Gene tells male fly how to go a-courtin'.

In pursuit of a mate, male fruit flies go through an elaborate courting ritual in which they chase, lick, and stroke a female and vibrate a wing to produce a love song. Researchers have now identified an insect gene, fruitless, that governs these and other aspects of male sexual behavior. Mutations in this one gene can turn the male fly's passion toward males as well as females, block him from singing his song of desire, or prevent successful copulation.

"To have a complex set of behaviors controlled by a single gene is quite exciting," says Steven A. Wasserman of the University of Texas Southwestern Medical Center in Dallas, who heads one of the four research teams that found and studied fruitless.

The collaboration, which also includes groups led by Bruce S. Baker of Stanford University, Jeffrey C. Hall of Brandeis University in Waltham, Mass., and Barbara J. Taylor of Oregon State University in Corvallis, reports its findings in the Dec. 13 Cell.

Interest in fruitless began in the early 1960s, when a scientist creating mutant fruit flies noticed oddities in the courting and mating behavior of some of his insects. A few of the male mutants actively courted both males and females. They also proved unable to mate successfully with females, thus earning them their name, fruitless.

Earlier this year, Japanese researchers finally unearthed part of the gene whose mutant form causes the unusual behavior of the fruitless male flies. Wasserman and his collaborators have now identified the complete normal gene, which is about 70 times larger than most other fruit fly genes. The gene is a complicated one, apparently able to generate several forms of a protein that regulates the activity of still-undetermined genes, says Baker.

When scientists created new mutations in fruitless, they found that the gene's influence extends beyond the male fly's sexual orientation and ability to copulate. Some mutations inhibit nearly all his courtship; others, though not depriving the fly of flight, check his ability to vibrate his wings for a love song. "They stick their wings out near a female but can't sing," says Hall.

The gene does more than govern sexual behavior. Some mutations in fruitless inhibit the development of a male-specific abdominal muscle. The most severe ones kill the insect, whatever its sex, very late in its pupal development. "We can't say fruitless only affects males or that it only affects behavior," cautions Hall.

Taylor's group found that fruitless functions in a mere 500 or so brain cells, less than 1 percent of the brain's total, and nowhere else in the fly. Since female flies with fruitless mutations display no obvious behavioral differences, researchers were surprised to see that the gene seems to work in both sexes. Brain cells with active fruitless genes are located in clusters, some of them in regions already implicated in aspects of courtship.

Is there a fruitless gene involved in human sexual behavior? Scientists, who have not yet found a human gene closely resembling the fly gene, tread carefully around that provocative question. "It's certainly possible there's going to be a human counterpart to fruitless. What is certain is if it's there, it won't control behavior, it will merely influence it," says Wasserman.

Ralph J. Greenspan, a fruit fly biologist at New York University, adds that it's difficult, if not impossible, to extrapolate the new insect findings to people. "Sexual behavior looks like it's pretty different, even in closely related organisms," he notes.