Monday, July 27, 2009

Parasitic worms make sex worthwhile

How's that for a title? That came from the PR department of Indiana University. The actual title of the article that appears in the journal Current Biology is The Geographic Mosaic of Sex and the Red Queen. Quite a bit of difference between the two titles, eh?

The article is based on a very elegantly designed series of experiments conducted on New Zealand's South Island, in Lake Alexandrina and Lake Kaniere. The test subject was snails, similar to the one I photographed on Saturday, but these snails are fully aquatic and often infested with worms. Ewww.

The parasitic worms are a genus of trematode worms called Microphallus, and infest the snails in both lakes. What makes this interesting is that the worms need to be ingested by ducks for the next stage of their life cycle. That can only happen if the snails live in shallow water where the ducks feed. Snails dwelling in deeper water still get infested with the worms, but it ends there. The worms never mature (since they don't get eaten by ducks) and thus can't reproduce.

Added to this is snails can reproduce with and without sex. Reproducing asexually requires less energy than reproducing sexually, further a snail can produce more offspring asexually. So you'd think that evolution would select against sexual reproduction... but then there's those darn parasitic worms to consider.

Because the life cycle of the parasitic worms is completed in the shallows but not in deeper water, evolution is only going to affect the shallows dwelling snails. In their case there's coevolution going on. As the snail develops defenses against the worms, they in turn develop new offenses against the snails. The snails in deeper water don't have to play that game. As a result, the snails that live in shallow water, where there's a strong evolutionary game being played with the worms, generally reproduce sexually. This allows the snails to combine and recombine genes to find the best strategy to defeat the worms, who are also doing the same thing.

And this is why the second lake is included in the experiment. Since there's no connection between the two lakes, the snails and worms in each lake have evolved defenses and offenses that are different from one another. Not surprisingly then, the researchers found that snails from one lake had little defense against the worms from the other lake. Not having coevolved with the parasite, the snails were virtually defenseless. Anyway, that's enough about snails and the worms that love them.

1 comment:

jan said...

I'm delighted to find that my alma Mater, Indiana University, is writing headlines in keeping with the pioneer sex studies of Albert Kinsey that were done at IU.