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Changing one gene swaps fly sex roles, scientists report

The gene leads to the production of a molecule called Fruitless, a protein. Different molecules of this sort can act in the brain or nervous system to promote different types of behavior.

Male and female flies have different versions of the fruitless gene. The male form is critical for the male courtship ritual and males’ preference for mating with females, previous studies have shown. 

In the new paper, Barry J. Dickson and Ebru Demir of the Institute of Molecular Biotechnology of the Austrian Academy of Sciences said they showed just how intimately the gene is linked to these stereotypically male behaviors. They found that female flies with the male version of fruitless behave like males, directing at other females a sexual display nearly identical to their male counterparts. 

Female flies with the male version of the protein also make amorous advances toward male flies that produce female pheromones, types of hormones that promote sexual attraction. 

“We have been able to reverse the sex roles” in the courtship of these flies, Dickson and Demir said. 

Dickson and Demir created male versions of fruitless in female flies and female versions in male flies. Males with the female version barely court at all, according to the researchers. 

Males with the female molecule were also more likely to court other males than flies with the male form, suggesting that male-specific fruitless “not only promotes male-female courtship, it also inhibits male-male courtship,” the researchers said. 

Dickson and Demir call fruitless a “switch gene” that is both necessary and sufficient to produce a behavior. Switch genes that trigger the development of a particular anatomical feature like wing structure have been studied extensively, but there are very few studies of switch genes that control a complex behavior, the researchers noted. 

In part, this is because finding behavioral switch genes can be difficult. The key, said Dickson, is showing that a specific gene is sufficient to produce a particular behavior. “This means showing that gene X is sufficient to create the potential for behavior Y in an otherwise normal animal. It is the ‘otherwise normal’ part that is tricky,” he said. 

“Putting gene X into another species and expecting to see a behavior is unrealistic,” Dixon remarked. “A ‘flight’ gene from Drosophila [the fruit fly], if it existed, is not going to make a mouse fly.” He noted that only members of the same species would be expected to share the same set of “normal” behaviors. 

“So you need to put gene X in a normal animal of the same species that doesn’t normally do Y. This is really only possible with sex-specific behaviors” like courtship, he said.

There is “something of a debate going on,” he added, “between the view that single genes can have profound effects on behavior, versus the more holistic view that behavior is so complex that we can never learn anything meaningful about a behavior by studying the action of a single gene.”

Still, studies show a single gene can trigger the development of complex anatomical structures like eyes or limbs, by influencing sometimes hundreds of other genes, Dickson notes.

“I don’t see any good reason why innate behaviors, which are a consequence of how the nervous system is built, should be any different. Indeed, I think that is what our work shows,” he said.

Dickson and colleagues have already begun collaborations with other researchers to find out how the fruitless gene might be involved in other behavioral patterns like aggression. “I think it is going to be fascinating to try to figure out how a fly decides between ‘love’ and ‘war’,” and what fruitless may have to do with this, he said.

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