Scientists have discovered two viruses that were preserved for 700 years in frozen reindeer poop collected from northern Canada. One of the viruses had probably infected plant material that passed through the animal’s digestive system; the other appears to have infected insects that swarmed around the reindeer, also known as caribou, in the summer months.
The plant-infecting virus was in good enough shape that the scientists were able to make a copy of it and use that to infect a modern-day plant.
Neither virus poses a danger to humans, the researchers said, but these rare ancient bits of viral material could provide clues to how viruses have evolved over time.
A paper describing the findings was published Monday in the journal Proceedings of the National Academy of Sciences.
Only a handful of ancient DNA and RNA viruses have been found that were preserved well enough to read their genetic code, the researchers say. In the case of the two viruses described in the paper, the intact state of the viral material was due to a specific mix of environmental conditions and animal behavior.
Each summer, the reindeer of northern Canada are plagued by swarms of biting insects. To escape their attackers, and to cool off from the heat, the animals seek out small frozen patches where the ice never melts. Ice cores pulled from these patches show that the reindeer have been using them as retreats and latrines for at least 5,000 years.
“It is an ecological phenomenon that is very unique,” said Terry Fei Fan Ng, a postdoctoral fellow at UC San Francisco, who led the study. “The reindeer are constantly defecating onto the ice, where the biological material gets preserved so nicely that we can carbon-date it to precise years.”
The lab that Ng works in specializes in detecting new viruses in animals and humans. In this case, however, he and his colleagues used their modern-day techniques to search for ancient viruses.
They were initially hoping to find viruses that infected the caribou themselves, but those have proved elusive. Instead, they came across two viruses that had similarities to those that affect plants and those that affect insects.
“It’s actually not that surprising,” said Ng. “A lot of the viruses that we shed in our feces are from our diet.”
The researchers also report that while they found fragments of caribou DNA in the feces, it was not as well-preserved as the two viruses.
“The viral DNA is amazingly intact and really long,” said Ng. “We think that is because it has a protein shell called a capsid on its outside that protects it like a pouch. So that protein capsule and the freezing temperature is what protected it from degradation.”
In addition to scouring the 700-year-old frozen feces for viruses, the researchers also analyzed four older samples, including one that was 3,230 years.
The older samples did not reveal any viral materials. Whether that’s because there were no viruses in that fecal material or because the virus material became too degraded over time is not clear, the scientists say.
Ng said that there may be other environments where viruses found in biological material could stand the test of time. As an example, he cited places with low humidity.
Still, he said, the ice patches of northern Canada are pretty special because they never thaw, and because the animals return there year after year.
“You can still see them standing there,” he said. “It’s like they are having a party.”