Semen, the stuff that ferries sperm, contains proteins that evolve swiftly to help males compete for paternity, suggests new research on fruit flies.

The high-stakes competition between male fruit flies vying for a top mating spot with females could be driving the fast-evolving proteins, according to the study published in the July 29 issue of the journal PLoS Biology.

"They change with the quickness we would expect for the immune system, which has to respond fast to new pathogens," said researcher Michael MacCoss of University of Washington's Department of Genome Sciences in Seattle.

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Though the study results are based on fruit flies, they do have implications for humans and other animals that also reproduce sexually.

"We think that if we can understand what are the pressures that are driving the rapid changes of these proteins in a system that's easier to study, like flies, we can get a handle on what may be causing the changes in humans," said lead study scientist Geoffrey Findlay, also of UW.

Findlay's team compared DNA sequences of semen proteins between closely related species of fruit flies, finding that the same proteins in different species had different arrangements.

They also identified semen proteins that were unique to certain species.

They discovered more than 80 previously unknown proteins that play a role in reproduction, confirming 70 others that had been predicted to be involved, that get transferred to female fruit flies via semen.

Semen advantage

Female fruit flies mate with several males during their lifetimes, storing the sperm at different times in specialized organs.

"If you've got sperm from different males, all competing for fertilization success, than anything a male can do to increase the chances of his sperm winning and decrease the chances of other males' sperm winning is going to be advantageous," Findlay told LiveScience. "It could be that these proteins have evolved to increase that fitness."

Semen proteins could, for instance, cue the female to lay eggs immediately upon getting the semen goods, and could make females less likely to mate again with another male, the researchers explain.

Perhaps certain proteins "disarm" the seminal proteins transferred to females by other males, they say.

Staying on top

There's a little twist, however. If a mutation imparts super-hero qualities to a male's sperm, over time that mutation would become a part of every male fruit fly's reproductive repertoire.

The researchers found that to counter this plateau of fitness, male fruit flies may have acquired duplicates of proteins.

"One copy can be held constant and in the other copy the mutation can play around with that copy and if it happens to hit on a beneficial mutation in the second copy, then that copy can diverge and maybe provide a new advantage," Findlay said.

They found 15 such duplications in the male reproductive genes.

He added, "We think this is evidence of the continuing arms race, or sort of struggle, for reproductive success.

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