Chemical keeps male sex drive in check
A single pheromone ensures a male fruitfly's urge to mate targets the right sex.
Male fruitflies will try to mate with nearly anything with a pair of wings — as long as it doesn't have a funky smell, work by a team of scientists in Canada suggests.
Hydrocarbons on the fly's exoskeleton, or cuticle, are known to act as pheromones, providing signals during courtship and helping flies identify members of their own species. But the details have been fuzzy because it's difficult to tinker with the mixture of hydrocarbons that decorate the flies.
Joel Levine and his colleagues at the University of Toronto at Mississauga found their way around this obstacle by engineering flies that could not produce hydrocarbon pheromones. Normal males voraciously pursued flies with this cuticular blank slate — regardless of sex or species — allowing the authors to determine which chemicals would nudge males toward more productive matings. The team's findings are published in Nature today1.
"Just by manipulating one pheromone you can completely change the way they behave," says Levine. "Recognition of sex and species is not about the genes of the individual; it's about these cues."
A number of studies have suggested that hydrocarbons might attract mates of the same species and opposite sex, and repel the rest. But the precise relationship between hydrocarbons and behaviour is unclear, as flies' cuticles bear a complex blend of hydrocarbons, some of which have no direct role in sex.
To remove all the hydrocarbons from the fly's cuticle, the authors targeted specialized cells, called oenocytes, that line the inner surface of the cuticle and are thought to synthesize the chemicals. They used a genetic trick to make the oenocytes self-destruct when the flies became adults.
As expected, flies without oenocytes lacked virtually all hydrocarbons. Yet despite the link between hydrocarbons and courtship, there was little change in the courtship behaviour of oenocyte-less males and normal females, and vice versa, though normal females were less attracted to oenocyte-less males.
"That was a big surprise," says Levine. "A lot of articles talk about how important hydrocarbons are for courtship. But no one had ever removed them with this specificity."
Intriguingly, normal males preferred to mate with females that lacked hydrocarbons over normal females. Even males from three different species of fruitfly tried to mate with the hydrocarbon-less females.
Males also attempted to mate with hydrocarbon-less males. The most extreme behaviours occurred when two hydrocarbon-less males came together: Both would furiously try to mate with each other (see video).
"It's amazing what you see," says Levine. "There are some pretty crude movies, with a male attempting and often succeeding to insert its penis somewhere into the head of the other male. It just goes on and on."
The authors were able to renew the repulsion males usually experienced towards each other by adding the hydrocarbon 7-T to the oenocyte-less males, thus confirming an earlier hypothesis that 7-T deterred male-male mating2. Levine thinks the female's hydrocarbon set may slow the male down, giving the female time to judge whether he would be a good mate.
But female pheromones aren't all off-putting. One female-specific hydrocarbon, 7,11-HD, is thought to be an aphrodisiac — though the relationship is more complex than that. Levine found that females with only 7,11-HD were no more attractive to males than females lacking all hydrocarbons.
7,11-HD was also able to deter males from other species from attempting to mate with the females, though 7-T managed to help counteract this effect in one species. "The biggest surprise is that the same signaling mechanisms allow a fly to recognize sex and species," says Levine.
The technique and findings take the field significantly forward, says biologist Matthew Cobb of the University of Manchester, UK. "It confirms and extends some research I was involved in 10 years ago ... because the Levine group has used a far more sophisticated and precise way of eliminating pheromones," he says.
Cobb says Levine's new genetic tool will help the field address outstanding questions, such as how males can respond to females despite constantly perceiving the 7-T on their own bodies, or how other signals besides hydrocarbons function in courtship behaviour.
"In reality, the situation is far more complex than we're showing," says Levine. "This is a proof of principle."
- Billeter, J-C. et al. Nature 461 987-992 (2009).
- Ferveur, J-F. et al. Proc. Biol. Sci. 263 967–973 (1996).