteria and archaea sweep these spacers away from viruses that infect them, and so the cells maintain a memory bank of viral DNA that helps them recognize viruses, should they attack again. “It’s an adaptive immune system that remembers those previous infections,” said Rambo, who is now a postdoctoral researcher at the USDA Agricultural Research Service.

Rambo and his colleagues hunted in the Guaymas Basin in the Gulf of California — the body of water between Baja California and mainland Mexico — for such DNA spacers in Asgard archaea specimens taken from sediment. near hydrothermal vents, about 1.25 miles (2 kilometers) below the water surface. The team matched the spacers they found to longer stretches of viral DNA collected from the deep-sea environment.

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“It’s quite easy to sequence viruses from deep-sea sediments…but the challenge is to recognize which hosts these viruses infect,” said Mart Krupovic, head of the archaeal virology unit at the Institut Pasteur in Paris and co-author of the study. other of them (opens in a new tab) studies (opens in a new tab). “CRISPR spacer matching is the most practical, compelling, and reliable approach to host assignment.”

Ultimately, Rambo’s team discovered six viruses that infect two types of archaea in Asgard, named Lokiarchaeota and Helarchaeota for the Norse god Loki and goddess Hel, respectively. The researchers named the new viruses after Norse mythological creatures, including the giant wolf Fenrir and the dragon Nidhogg.

illustration of crispr-cas9 cutting some DNA from a strand

An illustration of the CRISPR-Cas9 system (Image credit: Shutterstock)

Similarly, in one study, Krupovic and his colleagues discovered two viruses which they named Huginn and Muninn, after the two crows that serve as scouts for the Norse god Odin; these viruses were discovered in an Asgard genome taken from a hot spring in Yellowstone National Park.

In the final study, Krupovic and his co-authors found viruses in deep-sea sediment collected from the Shimokita Peninsula, the northeast cape of the Japanese island of Honshū, as well as two other sites in the Pacific and one in the Indian Ocean. In these samples, they found three family-level groups of viruses, which they named after the three Norns – Wyrd, Verdandi and Skuld – who are supernatural beings who determine the destinies of gods and mortals in mythology. Nordic.

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By working from viral DNA, the researchers were able to deduce what kinds of proteins the different genes code for and, therefore, what viruses might look like and function.

For example, viruses named for Norn Verdandi likely have tails extending from their outer shells, or capsids, and viruses named for Wyrd are likely lemon-shaped, Krupovic and his colleagues determined. Rambo’s team also found evidence that Nidhogg viruses might be able to hijack key proteins in their host cells, which would help the viruses produce new copies of themselves. (Viruses that infect eukaryotic cells hijack their hosts in the same way.)

In the end, the researchers were only able to understand the functions of some of the viruses’ genes; the functions of the vast majority of genes are still unknown, Erdmann said. Also, because CRISPR doesn’t work against all viruses, many other viruses infecting Asgard are likely yet to be discovered, she said.

One way to find these hidden viruses would be to grow Asgard archaea in the lab and isolate any viruses found in their cells. “However, cultivating the Archaea of ​​Asgard has proven to be very difficult,” Erdmann noted. To date, only one research group has Successfully Cultivated Archaea of ​​Asgard (opens in a new tab), and it took them 12 long years to do it. This is partly because archaeal cells take weeks to replicate. (By comparison, the bacterium Escherichia colifor example, takes about 20 minutes, according to Science News (opens in a new tab)).

Until more Asgard can be grown in the lab, matching CRISPR spacers is probably the most efficient way to find more viruses, Krupovic said. And as more viruses are discovered, their role in the emergence of eukaryotes — including humans — may become clearer, Rambo told Live Science.

Originally posted on Live Science.

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