Is the same gene that gave you a large (or small) penis behind your winning (or failed) sexual behavior?

Biologists at Brown University would say yes after their investigation in fruit flies. Males are smaller and darker than females and their forelegs hair-like bristles are shorter, thicker (not to mention, of course, male genitalia).

"Doublesex" is the gene behind these body differences but, as they have proven, of behavioral differences as well. Doublesex was found to interact with the gene "fruitless" to establish the wing-shaking come-ons and flirtatious flights that mark male and female courtship. "What we found here, and what is becoming increasingly clear in the field, is that genetic interactions that influence behavior are more complex than we thought," said Michael McKeown, a biologist.

"In the case of sex-differences in flies, there isn't a simple two-track genetic system - one that shapes body and one that shapes behavior. Doublesex and fruitless act together to help regulate behavior in the context of other developmental genes."

Geneticists have been using the fruit fly as a model to see how genes build a nervous system that, in turn, controls complex behaviors. As humans and flies have thousands of genes in common, these researches could explain how genes act in complex human behavior, from aggression to alcoholism. Recently, the same team found a gene in fruit flies which is very similar and provokes the same effects as the human gene of Alzheimer (senile dementia).

The fruitless gene, active only in males, was known to be involved in controlling courtship and sexual receptivity by repressing female behavior and activating male behavior, together with other still unknown genes. The Brown team suspected that doublesex could be amongst them. Doublesex is difficult to manipulate as when both copies of the gene are removed, flies have the physical features of both sexes.

These mutant males/females are not recognized by normal males or females and none can mate, so the researchers were not able to define their behavior as gender appropriate. But flies missing one of two copies of doublesex developed intact sexual equipment. The researchers also reduced the activity of the fruitless gene as well as one called "retained."

Doublesex helped the males act like males during courting: chasing females, "sing" love songs (by wing buzz), tapping or licking their intended mates. In females, doublesex worked together with the gene "retained" to make them more receptive to this wooing.

Females with two copies of doublesex were more likely to listen to love songs and to copulate. But just one doublesex or retained turned females acting like males, even though they lack the male-behavior-inducing activity of fruitless.

Other experiments with "fruitless" and "retained" found critical overlaps in the "mind" and "body" pathways. "Retained" acts in both sexes, repressing male courting behavior and boosting female receptivity. "Fruitless" and "doublesex" act like a switch system, to affect this sexual behavior. "The big story is the crossover between the 'mind' and 'body' pathways," McKeown said. "If sexual behaviors are genetically controlled in humans, I expect that this system would be just as much, if not more, complicated."