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X-rated worm movies reveal sex secrets

Sperm design and mating behaviour co-evolve.

By watching countless hours of hermaphroditic worm sex, Lukas Schärer and his wife Dita Vizoso, evolutionary biologists at the University of Basel in Switzerland and their colleagues, have discovered evidence for a theory that has eluded testing for nearly a century: sex shapes sperm. Their findings, including videos of the mating worms, are published today in the Proceedings of the National Academy of Sciences

Sperm are the most diverse of animal cells, variously adorned with tails, hairs, hooks, bristles and more. "But we don't know what any of those doodads do," says Scott Pitnick, an evolutionary biologist at Syracuse University in New York. Fertilization is not easy to observe, and predictions about the function of sperm design are even harder to test, so it took a group of transparent and rather kinky flatworms to unravel a piece of the puzzle. The creatures are simultaneous hermaphrodites: each has both male and female genitalia. The worms are about the size of a comma, but readily mate under a microscope.

The heterosexual world of animal reproduction is populated primarily by males eager to mate and females more concerned with finding a superior partner, but simultaneous hermaphrodites face antagonistic desires at the same time. Flatworm species in the genus Macrostomum solve the conflict by allowing eager sex to come first, and selectivity to follow.

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