Credit: POINT OF VIEW

About 5 years ago, Barry Dickson's group at the Research Institute of Molecular Pathology in Vienna started studying the mating habits of the fruitfly Drosophila. His goal was to unravel the neural circuits in the brain that control behaviour, and mating seemed like a good starting point. “It is a robust behaviour,” says Dickson. “It is something flies are really good at doing.”

In flies, gender-specific behaviour is controlled by a gene called fruitless, which is expressed in the nervous system. In 2005, Dickson and others found that fruitless is expressed in three out of the 50 types of neurons that sense odors. He thought these 'fruitless' neurons might detect sex pheromones — chemicals produced by animals to let other members of the species know things like the animal's sex or willingness to mate.

Dickson had a clue as to the pheromone involved. One of the three types of fruitless neurons also expressed a receptor thought to sense the male sex pheromone 11-cis-vaccenyl acetate (cVA). Graduate student Amina Kutrovic engineered flies lacking the putative cVA receptor, and with the help of postdoc Alexandre Widmer examined the flies' behaviour.

Normally, male flies court females and not males, and females mate with males. But mutant male flies lacking the cVA receptor often courted males, and mutant females were less willing to have sex. So it appeared that activating the receptor for cVA had opposing functions: turning males off and females on.

To test this idea further, the team engineered male flies to produce a receptor for a female moth sex pheromone in the same neurons that normally express the cVA receptor. They then rubbed the moth pheromone on female fly abdomens so that the female flies now smelled like female moths. Male flies expressing the moth pheromone receptor shunned these females, whereas those without it were happy to court them (see page 542). This seemingly counterintuitive result told the scientists that the activation of neurons normally expressing the cVA receptor is responsible, and sufficient, for suppressing courtship by males.

Everyday odours, such as the smell of rotting fruit, activate many different receptors on different neurons, and the odour identity is encoded by the combination of activated receptors. But odours of particular biological significance, such as pheromones, may activate a single class of olfactory neurons, thus communicating an unambiguous signal to the brain.

Dickson believes that the difference in responses to cVA between males and females depends on how the signal is processed in the brain. “The sensory processing appears to be the same in both sexes,” says Dickson. “We don't know exactly where the difference arises, but it does not seem to be in the olfactory neurons themselves.”

His group is now tracking down the higher-order pathways that process the signals from the cVA receptor. “We want to know how such chemical signals activate neural circuits to elicit behaviour,” he explains. “The fruitless and Or67d genes are not present in humans, and so these findings cannot be directly translated to human behaviour. But we may be able to extend the results to other species by looking for analogy rather than homology.”