Female fruit flies have a busy sex life. They mate once every couple of days, and store male sperm in their reproductive tract for up to a week. For males, this means that the drive to produce offspring doesn't end with mating; their sperm may have to compete with that of several other males. Now, by studying a mutant breed of fruit flies with fluorescent sperm, biologists are getting their first look at how "survival of the fittest" works inside the female reproductive tract.
"There's a whole world of bizarre phenomena that go on between copulation and fertilization," reveals evolutionary biologist Jerry Coyne of the University of Chicago. Several hundred sperm, each of them about 2 millimeters long--almost as long as the fly itself--wait around in a jostling, writhing, spaghetti-like mass before they can reach their goal. For some reason, the most recent arrivals seem to have an advantage. In laboratory experiments, when a female fly mates with two different partners, over 80% of the progeny are fathered by the second one.
To see why the latecomers had an edge, Coyne and graduate student Catherine Price mated some females first with a fly whose sperm contained green fluorescent protein (GFP), and then with a normal male. Another group of females were mated only with one GFP-containing male each. Then they dissected the females and counted the number of sperm of each mate in her reproductive tract. As they report in tomorrow's Nature, they found many fewer glow-in-the-dark sperm in the females with two mates than in the females with only one. Moreover, the sperm from the two mates were thoroughly mixed together. Taken together, these observations suggest that the second male's sperm win by driving out the first male's, not merely by occupying a more favorable position, says Coyne. Further experiments suggested that the seminal fluid may help; It seems to incapacitate older sperm.
Other researchers on sexual selection are very enthusiastic about the trick for telling sperm apart. "The GFP construct is somewhat of a dream come true," says Scott Pitnick, an evolutionary biologist at the University of Syracuse. "The details of the interaction between the sperm and the female, or between the sperm and sperm inside the female, are a great new frontier in evolutionary biology." But Pitnick cautions that Coyne and Price's results on genetically altered flies are not necessarily representative for normal flies. "The GFP males transfer only about 85 sperm per copulation, as opposed to about 5000 for a normal fly," says Pitnick. "They're not fit males."