Battle of the sexes offers evolutionary insights

May 03, 2012
Karl Grieshop uses a precision laser system to trim the claw-like spines of male fruit flies. 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. Credit: Jay Yocis, University of Cincinnati

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 ," 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 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 environments where fruit flies most often mate.

"The mating system of these flies is best described as 'scramble competition,'" Grieshop said. "They swarm on rotting fruit. Some females are receptive. Others are not. The premium goes to males that can mate efficiently with many females before they are usurped by sexual rivals"

In such a chaotic environment females are known to exercise choice prior to mating according to the scent of pheromones, sounds of mating calls, or the visual cues of courtship dances, Grieshop said. But with regard to the surgical treatments of these genital spines, females apparently do not discriminate between competing male treatments.

"It appears that these spines promote male copulation success in this sort of environment," Grieshop said. "Identifying the precise function of a trait that varies across species, such as these , provides insight into the evolutionary pressures that caused them to evolve and also how new species may arise. Most adaptive functions of genitalia so far discovered only make a difference after mating has begun. For genitalia to make a difference before copulation is unusual. This puts genital traits on the same playing field as so-called 'secondary sexual traits,' like coloration or other adornments."

The research, he said, encourages the scientific community to consider male genital trait evolution as being similar to that of secondary sexual traits, which may facilitate the understanding of one of the greatest unknowns in evolutionary biology: why male genitalia are so incredibly variable across species.

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User comments : 3

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Argiod
1 / 5 (1) May 03, 2012
Unless they can find a way to improve human sexual performance; I see no use for the study of the sexual dynamics of flies.
tkjtkj
not rated yet May 04, 2012
Argiod :
I'm afraid you've missed the point: the study is not concerned with affecting sexual performance. The study is about understanding the processes of evolution.
slayerwulfe
not rated yet May 06, 2012
fruit fly's as if that isn't prominent enough in 60? yrs of research. all male mammals all of us/them have breast glands, what other proof do you need. in female look what is right below the clitoris and then look at male an extension of this and females do have ejaculations. why is everyone so afraid of the truth. are you all islamist.

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