Marine biologists have made an unexpected discovery by finding remnants of ancient RNA viruses embedded in the DNA of single-celled algae known as dinoflagellates, which live inside reef-building corals. These symbionts provide corals with their bright colors, and the viral fragments found date back as far as 160 million years ago. The research, which appears in the journal Communications Biology, could highlight ways in which corals and their partners fight off viral infections today.
Endogenous viral elements (EVEs) are known to appear widely in the genomes of coral symbionts. This recent finding underscores observations that viruses other than retroviruses can integrate fragments of their genetic code into their hosts’ genomes. That an RNA virus appears at all in coral symbionts surprised the researchers, as most RNA viruses are not known for embedding themselves in the DNA of the organisms they infect. The researchers did not find EVEs from RNA viruses in samples of filtered seawater or in the genomes of dinoflagellate-free stony corals, hydrocorals or jellyfish.
However, EVEs were pervasive in coral symbionts that were collected from dozens of coral reef sites, meaning the pathogenic viruses were—and likely remain—picky about which organisms they target as hosts. The researchers found ancient viral fragments in the dinoflagellates living in the corals, which were previously unknown.
“There’s a huge diversity of viruses on the planet,” says Adrienne Correa, an assistant professor of biosciences and co-author of the study. “Some we know a lot about, but most viruses haven’t been characterized. We might be able to detect them, but we don’t know who serves as their hosts.”
The discovery that the EVEs have been conserved for millions of years suggests they may somehow be beneficial to the coral symbionts and that there is some kind of mechanism that drives the genomic integration of the EVEs. “There are a lot of avenues we can pursue next, like whether these elements are being used for antiviral mechanisms within dinoflagellates, and how they are likely to affect reef health, especially as oceans warm,” says Alex Veglia, a graduate student in Correa’s research group.
The researchers are exploring the possibility that these EVEs could be used as antiviral mechanisms in the dinoflagellates, which may increase their chances of fighting contemporary RNA viruses. Previous studies from Rice University have shown an increase in RNA viral infections when corals underwent thermal stress, due to climate change.
Another scientist involved in the study, marine ecologist Rebecca Vega Thurber from Oregon State University, notes that the detrimental effects of the virus are highly dependent on the environment: “One of the reasons we’re seeing viral impacts in coral reefs is because the corals are under stress,” she says. “They might not have as much of an impact when the reef is healthy and the corals are performing well. But when the corals are stressed, it’s possible that these viruses are taking advantage of that.”
Regardless of the virus’s effects, the discovery of ancient embedded RNA virus fragments advances understanding of the relationship between viral infections and their hosts. With climate change causing increased stress on coral reefs, further studies on genomic integration of EVEs could provide insight on how to safeguard the health of corals and their partners.
