Made from a hodgepodge of genetic bits and pieces, the newly discovered Marseillevirus is the world's largest virus.

But fame is fleeting: It's almost sure to be supplanted by another, even bigger virus. What's really special about Marseillevirus is where it comes from. Like other giant viruses, it was found inside amoebas — lowly, single-celled organisms that devour anything they can absorb. Their voracious appetites make them incubators of genetic remixing among their prey, and may hint at processes that spawned complex life.

"What we find is that inside the amoeba, a virus can meet bacteria, archaea and prokaryotes. A whole new repertoire of an organism can be composed," said Didier Raoult, a microbiologist at the University of the Mediterranean in Marseille, France.

Six years ago, Raoult and his colleagues described the mimivirus, a virus so big they originally thought it was a microbe. Then they found the mamavirus, which was even bigger — so big that it could be infected by other viruses, which wasn't even known to be possible. The Marseillevirus, described Monday in the Proceedings of the National Academy of Sciences, is even bigger.

This string of discoveries — and there are many more that the researchers have yet to describe in the formal literature — shows that giant viruses are not an oddity, but a branch of the organismal tree that scientists are just starting to explore. And all the giant viruses have been found inside amoebas, a group of single-celled animals so common that it's easy to overlook their uniqueness.

The largest genome in the world, for example, belongs to an amoeba. "It's 200 times bigger than the human genome," said Raoult.

Such size comes from their eating habits. Amoebas absorb just about anything they can, from viruses to bacteria and archaea. Sometimes their food survives inside them. Through the free-for-all mixing process known as horizontal gene transfer, amoebas and their residents swap genes, giving rise to massive amoeba genes, giant viruses and mutant bacteria.

"It's a whole world in there," said Raoult.

According to Raoult, the amoeba melting pots likely had analogues billions of years ago, when eukaryotes — complex cells, with a nucleus and other sophisticated machinery — had yet to evolve. How they evolved is a scientific mystery, but Raoult thinks that simple forerunners of modern amoebas may have provided the necessary incubators for eukaryotic evolution.

Whether this actually happened may never be known, but Raoult added that it's definitely continuing today. "We have this idea that everything is derived from something with very old roots. But there is still creativity going on, creating new origins," he said.

Image: An amoeba containing Marseillevirus in (a) and (b), a Marseillevirus replicates itself in (c) and (d), while (e), (f) and (g) are reconstructions of electron micrographs of Marseillevirus/PNAS.

See Also:

Citation: "Giant Marseillevirus highlights the role of amoebae as a melting pot in emergence of chimeric microorganisms." By Mickael Boyer, Natalya Yutin, Isabelle Pagnier, Lina Barrassi, Ghislain Fournousa Leon Espinosa, Catherine Robert, Saïd Azza, Siyang Sun, Michael G. Rossmann, Marie Suzan-Monti, Bernard La Scola, Eugene V. Koonin, and Didier Raoult. Proceedings of the National Academy of Sciences, Vol. 106 No. 48, Dec. 7, 2009.

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecosystem and planetary tipping points.