Are Ancient Viruses Lurking In Polar Ice?

by Matt Walker

The virus, found deep within the Greenland icepack, is known as a tomato mosaic tobamovirus (ToMV), a common plant pathogen. The discovery suggests that other viruses, such as ancient strains of flu, polio and smallpox, may also be entombed and could make a comeback. "We don't know the survival rate, or how often they get back into the environment. But it certainly is possible," says Tom Starmer of Syracuse University in New York.

Starmer's colleagues Scott Rogers and John Castello of the State University of New York in Syracuse had earlier found ToMV in clouds and fog. The virus can survive in such environments because it belongs to a family with particularly tough protein coats. "Since it's widespread, moves in the atmosphere and is very stable, we deduced that we would find it in the Arctic ice," says Rogers.

So under sterile conditions, the researchers examined four cores containing ice between 500 and 140,000 years old from three sites in Greenland. To prevent contamination, they first disinfected the outside of each core using ultraviolet light, then searched the inner core for RNA from the tomato pathogen using the polymerase chain reaction (PCR). Sure enough, the telltale RNA was present. What's more, sequencing revealed 15 strains of the virus.

The frozen viruses were probably still infectious. PCR destroys organisms as it amplifies their genetic material, so the researchers can't be sure the viruses had retained the protein coats that help them invade cells. But Castello says they probably needed their coats to survive for hundreds of years in the ice.

The team says that a brief rise in temperature could unleash the entombed viruses. "The ice is melting constantly around the poles," says Rogers. If released, they could cause outbreaks of disease. "If you've got these things lying in the ice for a thousand years or more and their usual host has not had to deal with them, this may be a source of epidemics," says Alvin Smith, a virologist at Oregon State University in Corvallis. He has already found evidence that caliciviruses, which can cause diarrhoea, periodically re-emerge from the oceans, causing new infections.

The findings may answer some puzzling questions, Smith adds. Viruses evolve fast, continually changing their protein coats. Yet identical caliciviruses have appeared at 20-year intervals on opposite sides of the U.S., he says. "How did these viruses stay the same for so long? Where have they been hanging out?" He thinks they might have spent years trapped in polar ice before re-emerging to strike again.

Such exchanges could put a spanner in the works of viral evolution studies. Castello says that instead of representing the endpoint of steady evolution, modern populations of viruses might be a complex mishmash of highly evolved ones and others that have taken a holiday from evolution in cold storage. "Perhaps they are actually ancient populations that have been recycled over and over again," he says. "How can you do evolutionary studies if that's true?"

The New York team plans to use other techniques to find out if viruses in other cores are still viable. As well as Arctic ice, they will look at ice from Antarctica that is up to 400,000 years old.