Homosexual behaviour may not be "solely genetically programmed" and could be affected by the environment in which one is brought up, according to a new study in fruit flies.
Researchers showed that the sexual preferences of male fruit flies with a mutant version of a gene known to affect male sexual behaviour can vary depending on whether the flies are reared in groups or alone.
The neurons that express the fruitless (fru) gene "basically govern the whole aspect of male sexual behaviour," said Daisuke Yamamoto, a professor at Tohoku University in Japan, who conducted the study with postdoctoral fellow Soh Kohatsu.
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.
Researchers wanted to analyse the role of vision in the courtship behaviour of normal and mutant fruit flies.
They 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, this behaviour was only displayed in mutant males reared in groups.
"We found that this kind of visually induced courtship behaviour in the fru mutant males was blocked by isolating them right after their emergence from the pupa," said Yamamoto.
The males reared by themselves did not react to the light spots, he said.
Yamamoto said that 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.
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 said.