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Fruit flies were genetically altered so their sperm heads were fluorescent red or green allowing scientists to observe in striking detail what happens to live sperm inside the female.

“Our first goal with these flies was to tackle the mechanisms underlying sperm competition,” says Scott Pitnick, professor of biology at Syracuse University.

“Whenever a female mates with more than one male—and female promiscuity is more the rule than the exception in nature—there are conflicts between the sexes over paternity, as well as competition between rival ejaculates to fertilize eggs. Such postcopulatory sexual selection is a powerful force for evolutionary change.”

Pitnick explains that major advances in reproductive biology came with the advent of molecular tools that determine paternity. “Until now, the door to most of the mechanisms responsible for patterns of paternity has been closed. But not anymore.”

Details of the study appear in Science Magazine.

By quantifying sperm movement and fate within females inseminated by a green-sperm male and a red-sperm male (including real-time analyses of sperm motility in vivo), Pitnick’s colleague Mollie Manier was able to unambiguously discriminate among hypothesized mechanisms underlying sperm precedence.

“Despite nearly a century of intensive and innovative work on the reproductive biology of the fruit fly [Drosophila melanogaster], much of what we know about the female reproductive tract is a mystery,” continues Pitnick.

“Our jaws hit the floor the first time we looked through a microscope and saw these glowing sperm. It turns out that they are constantly on the move within the female’s specialized sperm-storage organs and exhibit surprisingly complex behavior.”

Pitnick says his team has created similar glowing sperm populations for other species, including ones that hybridize, so he can observe what happens when sperm and the female are evolutionarily mismatched.

“I suspect we have just scratched the surface of using this material,” he says.

Syracuse University news: www.syr.edu/news/