Coral-associated viruses have been extensively researched in order to better understand their diversity, ecology, and potential impacts on coral reefs. In coral samples, scientists discovered a variety of viruses, including herpes-like viruses, retroviruses, and single-stranded RNA viruses. These viruses can be integrated into the genome of the coral host or exist as episomal elements.
The symbiotic organisms that live in corals and give them their vibrant colors contain fragments of ancient RNA viruses dating back up to 160 million years. An international team of marine biologists discovered ancient RNA virus remnants embedded in the DNA of symbiotic organisms living inside reef-building corals.
The RNA fragments came from viruses that infected the symbionts 160 million years ago. The discovery is described in an open-access study published in the journal Nature Communications Biology, and it could help scientists better understand how corals and their partners fight viral infections today. However, the discovery was surprising because most RNA viruses are not known to embed themselves in the DNA of the organisms they infect.
Endogenous viral elements, or EVEs, were discovered to be abundant in the genomes of coral symbionts. Dinoflagellates are single-celled algae that live inside corals and give them their vibrant colors. The EVE discovery supports recent findings that viruses other than retroviruses can integrate fragments of their genetic code into the genomes of their hosts.
There’s a huge diversity of viruses on the planet. We know a lot about some of them, but most viruses haven’t been characterized. We may be able to detect them, but we don’t know who hosts them.
Adrienne Correa
“So why did it get in there?” asked study co-author Adrienne Correa of Rice University. “It could just be an accident, but people are starting to find that these ‘accidents’ are more frequent than scientists had previously believed, and they’ve been found across all kinds of hosts, from bats to ants to plants to algae.”
That an RNA virus appears at all in coral symbionts was also a surprise.
“This is what made this project so interesting to me,” said study lead author Alex Veglia, a graduate student in Correa’s research group. “Based on what we know, there’s really no reason for this virus to be in the genomes of symbionts.”
Correa, Veglia, and two Oregon State University scientists, postdoctoral scholar Kalia Bistolas and marine ecologist Rebecca Vega Thurber, led the study, which was funded by the Tara Ocean Foundation and the National Science Foundation. The findings can help scientists better understand the ecological and economic impacts of viruses on reef health.
EVEs from RNA viruses were not found in filtered seawater samples or the genomes of dinoflagellate-free stony corals, hydrocorals, or jellyfish. However, EVEs were found in coral symbionts collected from dozens of coral reef sites, indicating that the pathogenic viruses were – and probably still are – picky about their host hosts.
“There’s a huge diversity of viruses on the planet,” says Correa, an assistant professor of biosciences. “We know a lot about some of them, but most viruses haven’t been characterized.” We may be able to detect them, but we don’t know who hosts them.”
She said viruses, including retroviruses, have many ways to replicate by infecting hosts. “One reason our study is cool is because this RNA virus is not a retrovirus,” Correa said. “Given that, you wouldn’t expect it to integrate into host DNA.
“We’ve seen a lot of viruses in coral colonies for a long time, but it’s been difficult to tell what they were infecting,” Correa said. “As a result, this is most likely the best and most concrete information we have for the actual host of a coral colony-associated virus.” Now we can ask why the symbiont retains that DNA or a portion of the genome. “How come it wasn’t lost a long time ago?”
The fact that the EVEs have been conserved for millions of years suggests that they may be beneficial to coral symbionts and that there is some kind of mechanism driving the EVEs’ genomic integration.
“There are a lot of avenues we can pursue next, such as whether these elements are being used for antiviral mechanisms within dinoflagellates, and how they are likely to affect reef health, especially as oceans warm,” Veglia said.
“Is it more likely that Symbiodiniaceae species will contain this endogenous viral element if the temperature of seawater rises?” “Does having EVEs in their genomes improve their chances of surviving infections from modern RNA viruses?” he asked.
“In another paper, we demonstrated that thermal stress increased RNA viral infections in corals.” As a result, there are numerous moving parts. And this is yet another useful piece of the puzzle.”
