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Glow-in-the-dark sperm shed light on sexual selection

By genetically altering fruit flies so that the heads of their sperm were fluorescent green or red, scientists were able to observe “in striking detail what happens to live sperm inside the female”, Syracuse University reported today.

“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,” said Scott Pitnick, professor of biology in Syracuse University’s College of Arts and Sciences and one of the authors of the study.

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Green sperm can be seen mixing with red sperm heads in the reproductive tract of a female fruit fly. Scroll down for the full image.

Picture by Scott Pitnick, Syracuse University

“The findings may have huge implications for the fields of reproductive biology, sexual selection and speciation,” the university said in a statement about the research, published in the March 18, 2010 issue of the journal Science.

The glowing sperm enabled Pitnick and colleagues Mollie Manier and John Belote to make direct observations of how sperm from multiple male Drosophila flies compete within a female’s reproductive tract.

The brightly colored sperm allowed them to identify how the distribution of sperm from rival males changes within female sperm storage organs over time, Science said in a summary about the research paper.

“The researchers found that sperm move through female storage organs at a more equal pace than expected; with sperm from the most recent copulation ousting, but not damaging, sperm from previous males. Furthermore, the race to fertilize the female’s eggs was equally competitive.

“That is, the use of sperm from different males for the fertilization of eggs was directly related to their abundance over long-term storage—no sperm was favored over another,” Science explained.

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Picture by Scott Pitnick, Syracuse University

The findings may help biologists understand sperm competition and sexual selection, which affects genetic diversity, Science said.

According to Pitnick, many advances in reproductive and evolutionary biology have been constrained by the inability to discriminate competing sperm of different males and by the challenges of observing live sperm inside the female reproductive tract.

“The solution? Glow-in-the-dark sperm,” the university statement said.

“Our first goal with these flies was to tackle the mechanisms underlying sperm competition,” Pitnick said.

“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.”

By quantifying sperm movement and fate within females inseminated by a green-sperm male and a red-sperm male, Manier and colleagues “were able to unambiguously discriminate among hypothesized mechanisms underlying sperm precedence,” the university said.

Pitnick said 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 said.