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Research Finds Role For Parasites In Evolution Of Sex

July 7, 2009

What’s so great about sex? From an evolutionary perspective, the
answer is not as obvious as one might think. An article published in
the July issue of the American Naturalist suggests that sex may have evolved in part as a defense against parasites.

Despite
its central role in biology, sex is a bit of an evolutionary mystery.
Reproducing without sex””like microbes, some plants and even a few
reptiles””would seem like a better 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
hypothesis is that parasites keep asexual organisms from getting too
plentiful. When an asexual creature reproduces, it makes 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,
often with different parasite vulnerabilities. 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.

The scenario works on mathematical models, but there have been few attempts to see if it holds in nature.

Enter Potamopyrgus antipodarum,
a snail common in fresh water lakes in New Zealand. What makes these
snails interesting is that there are sexual and asexual versions. They
provide scientists with an opportunity to compare the two versions
side-by-side in nature.

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,
Bloomington began observing several populations of these snails for ten
years starting in 1994. They monitored the number of sexuals, the
number asexuals, and the rates of parasite infection for both.

The
team found that clones that were plentiful at the beginning of the
study became more susceptible to parasites over time. As parasite
infections increased, the once plentiful clones dwindled dramatically
in number. Some clonal types disappeared entirely. 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.”

So we may well have to thank parasites””in spite of their nasty reputation””for the joy of sex.

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