Gay fruit flies? Really?
Yes really. In fruit flies, there is a gene (the fru gene) that influences the mating behaviours of males. Flies that are genetically engineered to have a faulty fru gene, show no interest in mating with females. Normal male fruit flies tap the abdomen of a female to get a whiff of her sex pheromones before pursuing her to mate. In contrast, males with a mutant version of the fru gene show no interest in females; instead, they set off in vigorous pursuit of other males.
The neurons that express the fruitless (fru) gene “basically govern the whole aspect of male sexual behavior,” explains neurogenetics professor Daisuke Yamamoto, who conducted the study with postdoctoral fellow Soh Kohatsu at Tohoku University.
He optically stimulated neurons in a region of the fruit fly brain known to control courtship decision-making. The fruit flies were shown spots of white light flashing across a screen that represented walking females.
Normal fruit flies courted the spots only after priming with pheromones, but mutant males did not need pheromone priming or direct brain stimulation. The mutant fruit flies immediately followed the moving light spots and vibrated their wings in courtship.
However, only mutant males reared in groups displayed this behaviour. “We found that this kind of visually induced courtship behavior in the fru mutant males was blocked by isolating them right after their emergence from the pupa,” says Yamamoto. The males reared by themselves did not react to the light spots, he says.
Two Tohoku University scientists have discovered that homosexual behavior in certain groups of male fruit flies can be altered by their environment. Specifically, they have shown that the sexual preferences of male fruit flies with a mutant version of the fru gene can vary depending on whether the flies are reared in groups or alone.
Yamamoto admits he was “terribly surprised” by the results, because he had previously never doubted that male-to-male courtship in fru mutant males was “solely genetically programmed”. It appears that social interaction activates neurons that make mutant males hypersensitive to visual stimuli, overcoming their genetic predisposition to homosexuality.
While Yamamoto is cautious about drawing conclusions on human sexual orientation from studies of fruit flies, he believes some aspects of sexual orientation in humans could have a similar mechanistic basis to that of flies. “Our study offers a conceptual basis to explain how nature and nurture interact in shaping human sexual orientation,” he says.