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Female Flies Decide Sperm Wars

Love may seem like war sometimes, but within the reproductive tract of female fruit flies, a true battle rages: Sperm from different male flies compete head-to-head for a chance to fertilize the precious eggs. Far from being innocent bystanders in these sperm wars, the females are actually masterminding them, scientists report in tomorrow's Science--for their own self-defense.

When a female mates with more than one male, the suitors do not have an equal shot at siring her offspring. That's because in many animal species (including humans, according to some researchers) some males have more competitive sperm than others. Certain kinds of male fruit flies, for instance, use a cocktail of proteins in their ejaculate as a secret weapon to gain the edge; these proteins induce the female to increase her sperm uptake or her rate of egg-laying, or even to prevent her from considering any new suitors.

But such tricks may harm the female's interest, says biologist Andrew Clark of Pennsylvania State University in University Park; cranking up egg production may reduce her life-span and staying single could prevent her from choosing the most interesting mates. Puzzled by this potential liability, Clark and colleagues from the University of Texas, Austin, and the University of California, Davis, decided to test whether females are able to withstand a male's attempted manipulations.

The researchers took female fruit flies from six different strains and mated each with two males: one guy, with an eye-color mutation, got lucky every time; the other was a male picked from one of the six strains. By checking how many offspring carried the mutant eye-color, they discovered that some males were better at sperm competition than others. The males' reproductive success depended not only on their own genes, but also on those of the female. Clark says that "some females just ignore the signals of some males, and others don't."

The females' ability to influence sperm wars may have checked the male evolutionary tricks, says Allen Orr, an evolutionary geneticist at the University of Rochester in New York. "It's a very nice experiment," he says.