Male flies help the females to bank sperm.

Long before people had the audacity to freeze sperm for use later, a variety of female insects, reptiles, birds, and even a few mammals evolved the ability to store sperm for periods ranging from weeks to months to years. Investigators have speculated that this odd talent gives sperm more time to compete with one another or allows females to avoid the rigors of frequent mating.

Female animals that hoard sperm often have specialized organs or regions of their reproductive tract devoted to the task. In at least one insect species, however, the male appears to provide critical assistance.

Seminal fluid produced by male fruit flies (Drosophila melanogaster) contains, in addition to sperm, a protein crucial to a female fly's ability to retain sperm, report Deborah M. Neubaum and Mariana F. Wolfner of Cornell University at this week's International Congress of Developmental Biology in Snowbird, Utah. The investigators found that after insemination, this protein, called Acp36DE, accumulates in the oviduct near the entrances to the female's sperm-storing organs.

Using a DNA-damaging chemical, the researchers created mutant fruit flies and identified those in which the gene for Acp36DE had been disabled.

Normal females that mated with males of this mutant strain produced 90 percent fewer offspring than usual. A close look showed that after mating with an Acp36DE-deficient male, a female fly typically stored fewer than 50 sperm. Ordinarily, she'd hold onto 300 to 600, says Neubaum.

The female flies seemed to realize that they had banked fewer sperm. After a normal initial burst of egg laying triggered by mating, the insects slowed their rate of egg production. When a female has stored lots of sperm, she usually maintains a high rate of production, says Neubaum. The females mated to Acp36DE-mutant males also became receptive to subsequent mating much more quickly than did females that had just mated with normal males.

The investigators are now trying to explain how Acp36DE promotes the stashing of sperm. The mechanism may take advantage of the tight binding observed between the protein and sperm. Alternatively, Acp36DE may simply direct sperm into the female fly's storage areas by blocking the oviduct. "This plug only lasts for a few hours. That's enough time, however, for most sperm to get stored," says Neubaum.