After mating, males of many species take steps to keep their partners from having sex with other males. But some females may have evolved tricks to foil these measures. Researchers studying the fruit fly Drosophila melanogaster have discovered that when females expel excess sperm after mating, they also get rid of a key pheromone that males leave behind to repel other would-be suitors. The findings, published on August 3 in Nature Communications, describe one of the first known observations of female defense in the animal kingdom against chemical mate-guarding.
“It’s a nice example that there’s a lot more complexity to sexual conflict responses than we thought,” says evolutionary biologist Allen Moore at the University of Georgia in Athens, who did not take part in the research.
Mate-guarding, which can take the form of behavioral threats, physical barriers, or chemical warfare, is thought to reflect an evolutionary conflict between males and females. Females can potentially increase the quality or quantity of their offspring by seeking out more mates. Males, on the other hand, are driven to ensure that females invest energy and resources in their own offspring. In Drosophila, the stakes are especially high: sperm from the last male to mate with a female fertilize the majority of her available eggs.
Male flies are known mark their mates with a malodorous mix of pheromones, the main ingredients of which are thought to be cis-vaccenyl acetate (cVA) and (Z)-7- Tricosene (7-T). Neurogeneticist Jean-Christophe Billeter and his colleague Meghan Laturney at the University of Groningen in the Netherlands discovered that during sex, males deposit cVA inside the female reproductive tract while they transfer 7-T onto the female’s exterior. Within six hours of mating, females eject the excess sperm that can’t be tucked away into specialized receptacles. Billeter and Laturney found that in the process, females also jettison more than 80% of the cVA acquired during mating.
Sperm ejection and the loss of cVA appeared to restore the females’ sexual attractiveness; whereas male flies courted mated females less vigorously than virgin females, this bias faded after mated females had ejected sperm. To isolate and directly test the contributions of cVA and 7-T to female attractiveness, Billeter and Laturney applied the molecules in varying concentrations and combinations to the bodies of virgin females (which had been decapitated to eliminate possible effects of female behavioral signals). At extremely high concentrations, cVA discouraged males from courting the perfumed females. But at naturally-occurring concentrations, the team found that only the combination of cVA and 7-T could create an anti-aphrodisiac effect. “By ejecting one of these pheromones, the female breaks this synergy and restores her attractiveness,” Billeter explains.
Previous studies had yielded conflicting results about whether cVA or 7-T could each act independently to turn off male suitors. “The fact that they found it was the combination of the two really explains some of the variability in the literature,” says reproductive biologist Mariana Wolfner at Cornell University in Ithaca, New York. “They did some really elegant experiments here.”
The team went on to show that females can change how quickly they eject sperm after mating, depending on social situations they encounter during mating. Sperm ejection occurred much earlier when females mated in a mixed housing group of six males and six females, compared to when they mated in an isolated pair.
“It’s a simple finding but changes the picture quite dramatically,” says Billeter. “We find that the females are not just passive victims of the males but can actively remove those male pheromones.” In future studies, he hopes to understand more about how the nervous system might help female flies strategically control their pheromonal profile, and therefore, their own sexual attractiveness.