Giant virus genomes discovered hidden in the DNA of common algae | Science

By Elizabeth PennisiNov 18, 2020 at 11:00 a.m.

In 2003, scientists discovered something huge, literally, in the world of viruses: viruses so large they could be seen with a standard microscope. These massive parasites were considered rare at the time, but have since turned out to be more common than expected. Now, researchers have found entire giant virus genomes incorporated into the genomes of several common algae. The finding suggests that this strange viral group is even more prolific – and potentially influential – than scientists thought.

“The amount of DNA and the diversity of genes these viruses bring to their hosts are staggering,” said Cedric Feschotte, genome biologist at Cornell University who was not involved in the work. This “great injection of genetic material” could influence everything from the host’s metabolism to its very survival.

Typical viruses do not have enough genes to live on their own. Instead, they have to rely on the machinery of their hosts, be it bacteria, human cells, or other organisms. Viruses reproduce by asking the host to replicate their genetic material and make the proteins needed to reproduce. So it was surprising earlier this year that researchers found that giant viruses contain genes they don’t seem to need – DNA segments that are important for cell metabolism, but not viral.

At the Virginia Polytechnic Institute and State University, microbiologist Frank Aylward and postdoctoral fellow Mohammad Moniruzzaman pursued this mystery by matching genes found in giant virus DNA to those already documented in others. genomes. Viral matches “kept showing up in algae genomes,” Aylward recalls. So the duo and their colleagues systematically examined genomes representing all of the sequenced DNA of the group of algae called Chlorophytes. An entire giant virus was genetically present in the DNA of a dozen of these species, the team reports today in Nature.

In total, viruses added between 78 and 1782 genes to algae. Two algae even had the entire genome of two giant viruses in their DNA, representing in one case 10% of the total number of genes in the alga.

It is not known why these viruses introduce their DNA into the genome of their host, instead of just replicating inside the cell. This can be a way for the virus to ensure that its genetic material is passed down from generation to generation. HIV and other viruses also integrate their genes into human DNA – one of the reasons they are difficult to eliminate by the immune system or by drugs.

Some of these giant viruses have probably been a part of algae for a long time, the researchers found, perhaps millions of years ago. This is because some viral DNAs have acquired non-coding DNA called introns in their genes. And some of their genes are now duplicated or missing, changes that are unlikely to occur in viruses that simply float inside algal cells.

“They clearly demonstrate that the viral sequences they identified are, in all likelihood, part of their host genomes,” says Matthias Fischer, environmental virologist at the Max Planck Institute for Medical Research.

“I am surprised that such a giant virus [incorporation] can occur and is widespread, ”adds Chuan Ku, a microbiologist at the Institute of Plant and Microbial Biology. Ku, who unveiled the life cycle of a giant virus that infects a small algae called Emiliania Huxleyi, said: “It would be interesting to investigate whether [incorporation] has lasting effects on [host] evolution of the genome. “

The viral DNA present in algae can even include genes hijacked from other algae. Giant viruses could therefore be a way to transfer genes between species, says Andrew Roger, evolutionary biologist at Dalhousie University. All of this new DNA may allow the host genome to take on new functions that improve the alga’s ability to survive and may have shaped the diversity and distribution of the group, he says.

“These interactions have continued from the very beginnings of life,” adds Fischer. “And they continue to play a big role in cellular evolution.”