In 2003, scientists discovered something huge, literally, in the virus world: viruses so big they could be seen with a standard microscope. These massive parasites were considered rare at the time, but they’ve since proved more common than anyone expected. Now, researchers have found entire giant virus genomes embedded in the genomes of several common algae. The find suggests this strange viral group is even more prolific—and potentially influential—than scientists thought.
“The sheer amount of DNA and the diversity of genes contributed by these viruses to their hosts is staggering,” says Cedric Feschotte, a genome biologist at Cornell University who was not involved with the work. This “big injection of genetic material” could influence everything from the host’s metabolism to its very survival.
Typical viruses don’t have enough genes to live on their own. Instead, they must rely on the machinery of their hosts, whether those be bacteria, human cells, or other organisms. Viruses reproduce by having the host replicate their genetic material and make proteins needed 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 important for cell, but not viral, metabolism.
At Virginia Polytechnic Institute and State University, microbiologist Frank Aylward and his postdoc Mohammad Moniruzzaman followed up on this mystery by matching genes found in giant virus DNA to those previously documented in other genomes. Viral matches “kept popping up in algal genomes,” Aylward recalls. So the duo and their colleagues systematically looked through genomes representing all of the sequenced DNA from 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 all, the viruses added between 78 and 1782 genes to the algae. Two algae even had the whole genomes of two giant viruses in their DNA, in one case making up 10% of the algae’s total gene count.
It’s not clear why these viruses sneak their DNA into their host’s genome, instead of just replicating inside the cell. It may be a way for the virus to ensure that its genetic material will be passed down from generation to generation. HIV and other viruses also integrate their genes into human DNA—one reason they are difficult to eliminate by the immune system or drugs.
Some of these giant viruses have likely been part of the algae for a long time, the researchers found, perhaps millions of years. Indeed, some viral DNA has acquired noncoding DNA called introns within their genes. And some of their genes are now duplicated or missing, changes that are unlikely to occur in viruses simply floating around inside algal cells.
“They make a solid case that the viral sequences they 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’m surprised that such giant virus [incorporation] can occur and is widespread,” adds Chuan Ku, a microbiologist at the Institute of Plant and Microbial Biology. Ku, who has unraveled the life cycle of a giant virus that infects tiny alga called Emiliania huxleyi, says, “It would be interesting to investigate if such [incorporation] has long-lasting effects on [host] genome evolution.”
The viral DNA present in algae can even include genes hijacked from other algae. The giant viruses may therefore be a way to transfer genes among species, says Andrew Roger, an evolutionary biologist at Dalhousie University. All of this new DNA can enable the host genome to take on new functions that improve the alga’s ability to survive and may have shaped the group’s diversity and distribution, he says.
“These interactions have been going on since the origins of life,” Fischer adds. “And they continue to play a big role in cellular evolution.”