Last year, researchers uncovered the largest virus yet discovered. With a genome that is over 700,000 base pairs long, the CroV virus has more DNA than some bacteria. Fortunately, it infects a small, unicellular organism that's very distantly related to humans. Now, the same research team is back, this time announcing that they've discovered a virus that attacks CroV, and may just have given rise to all transposable elements, sometimes known as jumping genes.

While studying CroV, the researchers discovered a much smaller virus that frequently accompanied it. The new virus, which they term Mavirus (for "Maverick virus") is still a healthy size, as far as most viruses are concerned, weighing in at just over 19,000 DNA bases, and encoding 20 genes. But Mavirus never appeared on its own; instead, it was only active in cells when the larger CroV was around, even though it could enter cells on its own. The authors conclude that it probably steals CroV's copying machinery for making more Maviruses; this is consistent with the fact that CroV infections slow down when Mavirus is around.

This isn't the first giant virus to be victimized by a smaller peer—there's even a term for this: virophage. But, when the authors looked at the 20 genes carried by Mavirus, they didn't look like the ones from another virophage; instead, they looked something like genes from a specific type of transposable element.

Transposable elements, or transposons, are stretches of DNA that can move around the genome, hopping from place to place. They're so effective at this that about a third of the human genome is composed of various forms of transposons, which don't appear to do anything very helpful, but require energy to copy.

The authors suggest, however, that transposons got their start by doing something useful. The Mavirus helps protect cells from CroV, so cells that permanently incorporate a copy into their genomes could be at a significant advantage. Once in the genome, however, the viral DNA would be free to evolve into something closer to a parasite. The authors predict that, if we look in the right places, we'll find virophages that correspond to most of the major families of transposons.

Science, 2011. DOI: 10.1126/science.1199412 (About DOIs).