In most species of insects, birds, and some arachnids, the second male to copulate with a female fathers most of her offspring. How does the sperm of the last male to mate maximize its chances of being fertilized? Researchers from the University of Chicago provided some answers by studying fruit flies. The female fruit fly mates with multiple males, storing the sperm in three specialized storage organs where it remains until it is required to fertilize her eggs. However the chances of becoming a father are not equal for all the males, the last one to mate with the female tends to father the most fruit flies. By labeling the sperm of the first mate with GFP researchers, were able to distinguish between sperm released by the first and second partners, see picture below. They found that the first male's sperm is displaced by that of the second partner. However, they could not find what happened to the sperm of the first male. Besides being displaced by the sperm of the second male, the sperm of the first mate is also incapacitated by that of the second partner. This effect is more pronounced the longer the sperm is stored in the seminal receptacle of the female. At this point no one knows why the sperm from the second mate nearly always beats out that from the first one. Is there an evolutionary reason? This has yet to be determined.
(Photocredit: Cathy Fernandez and Jerry Coyne, Copyright permission granted by Nature's publishing group)