A mere change in temperature is all it takes for the males of a new line of genetically engineered flies to switch from being heterosexual to suddenly courting other males.
The switch occurs within minutes and is fully reversible, meaning scientists have for the first time a system they can manipulate to study how some brain regions may be involved in determining sexual orientation.
Whether the secrets unveiled by these peculiar flies will shed any light on human sexual preference is still unclear. Some of the neurons affected by the genetic changes are involved in sensing pheromones. These play an important role in fly courtship behaviour, but their influence in human sexual behaviour is unknown.
Toshihiro Kitamoto of the Beckman Research Institute of the City of Hope in California created the flies, but is reluctant to extrapolate the findings: “It’s a very sensitive issue.”
Others are more willing to express enthusiasm. Dean Hamer of the National Institutes of Health says the research should inspire other scientists hunting for genes involved in sexual orientation in humans.
There is no reason to think similar features will not be present in mammals, he says, noting that homosexual behavior is widespread among many animal species and suggests a strong genetic influence.
Hamer’s team has published two highly publicised studies suggesting there is a “gay gene” on human chromosome X, though a different group failed to reproduce their results.
But many disagree. Sexual orientation in humans is mostly a social behaviour, contends Ruth Hubbard from the Council for Responsible Genetics. “To try to find analogies in flies, who have their own agendas, is just plain silly,” she says.
Nonetheless, in flies there is no doubt that genetics plays a major determining role. A handful of genes have been identified that, when mutated, lead male flies to lose their preference for females and instead go after other males.
It has been previously suggested that some regions of the nervous system are connected to sexual orientation, but there was no way to focus on them. Now Kitamoto has genetically engineered flies so that only a subset of their neurons stopped working.
The mutation is temperature sensitive, meaning neurons carrying the mutation suddenly become inactive above 30°C. At the normal 19°C, males are heterosexual. But ramp up the heat above the critical temperature and in about two minutes their behaviour changes.
When put in a chamber with virgin females, the males become largely disinterested. Add them instead to a vial with other males and they pursue them vigorously. Flip the temperature back to normal and the flies become heterosexual again.
Kitamoto does not yet know why the flies change their preference so quickly, but says the system should become a valuable tool to define further which regions of the brain, and which genes, interact in determining sexual orientation.
Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.202489099)