The joy of sex – courtesy of parasites

By Will Parker on July 7, 2009 in News

Why do we have sex? From an evolutionary perspective, the answer is not as obvious as we might think. And now, a fascinating new study inAmerican Naturalist suggests that sex may have evolved primarily as a defense against parasites.

Sex is something of an evolutionary mystery to biologists, as reproducing without sex – as microbes, some plants and even a few reptiles do – would seem like a much more efficient way to go. Every individual in an asexual species has the ability to reproduce on its own. But in sexual species, two individuals have to combine in order to reproduce one offspring. That gives each generation of asexuals twice the reproductive capacity of sexuals. Why then is sex the dominant strategy when the do-it-yourself approach is so much more efficient?

One theory is that parasites keep asexual organisms from getting too plentiful. When an asexual creature reproduces, it creates clones – exact genetic copies of itself. Since each clone has the same genes, each has the same genetic vulnerabilities to parasites. If a parasite emerges that can exploit those vulnerabilities, it can wipe out the whole population. On the other hand, sexual offspring are genetically unique. So a parasite that can destroy some can’t necessarily destroy all. That, in theory, should help sexual populations maintain stability, while asexual populations face extinction at the hands of parasites.

Now, thanks to Potamopyrgus antipodarum, a snail common in fresh water lakes in New Zealand, scientists have a chance to test this parasite theory. The snails exist in both sexual and asexual versions, thus providing biologists with an opportunity to compare the two versions side-by-side in nature.

Researchers Jukka Jokela, of the Swiss Federal Institute of Aquatic Science and Technology, Mark Dybdahl, of the University of Washington, and Curtis Lively of Indian University, began observing several populations of these snails for ten years beginning in 1994. They monitored the number of sexuals, the number asexuals, and the rates of parasitic infection for both.

They found that while clones were plentiful at the beginning of the study, they became more susceptible to parasites over time. And as parasite infections increased, the once plentiful clones dwindled dramatically in number. Meanwhile, sexual snail populations remained much more stable over time. This, the authors say, is exactly the pattern predicted by the parasite hypothesis.

“The rise and fall of these female-only lineages was surprisingly fast and consistent with the prediction of the parasite hypothesis for sex,” Jokela said. “These results suggest that sexual reproduction provides an evolutionary advantage in parasite rich environments.”