Researchers at the University of Chicago have discovered two offensive mechanisms male fruit flies use to ensure that more of their genes get passed on to the next generation: displacement and incapacitation of a previous male's sperm.

In most insect species, the second male to copulate with a female sires most of her offspring. Scientists have long puzzled over this strange phenomenon, also seen in birds and some arachnids.

Female fruit flies mate with multiple males, storing the sperm in three specialized storage organs (the long, tubular seminal receptacle and two mushroom-shaped spermatheace) and using it as needed to fertilize eggs. However, the odds of becoming a father aren't equal for all the males: the last fruit fly to mate with the female tends to sire the most offspring.

"Not only are the flies in competition with each other to mate with a female, but their sperm are in competition to fertilize the eggs once inside the female," says Catherine Price, PhD, first author of the July 29 Nature paper. "This leads us to believe that the males have evolved mechanisms for encouraging females to use their sperm, and females may have evolved means of mediating competition between sperm from different males."

Price and Jerry Coyne, PhD, professor of ecology & evolution at the University of Chicago and an author of the paper, concentrated on mechanisms the male uses for ensuring paternity, focusing on the apparent displacement and incapacitation of stored sperm by the ejaculate of later- mating males.

The researchers obtained male fruit flies whose sperm had been labeled with green fluorescent protein (GFP), enabling them for the first time to distinguish first from second male sperm in the female's reproductive tract.

When they mated females to males with the GFP-labeled sperm, and then to males without the label, they counted far less fluorescent sperm in the seminal receptacle than
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Contact: Sharon Parmet
sparmet@mcis.bsd.uchicago.edu
773-702-6241
University of Chicago Medical Center
29-Jul-1999