Drosophila melanogaster image showing sexual dimorphism and mating behavior. New research shows odours from food stimulate neural networks needed to initiate courtship.
Credit: wikimedia commons
CAMBRIDGE: Courtship behaviour in male fruit flies is elicited by odours from their food that stimulate neural networks in a similar way to pheromones, research has shown.
The findings, published today in Nature, reveal a sensory pathway in fruit flies (Drosophila melanogaster) that links feeding with mating.
It's an evolutionary mechanism that ensures eggs are laid where resources are guaranteed for offspring, and it may shed some light on the intrigue of aphrodisiacs for humans.
"This is a sensory pathway for a food aphrodisiac," explained geneticist Richard Benton who led the research at the University of Lausanne, Switzerland. "I think [this] chimes slightly with the fact that for humans, many foods are often associated with aphrodisiac properties."
Fruitless neural networks complex
Fruit flies are commonly used to understand how genes and neurons control behaviour. But while the neural networks involved in the courtship behaviour of males have been identified, the signals that control them have not.
Many of the instrumental neurons express a protein called 'Fruitless', which is essential for courtship behaviour. These Fruitless-expressing cells include some olfactory sensory neurons.
It seemed likely that, as in many other animals, these olfactory neurons respond to pheromones - airborne chemical signals that animals release to attract potential partners and trigger mating behaviour. Benton and his group demonstrate that it is not that simple.
Coupling sex with food
Olfactory neurons expressing a receptor protein called IR84a alongside Fruitless were tested with fly pheromones, but there was no response. "Instead they responded to odours from fruits and other food sources that Drosophila feed on," explained Benton.
To work out if these food odours affect courtship behaviour in a similar way to pheromones, the group made mutant flies that lacked IR84a receptors. The mutants were clearly different to normal flies.
"These male flies show greatly decreased courtship," said Benton, "suggesting that [the receptor] is important to promote courtship behaviour."
Increased courtship behaviour of normal flies when their chambers were perfumed with the food odours confirmed that the chemicals do indeed make for frisky flies.
Benton said that coupling food with sex is a mechanism that helps ensure nesting sites have sufficient food resources nearby.
In contrast to pheromones, which often act over long distances, the food odour chemicals are only detected at short range.
This ensures that courtship behaviour is not triggered before reaching the food. "It's conceptually a different strategy for getting males and females together," says Benton.
Fly aphrodisiacs in perfume
Barry Dickson, who discovered Fruitless and is scientific director at the Research Institute of Molecular Pathology in Vienna, is excited by the findings.
"We assumed that those [fruitless-expressing] neurons would all express pheromone receptors," he said, "That turned out to be true for one [receptor]. This is the second one, and it's really quite a surprise that it doesn't detect the fly at all."
Dickson thinks that, while this receptor is specific to Drosophila, the principle may apply to other animals.
"Probably [in] many species, environmental stimuli other than pheromones can elicit feelings of sexual arousal and mating behaviour."
Similar process in humans?
Benton agrees, but is slightly more cautious: "I think it's too soon to say whether there will be homologous mechanisms in other animals, but if it occurred once in evolution, it's very likely to have occurred in other animals."
He does not rule out the possibility that similar processes might even play a role in humans, particularly since the same chemicals as the fly aphrodisiacs are used in perfumes.
"We found this rather intriguing that what smells nice to humans also smells nice to flies," he said. "I wouldn't like to go as far as to say that we have a receptor that's triggering our sexual behaviour, but it may be part of a more general sensory mechanism that's promoting a sense of attraction."
