Parasitic worms make sex worthwhile( BLOOMINGTON Ind. -- The coevolutiona...)

BLOOMINGTON, Ind. -- The coevolutionary struggle between a New Zealand snail and its worm parasite makes sex advantageous for the snail, whose females favor asexual reproduction in the absence of parasites, say Indiana University Bloomington and Swiss Federal Institute of Technology biologists in this week's Current Biology.

The scientists' report represents direct experimental evidence for the "Red Queen Hypothesis" of sex, which suggests sexual reproduction allows host species to avoid infection by their coevolving parasites by producing genetically variable offspring.

"We have shown that parasites can play a role in the maintenance of sex -- and on an incredibly small scale," said Kayla King, IU Biology Ph.D. student and lead author of the paper. "The females that reproduce asexually aren't as exposed to the parasites, and are just a few meters away."

The Current Biology report also supports the "Geographic Mosaic Theory," which says natural selection need not act uniformly on all members of a species, but can be intense in pockets of a population (hot spots) and absent elsewhere (cold spots).

"Asexual females can have a very large reproductive advantage, as all the individuals in the clone can directly produce offspring," said IU evolutionary biologist and coauthor Curt Lively, who has been working on the system for 25 years. "So evolutionary biologists have long wondered why clonal reproduction does not replace sexual reproduction in natural populations. Our studies suggest that interactions with parasites might be part of the answer, because genetically variable sexual individuals might be more likely to escape infection."

The biologists examined Potamopyrgus antipodarum, a common freshwater snail, in Lake Alexandrina and Lake Kaniere on New Zealand's South Island. The two lakes are on opposite sides of the Southern Alps mountain range, so the researchers assume that neither the worms
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