Geneticists identify 'master switch' that causes female flies to behave like males 15/6/05. By Stanford University Turning on a single male-specific gene produces a female fruit fly that displays male courtship behaviours: chasing other females, tapping their abdomens and performing wing-beating love serenades.

The results, published in the 15 June 2005 online edition of the journal Nature, show that a single gene can determine how females and males detect and respond differently to sexual cues.

"In these experiments we see all the steps of the male courtship ritual you could physically expect a female fly to do," says Bruce Baker, Professor in Biology at Stanford University. "It's a male's behavioural circuitry in a female body."

Baker and Stanford graduate student Devanand Manoli, and their collaborators at Brandeis and Oregon State universities, focused on a gene known as fruitless – one of approximately 13 000 genes in the DNA of the common fruit fly, Drosophila melanogaster.

The three laboratories had previously discovered that fruitless is the master gene controlling the male fruit fly's elaborate six-step courtship ritual. Last year, they showed that disabling the fruitless gene in a tiny group of cells in the brain of a male fruit fly was enough to prevent successful mating, by turning him into a bumbling, ineffective suitor.

In the new study, researchers asked whether the fruitless gene would be enough to elicit male courtship behaviour in female flies. They activated the gene in neural cells in the female fly's brain and sensory organs.

When paired with a virgin female fruit fly, the masculinised females showed male courtship behaviour: chasing the female fly and then tapping her on the abdomen. When a masculinised female was placed with a male, she responded to his advances with masculine rejection behaviour – wing-flicking and kicking – rather than the upturned posterior that is the normal female rejection response.

In a group setting, the masculinised female demonstrated even more aspects of male courtship behaviour. She vibrated her wings in a mating song and occasionally extended her proboscis. Both are male-specific courtship behaviours that lead up to sexual intercourse.

When the researchers took male flies and disabled the roughly 2 per cent of brain cells that express the fruitless gene, the males had no interest in females but otherwise behaved normally, walking, flying and grooming as usual. "That's surprising, that you could take a chunk of the brain, shut it down and get so little of an effect," says Baker. This behaviour implies that these brain cells do not affect the body's motor functions but are dedicated exclusively to sex.

The new findings provide further evidence that sexual behaviours are hard-wired in animals' genes. Although the fruitless gene has not yet been found in humans, a corollary may exist. At the cellular or the genetic level, fruit flies are similar to other animals, including humans.

"It wouldn't surprise me to learn that human sexual behaviours also have, underneath them, a basic circuitry in the nervous system that mediates attraction and mating," Baker notes, adding that recent research suggests specific genes build the circuits for instinctive behaviours, ranging from sex to aggression, in mammals. But human behaviour, he cautions, is less fixed than that of fruit flies.

Adapted from a news release by Stanford University .

Further reading

Manoli DS et al. Male-specific fruitless specifies the neural substrates of Drosophila courtship behaviour. Nature, published online 15 June 2005 | doi: 10.1038/nature03859. Abstract

Bruce Baker research page