'Sex, lasers and male competition' — '80s action flick or new fly study?

But University of Cincinnati biologist Michal Polak says Darwin’s book “The Descent of Man” only tells part of the story. Sometimes when the victor vanquishes his sexual rival, the quest to pass genes to the next generation is just beginning.

According to a new UC study published in the journal Current Biology, male fruit flies with the most impressive sexual ornamentation also have super sperm that can outcompete that of rivals in the post-mating fertilization game.

The study demonstrates UC's commitment to research as described in its strategic direction called Next Lives Here.

UC studied Drosophila bipectinata, a tiny red-eyed fruit fly from the South Pacific. The male’s forelegs have a distinctive “sex comb,” dark bristles that female fruit flies find appealing — like the colorful train of a male peacock. Scientists previously found that female flies prefer males with more robust sex combs, which the males use to grasp the female’s abdomen before mating.

What females gain from mating with multiple suitors is not always clear, Polak said. But when they do, postcopulatory sexual selection provides a competitive edge.

“It’s evident even in primates. Female chimpanzees and bonobos are promiscuous, so the males have large testes that produce big volumes of sperm,” Polak said.

“And you have species like gorillas where females are not promiscuous. Silverback males enforce monogamy. And lo and behold, their testes are much smaller relative to body size compared to chimps.”

And if you’re wondering, the relative size of human testes falls somewhere between gorillas and chimps, Polak said.

Polak, a professor of biology in UC’s College of Arts and Sciences, decided to study this species of fruit fly after encountering it while conducting fieldwork in Queensland, Australia.

“I was watching these flies mate on a fruit and looked under the microscope and saw these beautiful sex combs. I thought it would make a good model system to study,” Polak said.

“Sexual selection picks up on these traits and they become really exaggerated,” he said.

For their study, UC biologists artificially selected males with the largest and smallest sex combs in 11 successive generations of fruit flies to create high and low genetic lines.

Kassie Hooker from 2012 to 2015 worked in Polak’s lab as an undergraduate biology student, undertaking the arduous task of categorizing generations of male fruit flies based on the size of the sex combs on their legs. By counting the teeth in each comb, she separated the males with the largest and smallest sex combs to create distinct genetic lines.

“Many researchers use these physical characteristics as an indicator of genetic quality and reproductive fitness in male Drosophila,” Hooker said.

To show that the male fruit fly’s sex comb doesn’t provide any reproductive benefit in mating, researchers used ultraprecise lasers to trim the sex comb in the high line males to mimic those found in the low line males. But these postsurgical males continued to fertilize more eggs even when females mated with lower-line males first.

Hooker is pursuing her master’s degree in clinical and translational research in the UC College of Medicine Department of Surgery, where she works as a research coordinator.

“I credit my involvement as a member of the Polak Lab as the experience that sparked my interest in science and research as a career,” she said. “I can confidently say my experience in undergraduate research at UC has given me numerous opportunities and continues to help me grow in my career and in my life.”

The research was supported by the National Science Foundation.

UC assistant professor Joshua Benoit, a study co-author, analyzed the RNA of the flies and identified seminal fluid genes that may be responsible for giving high-line males a fertilization advantage.

“There aren’t many studies more interesting than this one,” Benoit said. “Sex, lasers, and male competition, which could describe most 1980s action movies.”

Darwin proposed the theory of sexual selection to account for the evolution of male weaponry and extravagant ornamental displays, Polak said. But UC’s study found a far more complex and interesting battle among the sexes.

“We established a link between Darwinian traits and the postcopulatory arena, which Darwin didn’t recognize was important in evolution,” Polak said.

Featured image at top: Kassie Hooker, left, a UC master's student, UC assistant professor Joshua Benoit and UC biology professor Michal Polak collaborated on a study that linked sexual ornaments called sex combs with a male's postcopulatory reproductive success. Photo/Andrew Higley/UC Creative + Brand