According to the new study, the DNA from the newly discovered viruses is similar to the DNA of virophages, suggesting that microbes may enjoy some protection from giant viruses thanks to “embedded” viruses residing in their genomes
While analyzing the genomes of single-celled microbes, a team of researchers made a startling discovery: thousands of previously unknown viruses were “hidden” in the microbes’ DNA.
The researchers found the DNA of more than 30,000 viruses embedded in the genomes of various single-celled microbes, they report in their new study. They explain that viral DNA can allow a host cell to replicate complete, functional viruses.
“We were very surprised by the amount of viruses we found through this analysis,” said lead author Christopher Bellas, an ecologist who studies viruses at the University of Innsbruck in Austria. “In some cases, up to 10 percent of a microbe’s DNA is found to be made up of hidden viruses.”
“These viruses do not appear to make their hosts sick and may be beneficial,” the researchers added. Some of the new viruses resemble virophages, a type of virus that infects other pathogenic viruses trying to infect a host cell.
“Why so many viruses are found in microbial genomes is still unclear,” says Bellas. “Our most convincing hypothesis is that they protect the cell from being infected with viruses that are dangerous for it.
Living on Earth means fighting viruses – the most common biological entities on the planet, collectively infecting every type of life form. They are very diverse, using many different tactics to exploit their cellular hosts.
Regardless of the semantic debates about whether viruses are alive, they certainly insert themselves into the lives of other living things. Some even replicate by adding their DNA to a host cell and becoming part of its genome.
When this occurs in a germ cell, it can result in endogenous viral elements (EVEs), or viral DNA, passed from one generation to the next in a host species.
Scientists have found EVE in a wide range of organisms, including animals, plants and fungi. For example, mammals carry different viral fragments in their DNA, and about 8 percent of the human genome consists of DNA from ancient viral infections. The study authors explain that most of these are no longer functional and are considered “genomic fossils.”
Research suggests that EVEs may be adaptive in humans and other organisms, possibly helping to fend off modern viruses.
This is true for many single-celled eukaryotes, the researchers point out, noting that these microbes are commonly infected and killed by giant viruses.
If a virophage already inhabits a host cell, it can reprogram a giant virus to build virophages instead of replicating, potentially saving the host.
According to the new study, the DNA from the newly discovered viruses is similar to the DNA of virophages, suggesting that the microbes may enjoy some protection from giant viruses thanks to “embedded” viruses residing in their genomes.
EVE research has so far focused mainly on animals and plants, the researchers wrote, with little attention to protists — eukaryotic organisms that are not animals, plants or fungi.
Discovering thousands of new viruses hidden in microbial DNA was not the original goal of Bellas and his colleagues, who planned to study a new group of viruses found in the waters of Gossenköllese, an alpine lake in the Austrian province of Tyrol.
“Initially with our research, we wanted to discover the origin of the new ‘polinton-like viruses,'” Bellas says.
“However, we didn’t know which organisms were commonly infected by these viruses. That’s why we conducted a large-scale study to test all microbes whose DNA sequences are known.”
To do this, they enlisted the help of Leo, a high-performance computer cluster at the University of Innsbruck that can analyze huge amounts of data.
Noticing genes from virophages and other viruses in many of the microbial genomes, the researchers decided to deepen the study by using Leo to systematically analyze all protist genomes.
They found EVEs “hidden in repetitive, hard-to-connect regions of unicellular eukaryotic genomes,” they write, noting that thousands of integrated viruses show that they make up a significant, previously unstudied portion of protist genomes.
The study also found evidence that many protist EVEs are not just genomic fossils but functional viruses, the researchers added, “suggesting that different arrays of these elements may be part of the host’s antiviral system.”
Source: sciencealert
Note: The study was published in the Proceedings of the National Academy of Sciences of the United States of America.
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