New research shows that genes exert powerful effect on sexual behavior

New research has shown that the manipulation of a single gene in female fruit flies can make their sexual behavior resemble that of males, in a study that demonstrates the power of individual genes and the profound impact of genetics on complex sexual behavior.

The findings were published today in the journal Nature by scientists from Oregon State University, Stanford University and Brandeis University.

The research was done with the gene "fruitless," which is present in both male and female fruit flies and some other insect species. Ordinarily, only in males does this gene result in the creation of proteins that guide male sexual behavior patterns - such as approaching females, tapping them, singing to them and performing little courtship dances.

However, through genetic manipulations the research group of university scientists was able to cause these same proteins to be produced in females, and when they were, the females showed classic patterns of male sexual behavior.

"When this genetic process was triggered in females, they acted as if they were masculinized," said Barbara Taylor, a professor of zoology at OSU. "And this was a single gene expressed in just a very small number of cells, controlling a surprisingly complex behavior. In a physical sense the females looked perfectly normal, but they acted like males and, if they were physically able to, I would not be surprised if they would have attempted to mate other females."

In related fashion, the researchers found, male fruit flies that had the "fruitless" gene inactivated failed to show normal male sexual behavior. But most of their other non-sexual behaviors, such as locomotion, flight or grooming, were unaffected.

According to Taylor, what's becoming clear is that genetic mechanisms set the stage for complex neuronal development that ultimately affect behavior.

"The only cells directly affected by the 'fruitless' gene are in the nervous system, in all other aspects of development there are no apparent physical differences caused by this gene," Taylor said. "In normal male fruit flies, the activation of the fruitless gene leads to neural connections that ultimately manifest themselves as complicated behavior, in this case relating to male sexuality." "In normal female fruit flies, that process doesn't take place," Taylor said. "But if you activate the fruitless gene in females, they behave as though they were males, showing that the same type of developmental connections and neural linkages begin to form as in males."

The research is somewhat surprising, the scientists said, because it shows how a single gene, which in this case they believe also controls the actions of other genes, can have such far reaching effects on complex behaviors, and be so focused in what in controls. The common fruit fly, Drosophila melanogaster, has about 13,000 genes in its complete genome.

Genetic mechanisms to build the nervous system may be especially important in fruit flies - most of the behaviors they exhibit are largely programmed by birth, and they are ready to mate within 24 hours after emerging from their pupa. But other more advanced animal species, Taylor said, may combine developmental experiences and social interactions with the still-powerful ability of single genes or small groups of genes to influence complex behaviors - ranging from a bird singing its song to the ability of a human to play a piano.

The study also suggests, Taylor said, that there is a strong biological basis for sexual behavior and orientation, clearly in fruit flies and almost certainly in most other animal species.

Humans have not been shown to have the "fruitless" gene, but they actually do have other genes in common with fruit flies, which have been preserved through millions of years of separate evolution. Humans and fruit flies, for instance, share the "doublesex" gene that controls the development of testicles, Taylor said. There are also a number of genes involved in immune system function that are the same in flies and humans.

"Research of this type is telling us quite a bit about the ways in which things we believe are developmental actually have a biological and genetic underpinning, and how the development of our nervous systems are affected by these genes," Taylor said.