Pheromones may not initiate sexual attraction and mating, as commonly believed, new research suggests. Instead, the chemical signals may help flies distinguish between different genders and species while choosing a mate, researchers report in this week's issue of Nature.

The cells in Drosophila melanogaster that produce pheromones are located in the abdomen. These 'oenocytes' are revealed by expression of a protein fluorescing green. Image: Jean-Christophe Billeter

"This is the first time a study has deciphered the chemical dialogue happening between flies as they mate," said Nicolas Gompel, a geneticist at the Institut de Biologie du Developpement de Marseille-Luminy in France, who was not involved with the research but wrote an accompanying commentary on the paper. "The research challenges the common belief that pheromones are essential to initiate courtship... and suggests that instead, [they] play a significant role in mate identification and selection."

Scientists have long understood that pheromones, hydrocarbons produced by an animal or insect that trigger reactions in another individual, play a role in mating. But they've been unable to identify the role of individual pheromones because the hydrocarbons are often secreted as blends, comprised of up to 30 molecules, and function in conjunction with other signals. In order to study individual pheromones, Jean-Christophe Billeter from the University of Toronto at Mississauga and his colleagues genetically engineered adult Drosophila melanogaster without oenocytes, cells that secrete hydrocarbons. The oenocyte-less flies were "blank slates," said Joel Levine, a geneticist from the University of Toronto and coauthor of the study, allowing the team to imprint one pheromone at a time.

Because pheromones have been generally believed to stimulate mating, Billeter and his colleagues expected that flies lacking hydrocarbons would be sexually unappealing to males. To their surprise, quite the opposite happened: wild-type males were hyperattracted to oenocyte-less flies. The wild-type males also ignored gender, choosing to mate with both unscented males and females over other wild-types. These results led the researchers to conclude that pheromones may not stimulate mating, but may instead act to slow down male mating attempts to allow the female to assess her partner's suitability. They also concluded that hydrocarbons help flies distinguish between sexes.

Billeter and his colleagues then tested the effect of individual pheromones on mate selection and copulation attempts. Researchers treated unscented D. melanogaster with wild-type levels of cVA, a hydrocarbon males are known to coat on females to deter further mating attempts. As expected, cVA effectively created a "chemical chastity belt," said Gompel.

Researchers then treated unscented flies with 7,11-heptacosadiene (7,11-HD), a pheromone thought to act as an aphrodisiac for flies. Although 7,11-HD alone did not stimulate additional mating attempts, when applied over cVA, it helped diminish the inhibiting effect of the compound, allowing females to broadcast their mating availability. The findings suggested that mating doesn't depend on just one pheromone relaying a message of availability, but instead on a complex mixture of attractive and aversive signals.

7,11-HD also seemed to act as a species barrier in mating. Unscented D. melanogaster females treated with the hydrocarbon attracted males of the same species, but deterred D. simulans and D. yakuba males.

"It's neat that one hydrocarbon (7, 11-HD) acts both as an aphrodisiac to males of D. melanogaster and also as a key compound causing males of other closely related species in the genus to reject her," said Tristram Wyatt, an evolutionary biologist at the University of Oxford. "It's surprising at first sight, but perhaps it's another example of evolution resulting in simple solutions, two effects for one."

Billeter's study isn't the first time that the pheromone-less flies have been shown to be hyperattractive. Fabrice Savarit from the Universite Paris Sud and colleagues reported similar results in 1999. "One of the main differences between our study and Billeter's is his team was able to cleanly get rid of oenocytes," said Matthew Cobb, an evolutionary neurobiologist at the University of Manchester in England and coauthor of the 1999 study. "We produced pheromone-less flies in a roundabout way by overexpressing the UAS-tra transgene. Billeter's methodology is much more precise and reproducible." Billeter's study was also able to produce pheromone-less male flies, something Cobb's team failed to do.

Wyatt agreed, noting that the "blank canvas" flies will be invaluable "in further researching the neural circuits involved in sex and species recognition in these flies."

Cobb also noted that while hydrocarbons excreted by oenocytes may not initiate mating in flies, other pheromones might. In their 1999 study, Savarit and his colleagues noticed there was still a small amount of pheromones on their supposedly hydrocarbon-less female flies. Cobb noticed a similar trend in Billeter's data. "It seems even after they wiped out all the oenocytes in flies, there is a tiny -- we're talking less than 10 millivolts -- but significant amount of hydrocarbons still present," he said. "These might be ancestral pheromones, or not, but either way it is very intriguing."




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