Flipping the switch on a single gene may be enough to turn a coy female fruit fly into a crooning Casanova, according to a new study.

Researchers found that altering a single gene in female fruit flies caused their sexual behavior to change and resemble that of males.

''In these experiments we see all the steps of the male courtship ritual you could physically expect a female fly to do,'' says researcher Bruce S. Baker, professor of biology at Stanford University, in a news release. ''It's a male's behavioral circuitry in a female body.''

Researchers say the results suggest that sexual behaviors that seemingly develop over time, like flirting and courtship rituals in flies and potentially in humans, may also have biological and genetic underpinnings.

''It wouldn't surprise me to learn that human sexual behaviors also have, underneath them, a basic circuitry in the nervous system that mediates attraction and mating,'' says Baker.

Read WebMD's "Is There a 'Gay Gene'?"

Sex in a Single Gene

In the study, which appears in the June 15 online issue ofNature, researchers looked at the effects of altering the gene known as "fruitless" on sexual behavior in fruit flies.

The proteins produced by the fruitless gene are made in all the sensory systems, which are involved in courtship. In the olfactory (sense of smell) system the gene is involved in the detection of pheromones - chemicals that can change courtship behavior, they write.

Both male and female fruit flies possess the fruitless gene, but researchers say that only in males does this gene result in the creation of proteins that influence male sexual behaviors. In male fruit flies, that means approaching females, tapping them, singing to them, and performing courtship dances.

Researchers manipulated this gene in a group of female fruit flies to trigger the production of these same proteins and found when the proteins were present, the females showed male patterns of sexual behavior.

"When this genetic process was triggered in females, they acted as if they were masculinized," says researcher Barbara Taylor, professor of zoology at Oregon State University, in the release. "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."

Similarly, when male fruit flies had their fruitless gene inactivated, they failed to show normal male sexual behavior, but other behaviors were unaffected.

Researchers say that although the fruitless gene has not been found in humans, there may be a similar gene because fruit flies share many other genes with humans.

They say the results are surprising because they show that a single gene can have far-reaching effects on complex behaviors.

Read WebMD's "Good Behavior May Start in the Genes"

By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCES: Manoli, D. Nature, June 15, 2005, online edition. News release, Oregon State University.