The Race for Solid Semen

from ScienceNOW web story.

Chimps are the most notorious swingers among the great apes. Their wanton sex lives, in which males compete to impregnate females, have led males to evolve huge testicles, three times the size of humans'. Now a study finds that the effects of promiscuity extend inside the testes as well. The research uncovered the genetic footprint of rapid evolution in a male protein that helps block the sperm of subsequent flings from entering the vagina.

It's not unusual for female primates to mate with more than one male in rapid succession. In response, males have evolved large testes that make voluminous ejaculates of sperm and cocktails of proteins. One of those proteins, called semenogelin, coagulates into a kind of vaginal plug (gelatinous in humans, solid in chimps) that keeps out sperm from the female's subsequent suitors. But another ingredient, an enzyme that rides in the front part of the ejaculate, can breach the plug.

That kind of reproductive head-to-head means that new and improved versions of the gene for semenogelin ought to constantly appear and spread as they give their owners the edge in sperm competition, says evolutionary biologist Sarah Kingan, then an undergraduate at Brown University in Providence, Rhode Island. This would lead to reduced variability in the gene, as more effective versions sweep through the population again and again, each time replacing outdated ones. To test if such an evolutionary sweep has actually happened, Kingan and her faculty advisors determined the DNA sequences of the gene in 12 humans, 10 chimps, and seven gorillas.

The results, published in this month's Journal of Molecular Evolution, indeed show a much lower variability of the gene in chimpanzees, with all individuals carrying the same version. Humans had slightly more variation, whereas each of the seven gorillas carried a unique version. In fact, some of the gorilla genes are so garbled that they may not be functional anymore. That's not surprising, says Kingan, as gorilla males monopolise their females and their sperm rarely, if ever, have to compete.

Geneticist Alberto Civetta of the University of Winnipeg, Canada, who studies sperm competition in insects and other invertebrates, is impressed with the new study, which he calls "very convincing." He looks forward to Kingan's yet unpublished study on another primate ejaculate gene, which she says also shows rapid evolution in chimpanzees.

--MENNO SCHILTHUIZEN

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