Not just epidemic pathogens: the diversity of RNA viruses is much greater than previously thought, as evidenced by the discovery of thousands of previously unknown viruses in the ocean. They added five new strains and 11 classes to the RNA virus family tree, doubling the number of RNA virus strains. A new branch of these viruses could also prove to be a critical link in the evolution of the virus, the researchers report in Science.
Viruses They are the oldest and most numerous organisms on our planet. They are found almost everywhere and are perfectly adapted for reproduction in almost all forms of life. Alone in our body one billion This is a parasitic cell. But so far only a small part of the total group of viruses is known. Attempts to catalog the atmosphere and their family tree To rebuild incomplete and incomplete.
Viral RNA hunt
This is especially true of RNA viruses – the group of viruses that Coronavirus SARS-CoV-2 And the causative agents of diseases such as Ebola, measles, mumps and hepatitis D. They are considered particularly original and, according to some theories, could have existed before the first DNA-bearing cells. “RNA viruses are important to our world, but we often only study a small portion of them: a few hundred species that infect humans, animals and plants,” explains lead author Matthew Sullivan of The Ohio State University.
That’s why Sullivan and colleagues conducted a large-scale search for viruses in a habitat that has not yet been screened for RNA viruses: the ocean. For their study, they analyzed the genetic data of more than 35,000 water samples from all the world’s oceans collected and sequenced as part of the Tara expedition. In order to identify RNA viruses, the researchers looked for a gene, RdRp, which is only present in these viruses, and thus represents a kind of recognition fingerprint.
In order to assign RdRp signatures to individual RNA virus species, the team developed a complex computer-aided assessment and comparison process. This helped them index the nearly 44,000 RdRp-containing RNA sequences detected in water samples.
New tribes, classes, and species
The result revealed a large number of previously completely unknown RNA viruses. “Almost all of the species we found were new,” Sullivan says. About 5,500 new species of these five can be assigned to the five strains and 20 classes of RNA viruses already known. Even among these viruses of known taxa identified in the ocean, 99.7 percent are new species, the researchers stated.
However, other RNA viruses cannot be assigned to any of the large known groups – they represent five completely new strains, for which the team proposed the names Taraviricota, Pomiviricota, Paraxenoviricota, Wamoviricota and Arctiviricota. This means that the number of strains in the RNA virus family tree doubled in one fell swoop. At least eleven new virus categories have been added at the level below. The formal family tree of RNA viruses must now be extended accordingly. “The results therefore change our ideas about the evolution and diversity of RNA viruses,” Sullivan and his team say.
The new discoveries also confirm that RNA viruses use primarily eukaryotic organisms as their host – and thus higher organisms such as plants, animals and humans. Accordingly, four of the five new RNA virus lineages and eight of the eleven new classes consist of cellular parasites that specialize in eukaryotes; Only a small minority infect prokaryotes such as bacteria.
The link is missing in the family tree of life
One of the newly identified strains of RNA viruses could shed new light on the evolution and emergence of viruses. Because the Taraviricota genome – a group of viruses found in all specimens and oceans – has been shown to be particularly original. According to the researchers, the RdRp sequences of this viral strain indicate that Taraviricota could be among the first representatives of orthoviruses – RNA viruses in the narrow sense.
“Our analyzes indicate with high probability that viruses of this newly proposed strain represent a missing link between retroviruses and orthoviruses,” Sullivan and colleagues write. Retro elements are genetic sections also found in the DNA of all living things that can change location and reproduce using special mechanisms. So they have long been suspected to be the remnants of viral genomes that were incorporated into DNA at some point in the course of evolution.
“Studies like this reveal connections between the viral and cellular worlds,” Jessica Labonte and Catherine Campbell of Texas A&M University wrote in an accompanying comment. “It opens the possibility of building a comprehensive family tree of life and understanding the origins and evolution of all life.” (Science, 2022; doi: 10.1126/science. abm5847)
Source: Ohio State University
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