A CHANGE in temperature is all it takes to flip the sexuality of males of a new line of genetically engineered flies. They switch suddenly from heterosexual behaviour and start chasing other males.
The change occurs within minutes and is completely reversible, giving scientists for the first time a system they can manipulate to see which genes or brain areas determine sexual orientation 鈥� in flies at least.
Whether these peculiar insects will shed any light on human sexual preference is still unclear. While several research groups continue to hunt for elusive 鈥済ay genes鈥�, others shun such searches as pointless, arguing that social factors or subtle hormonal changes encountered in the womb might determine sexual orientation instead.
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In flies, however, there is no doubt that genetics plays a big role. A handful of genes have been identified that, when mutated, cause male flies to lose interest in females and go after other males instead. Researchers have even identified areas of the flies鈥� nervous system involved in sexual orientation. But until now nobody had found a way to manipulate those regions to study their role in detail.
This week Toshihiro Kitamoto of the Beckman Research Institute near Los Angeles reports on a way to do just that, in a paper in Proceedings of the National Academy of Sciences (DOI:10.1073/ pnas.202489099). He and his colleagues have engineered a strain of the fruit fly Drosophila melanogaster so that certain neurons stop working above a particular temperature. They did this by activating a mutated gene called shibire in specific regions of the fly brain. This shuts down synaptic communications from those neurons at temperatures above 30 掳C.
At a cool 19 掳C, the males behaved as heterosexuals. But when the thermostat was turned up to 30 掳C, their behaviour changed in about two minutes flat. Put in a chamber with virgin females, the males were not as interested. But if they were confined with other males, they pursued them vigorously. When the temperature was lowered, the flies became heterosexual again.
Kitamoto doesn鈥檛 know yet why the flies change their behaviour so quickly, but says the secret must lie within the inactivated neurons. He suspects that some of these must be involved in sensing pheromones, which play a big role in courtship behaviour. He says the temperature-based system should become a valuable tool in pinpointing exactly which regions of the brain and which genes govern sexual orientation.
Kitamoto is cautious about extrapolating the findings to other organisms. 鈥淚t鈥檚 a very sensitive issue,鈥� he says. But others are more keen to speculate about the implications of the work. Researcher Dean Hamer of the National Institutes of Health in Maryland says the work should be a boost for scientists hunting for genes involved in human sexual orientation. He adds that flies and mammals may not be as different as we think in this respect.
Hamer鈥檚 team has published two high-profile studies suggesting there is a 鈥済ay gene鈥� on the human X chromosome, although a different group failed to confirm this later on. Hamer adds that homosexual behaviour is widespread among many animal species, suggesting a strong genetic influence for the trait. 鈥淭he emerging evidence is that there is in fact a genetic basis for complex behaviours like sexual orientation,鈥� says Hamer.
But many still disagree. Ruth Hubbard from the Council for Responsible Genetics contends that even though human sexuality may have some biological components, it鈥檚 also a social behaviour. 鈥淎nd trying to find analogies in flies, who have their own agendas, is just plain silly,鈥� she says.