Researchers have reconstituted a viral genome from centuries-old caribou feces frozen in the subarctic, and they’ve used the ancient viruses to infect lab plants. The findings, published in Proceedings of the National Academy of Sciences this week, offer a rare glimpse of viral evolution.
Ancient viruses provide snapshots of past diversity and a way to trace viral evolution, but their concentrations are low and intact samples are rarely successfully isolated from the environment. Cryogenically preserved samples in nature may be an untapped repository of preserved ancient viral genetic material.
A team led by Eric Delwart from the University of California, San Francisco, analyzed viral genetic material contained in an ice core obtained by drilling through layers of accumulated caribou feces up to 4,000 years old in a permanent ice patch in the Northwest Territories of Canada. Caribou gather on ice patches to escape pesky flying insects and heat in the summertime. After eating nearby veggies, they deposit feces that contain their DNA, partially digested plant material, as well as viruses—which can remain frozen for millennia. Here’s an aerial photograph of one such caribou congregation:
From a 700-year-old layer of the ice core, the team isolated the complete small circular genome of a DNA virus that was distantly related to plant and fungi-infecting viruses today. They named it aCFV, for ancient caribou feces associated virus. They also isolated a partial viral RNA genome that was related to an insect-infecting virus. They call this one Ancient Northwest Territories cripavirus, or aNCV.
These never-before-seen viruses either originated in plants eaten by caribou or insects attracted to fecal matter, and they were preserved at constant freezing temperatures within protective viral capsids.
The team used a “reverse genetic approach” to reconstitute the genome of the DNA virus. Then, to confirm that the virus infects plants, they inoculated the tobacco relative Nicotiana benthamiana with the ancient viral DNA. The inoculated plants displayed evidence of infection: The DNA virus replicated and systemically spread in the inoculated leaves (orange arrow) as well as their newly emerging leaves (white arrow).
As far as Delwart can tell, these viruses aren't dangerous, NPR reports. But as the climate warms and more ice melts, more caribou poo infected with ancient viruses might be making its way into the modern ecosystem.
Images: Brian Moorman (top), Glen MacKay (middle), Li-Fang Chen (bottom)