In a paper published May 3, in the journal Evolution, University of Cincinnati graduate student Karl Grieshop and Michal Polak, associate professor of biological sciences at UC, examine the role of genital spines in the reproductive success of a species of fruit fly. Their investigation identifies the specific type of advantage these spines bestow in the competition to reproduce.
"The leading hypothesis to explain the remarkable diversification of male genital traits is that such complexity evolves in response to sexual selection," Grieshop said. "Specifically, mechanisms of sexual selection operating during and after mating, such as sperm competition and cryptic female choice, have received the most support."
Grieshop and Polak have found compelling evidence that the spiny genitals of these male fruit flies provide the most benefit before the sexual act, rather than during mating or afterward. They achieved this understanding by using a precision laser surgery system to trim the claw-like spines of hundreds of male fruit flies and monitoring their success in a variety of mating situations.
Of the many species of fruit fly, they chose Drosophila ananassae because of the extraordinary length of these males' genital spines. Using the laser system, they removed the spines completely from some males, cut the spines in half on others, and merely blunted the spine tips on the final group.
Grieshop and Polak discovered that males with their spines completely removed were unable to copulate at all. Males with spines cut in half saw a profound reduction in sexual success. Those whose spines were merely blunted suffered a slight, non-significant reduction in copulation success, which was intensified to a statistically significant effect in sexually competitive environments. They likewise found the decrease in copulation success of partial-cut males was much stronger in competitive environments. And, it is in competitive e
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University of Cincinnati