Published online 29 July 1999 | Nature | doi:10.1038/news990729-6

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What happens when sperm from two different males meet up? Is there all-out war, or do they live in harmony? Females in many animal groups have multiple mates - often close enough together in time to cause the sperm to mix. While no battle-axes are visible, a known phenomenon is that the sperm from the second mate almost always triumphs, somehow disabling the first mate’s sperm. Just how does it manage this feat?

In a report in the 29 July issue of Nature, Jerry Coyne and colleagues from the University of Chicago, Illinois,show that fruitflies (Drosophila melanogaster) have two different weapons - displacement and incapacitation - to disable the sperm of the first mate. In displacement the incoming sperm physically push the sperm of the first mate out of a receptacle in the female where it is stored. Incapacitation is slightly different because it does not even require the sperm of the second mate. Instead, something in the seminal fluid destroys the ability of the first sperm to perform its duty of fertilising eggs.

Female fruitflies store sperm in three places: a long, tubular, seminal receptacle, and two mushroom-shaped sacs. The general set-up is that copulation with a male causes sperm to be held in all three receptacles. It hangs out there for many days as a parade of eggs goes by, allowing fertilisation to occur.

The researchers group used male flies that produce a special green fluorescent protein on the tail of their sperm. This allowed the researchers to keep tabs on which sperm was from which male in which receptacle. When the second mating occurred after either four or seven days, the sperm of the first was physically pushed out of the seminal receptacle, but not the sacs. The net result was that the second mate became the proud father of the majority of the offspring.

The group showed the presence of incapacitation by having the second mate sterilised so that it deposited seminal fluid, but no sperm. The sperm of the first male was not pushed out, but was no longer capable of fertilising the eggs. Somehow, something in the seminal fluid deactivated the first sperm. Oddly, incapacitation was much stronger if the second mating took place after seven days than if it occurred after two days.

One way to explain the difference in timing is that some aspect of the environment inside the female may play a role in altering the sperm so that they are susceptible to incapacitation by seminal fluid. Such a theory suggests that the female may have some control over the whole process of sperm preference, possibly choosing - for a reason we don’t understand - to have the second mate’s sperm take priority.

This preference “does seem to be a ubiquitous phenomenon,” comments Coyne. Researchers are still scratching their heads as to why this system of sperm priority has developed throughout the animal kingdom.