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

By Elizabeth Pennisi18 November 2020, 11:00

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 proved more common than previously expected. Now, researchers have found entire genomes of giant viruses embedded in 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 sheer amount of DNA and the diversity of genes these viruses provide to their hosts is astounding,” says Cedric Feschotte, a genome biologist at Cornell University who was not involved in the work. This “large injection of genetic material” could affect everything from the host’s metabolism to its very survival.

Typical viruses don’t have enough genes to live on their own. Instead, they have to rely on their hosts’ machinery, be it bacteria, human cells, or other organisms. Viruses reproduce by causing the host to replicate their genetic material and produce proteins necessary for copies of themselves. So it was surprising earlier this year when researchers discovered that giant viruses contain genes they don’t seem to need, namely, stretches of DNA that are important for cellular metabolism, but not viral.

At Virginia Polytechnic Institute and State University, microbiologist Frank Aylward and his postdoc Mohammad Moniruzzaman followed this mystery by matching genes found in the giant virus’s DNA to those previously documented in other genomes. The viral matches “kept appearing in the algae genomes,” recalls Aylward. Then the duo and their colleagues systematically examined the genomes that represent all the DNA sequenced by 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 Nature.

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

It is not clear why these viruses introduce their DNA into their host’s genome instead of replicating within the cell. It could 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 get rid of by the immune system or drugs.

Some of these giant viruses have likely been a part of algae for a long time, the researchers found, perhaps millions of years. In fact, some viral DNAs have acquired non-coding DNA called introns within 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 have shown that the viral sequences they have identified are, in all likelihood, part of their host genomes,” says Matthias Fischer, an environmental virologist at the Max Planck Institute for Medical Research.

“I am surprised that such a giant virus [incorporation] can occur and is very widespread, “adds Chuan Ku, a microbiologist at the Institute of Plant and Microbial Biology. Ku, who uncovered the life cycle of a giant virus that infects a tiny algae called Emiliania huxleyi, he says: “It would be interesting to investigate in that case [incorporation] has long lasting effects on [host] genome evolution. “

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

“These interactions have been going on since the origins of life,” adds Fischer. “And they continue to play an important role in cellular evolution.”