Fruit flies that lack the ability to make pheromones are sexually irresistible to normal male fruit flies, regardless of gender or species, Canadian researchers have found.

fruit-fly-pheromone

The pheromone-producing cells of this fruit fly were made to glow by adding a fluorescing green protein. ((Jean-Christophe Billeter))

The research shows that pheromones are not the chemical aphrodisiacs that many people consider them to be but are just one part of an elaborate language involving all the senses.

Researchers at the University of Toronto manipulated the genes of fruit flies — specifically, flies of the species familiar to genetics labs everywhere, Drosophila melanogaster — to remove the cells that produce the pheromones.

They did this to create a group of "blank slate" flies that they could apply pheromone chemicals to and study the results.

To their surprise, though, they found that the flies that produced no pheromones, both male and female, were extremely attractive to normal male flies, not just of D. melanogaster but also of other species of fruit fly.

Joel Levine of the University of Toronto said the result shows how pheromones, called cuticular hydrocarbons in the study, regulate sexual behaviour in fruit flies. They also influence their ability to recognize members of their own species and distinguish between males and females.

Superficially, male and female fruit flies, and fruit flies of other species in the genus Drosophila, are difficult to distinguish, and the flies rely on these signals to identify gender and species.

Removing pheromones resulted in strange sexual behaviour

Levine said the hydrocarbons represent just one signal in the flies' mechanism for recognizing and classifying other creatures, and they work differently among males and females.

"The default [behaviour] for males is to go for it," said Levine in an email to CBC News. "But while males are turned on by the absence of hydrocarbons, females are not at all interested in males who lack the hydrocarbons."

Female fruit flies are always attracted to male fruit flies that do produce pheromones, though.

"Other studies have shown that females are attracted by the male's song, produced by wing vibration," said Levine.

The male fruit flies without pheromones exhibited some unusual sexual behaviour, including attempting to copulate with each other's heads.

The scientists found that normal sexual behaviour was restored in the fruit flies when the "blank slate" flies were treated with a single synthetic pheromone.

"Clearly, the species barrier and the recognition of sex is gone when we eliminate the hydrocarbons, and it is restored by adding just one," said Levine.

The pheromone in this case represents a reproductive barrier that prevents the flies from attempting to mate with other flies of the wrong species.

The research appears this week in the journal Nature.