PREHISTORIC viruses are lying dormant in the polar ice caps鈥攁nd a bout
of warm weather could release them into the atmosphere, sparking new epidemics.
This chilling warning follows the discovery, for the first time, of an ancient
virus in Arctic ice.
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. 鈥淲e don鈥檛 know the survival rate, or how
often they get back into the environment. But it certainly is possible,鈥� says
Tom Starmer of Syracuse University in New York.
Starmer鈥檚 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. 鈥淪ince it鈥檚 widespread, moves in the atmosphere and is very
stable, we deduced that we would find it in the Arctic ice,鈥� says Rogers.
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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鈥檚 more, sequencing revealed 15 strains of the virus
(Polar Biology, vol 22, p 207).
The frozen viruses were probably still infectious. PCR destroys organisms as
it amplifies their genetic material, so the researchers can鈥檛 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. 鈥淭he ice is melting constantly around the poles,鈥� says Rogers. If
released, they could cause outbreaks of disease. 鈥淚f you鈥檝e 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,鈥� 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 US, he says. 鈥淗ow did
these viruses stay the same for so long? Where have they been hanging out?鈥� 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. 鈥淧erhaps
they are actually ancient populations that have been recycled over and over
again,鈥� he says. 鈥淗ow can you do evolutionary studies if that鈥檚 true?鈥�
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.