A single gene is responsible for mating actions and sexual orientation in Drosophila, that bountiful font of genetic research, the common fruit fly.
A pair of articles in the June 3 issue of the journal Cell powerfully reinforce the case for the innate biological nature of sexual orientation. After all, distant mothers, weak fathers, 'recruitment,' and embracing Jesus are not major factors in shaping the 'impressionable' fly pupa.
Barry J. Dickson and colleagues at the Institute of Molecular Biotechnology of the Austrian Academy of Sciences in Vienna focused on the 'fruitless' gene ( fru ) , which comes in four variations. They hypothesized that a switch gene for sexual behavior should act to specify either male or female behavior, irrespective of the physical sexual characteristics of the fly.
Through a complex series of gene splicing and breeding they generated lines of fruit flies that carried one of the four distinct variants of fru, and then they observed their behavior.
They found that the male flies with the female variant of the fru gene were significantly less likely to court and breed with females than were the males with the other variants of the gene.
They were also more likely to engage in courtship behavior with other males.
Among female flies, those with the male version of the gene were significantly less likely to mate with 'normal' male flies, while more than 40% of them engaged in male courtship behavior—a ritualistic series of wing beatings and body lickings prior to copulation—with 'normal' females.
The researchers carefully examined the bodies of the fly lines and could discern no physical changes that might explain these actions. The flies in every way looked like males and females of the species should look, but they did not behave the same as their sexual features would indicate.
The second paper looked at neurons in the fly brain that have been associated with the fru gene. It said, 'We could not detect any major anatomical differences in this circuit that might explain the different sexual behaviors of males and females, suggesting that the essential difference between the sexes lies in the functioning of this circuit, not its construction.'
It also showed how male and female specific pheromones—a chemical signaling system that parallels the sense of smell, is wired to the most primitive portion of the brain, and regulates sexual attraction—can be affected by male and female fru genetic variants.
Dickson says, 'We have shown that a single gene in the fruit fly is sufficient to determine all aspects of the flies' sexual orientation and behavior.' Their findings demonstrate that instinctive behavior can be regulated by the same genetic forces that regulate height or eye color.
'The notion of a behavioral switch gene implies that at least some instinctive behaviors develop according to the familiar genetic logic of morphological [ body ] development.' It does not deny the role of other genes and the environment in strengthening or weakening the influence of that gene in developing patterns of behavior.