Viral Beast: Meet the Supersized Pandoravirus

A virus as big as a bacterium is like a mouse as big as an elephant—and scientists just found one

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courtesy of Chantal Abergel and Jean-Michel Claverie

Claire Abergel is on vacation this week on Canada’s Gaspe Peninsula, but for some scientists, there’s no such thing as pure play. At some point she’ll make her way over to the Saint Lawrence River to take samples of sediments along the riverbank. Amoebas can often be found in such soggy locations, and Abergel, a biologist at Aix-Marseille Université in Marseille, France, is part of a team that has found some enormous surprises inside the amoebas.

That’s “enormous” in the physical sense. Abergel and her colleagues have just reported the discovery of the world’s most gigantic virus. It’s so large that if you  knew only its size—about a micrometer, or a millionth of a meter, across—and the complexity of its genome, you’d swear you were talking about a bacterium. When the scientists found a smaller cousin of this new virus a few years ago, says Abergel, “we didn’t know at first what we were looking at. We called it NLF, for ‘new life form.’”

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They finally determined that what they were looking at was new, all right, but only in the sense that viruses this large had never been identified before. That set them off hunting for more examples, and when they dredged up samples from off the coast of Chile, they found the new champion, which they’ve named Pandoravirus, and which they describe in the July 19 Science.

It’s not just Pandoravirus’s physical size that makes it so intriguing as much as its complement of genes—more than 2,500 of them, compared with just 13 in the influenza virus, for example. “That’s just huge,” says Abergel. Even more suprising: only a tiny fraction of the 2,500 have ever been seen in any other organism.

Figuring out what those genes do will be part of the ongoing research agenda, but figuring out where Pandoravirus sits on the tree of life is in many ways a more intriguing question. Scientists know that our own genomes include DNA from ancient viruses that infected us eons ago and became part of our genetic essence. Some have hypothesized that because they’re so genetically simple—or so we used to think, anyway—viruses might be Earth’s original life forms (though you’ve always been able to get a vigorous argument going among biologist about whether viruses are alive at all since they can’t independently reproduce, which is one of the accepted definitions of a living thing).

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But the discovery of the first giant viruses a decade or so ago confused the issue. “The question,” says Abergel, “is whether Pandoravirus might have evolved from a bacterium. And the answer is, maybe it could.” In fact, she says, it’s not crazy to hypothesize that all viruses might be the descendants of full-fledged bacteria—although there’s no proof at this point. “We don’t really understand where viruses come from,” she says, “and we don’t really understand what they are.”

What we now do know is that they can be a lot bigger than any virologist would have guessed a decade ago. There’s no evidence that the members of these jumbo lines could be harmful to humans, but we should be prepared for any surprises. The Pandoravirus itself, after all, was one of them.

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