Amidst the animal kingdom’s menagerie of sexual practices, those of Corbicula clams stand out.

A common freshwater genus about the size of a half-dollar, nearly all Corbicula species are clones. That’s odd, albeit not extraordinary. They’re also physically hermaphroditic but genetically male — again odd, but not extraordinary.

What’s really strange is that, once in a great while, they hijack fresh DNA from other clams.

“They can steal the eggs of other species,” said David Hillis, a University of Texas at Austin computational biologist whose Corbicula investigations are described May 23 in the Proceedings of the National Academy of Sciences. “Usually, the whole maternal genome is then kicked out. But sometimes they keep some of the genes, and incorporate them into their own genome.”

At a glimpse, Corbicula reproduction seems to be of the unexceptional, sperm-meets-egg-in-the-water marine variety. But in most cases, sperm and egg come from the same clam, which produces both. Then, after fertilization, egg genes are ejected from the embryo. Should currents happen to mix sperm and egg from different clams, the same happens. In either case the result is a clone descended from one original clam’s sperm.

Cloning’s great advantage is that it lets organisms quit worrying about finding the right mate, which is of course themselves, and channel all that time and energy elsewhere. Once considered an evolutionary aberration, cloning is now seen as a fairly common and successful reproductive strategy. But every self-cloning species is confronted by what Hillis calls the Xerox principle: with each round of copying, errors are introduced. Genomes become smudged and, over time, unreadable.

Cloners have evolved a variety of solutions to this problem. Some species of fungi, along with a fish called the Amazon molly, reproduce sexually just often enough to prevent their gene pools from drying. Certain all-female lizards can alternate between single and double-sexed–species status as needed, or rely on chromosome mix-up mechanisms that bootstrap them into genetic diversity. One class of animals can even absorb the DNA of its deceased (see sidebar). But how Corbicula stayed genetically vital was unknown.

In the new study, Hillis and colleagues scanned the genomes of 19 different Corbicula species from around the world, searching for patterns that could reveal the clams’ trick. They found odd spikes — groups of genes that belonged to one species, but inexplicably showed up in another.

According to Hillis and colleagues, the most plausible explanation involves rare fertilization events when sperm mets egg that doesn’t just come from another clam, but from another Corbicula species. Most of the time, development proceeds normally, with egg DNA jettisoned — but every so often, once in thousands or even millions of generations, some of the egg’s genes are allowed to stay. The clone’s lineage is replenished.

“This is the signature you’d expect to find to find from these genetic-capture events,” he said. “It’s exactly what we observe.”

‘When it comes to sexual systems, almost anything you can imagine, and a lot of things we never imagined, happen somewhere in nature.’

Hillis is now curious as to whether there’s a relationship between this reproductive habit and Corbicula’s propensity for sudden population booms, which in some areas has made it a pipe-clogging pest. He’s also interested in whether all-male species, which seem to be a rarity among single-sex reproducers, are actually more common than thought.

In a recent Systematic Biology paper, Hillis described how the cells of various organisms, from other mollusks to oak trees, contain DNA traces from other related species. These are generally explained as the genetic remnants of past hybridization events, but to Hillis the patterns don’t look right. He suspects that androgenesis — the technical name for father-only reproduction — is responsible.

“There are all kinds of interesting questions now about asexual systems. A lot of them we don’t know much about, and biologists never even spent any time thinking about them,” said Hillis. “When it comes to sexual systems, almost anything you can imagine, and a lot of things we never imagined, happen somewhere in nature. ”

Images: 1) A Corbicula clam shell (Wikipedia). 2) A Bdelloid rotifer (Wikipedia).

See Also:

Citation: “Rare gene capture in predominantly androgenetic species.” By Shannon M. Hedtke, Matthias Glaubrecht, and David M. Hillis. Proceedings of the National Academy of Sciences, Vol. 108 No. 21, May 24, 2011.