The world's most well-studied viruses are tiny viruses that make us sick, but Earth hosts a great diversity of viruses, many of them harmless, some of them giant and most of them poorly understood.

According to a new study, published Wednesday in the journal Nature, genetic fragments from giant viruses, especially those from the nucleo-cytoplasmic large DNA virus family, are surprisingly common in the genomes of green algae.

When viruses infect living organisms, they leave behind genetic fragments that remain long after the virus is vanquished. These fragments can become incorporated into the host's genome and passed down from generation to generation.

Numerous studies have explored the impacts of viral infections on the genomes of humans and other mammals, but until now, the effects of viral DNA on the evolution of eukaryotic organisms were a mystery.

"Viruses play a central role in the evolution of life on Earth. One way that they shape the evolution of cellular life is through a process called endogenization, where they introduce new genomic material into their hosts," study co-author Frank Aylward said in a news release.

"When a giant virus endogenizes into the genome of a host algae, it creates an enormous amount of raw material for evolution to work with," said Aylward, an assistant professor of biological sciences at Virginia Tech.

Because chlorophytes, a group of green algae common in lakes and ponds, are closely related to land plants, researchers hypothesized their relationship with giant viruses could offer insights into the role viral DNA played in the evolution of the earliest plants.

"We now know that endogenous viral elements are common across chlorophytes, which makes you think that plants might also interact with these giant viruses," said lead author Mohammad 'Monir' Moniruzzaman.

"There is some data that suggests that some early plants, like moss and ferns, did experience these endogenization events over the evolutionary timeline. But we are not exactly sure about the extent of this phenomenon in other early plants," said Moniruzzaman, a postdoctoral researcher in Aylward's lab.

For the study, Aylward, Moniruzzaman and their research partners sequenced genomes of various algae groups, looking for the genetic signatures of giant viruses. They found nearly one-third of the sequenced algae genomes contained the signatures of repeated viral endogenization events.

Researchers identified one algae species, Tetrabaena socialis, that derived 10 percent of its genes from a single virus in the nucleo-cytoplasmic large DNA virus family.

"These large endogenous viral elements are a lot more common than previously thought. Now that we have a systematic analysis, other researchers are really going to start to pay attention," Moniruzzaman said. "This study shows that endogenous viral elements are pretty common, and so it might possibly be a common mechanism of genome evolution. I think these results will broaden our view on the role of giant viruses as mere agents of host mortality to significant players in host genome evolution."

Moving forward, the researchers said they hope to determine why DNA fragments from giant viruses are so frequently integrated into the genomes of some algae species.

Aylward and Moniruzzaman said they are also curious to find out why many green algae species make no effort to purge foreign DNA from their genomes.

The answers to these questions could help scientists better understand whether -- and to what extent -- viral endogenization offers an evolutionary advantage to some algae groups.

"There might be a reason as to why the host is keeping these viral genomes within them," Moniruzzaman said. "It's not like these viral genes are causing the hosts to become unsuccessful or unable to survive in the environment. So that's the thing: Are the endogenous viral elements beneficial to the host? And how are they getting in there and staying in there?"

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