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Under normal circumstances, the white gene is active only in certain cells, including brain cells, and does nothing to disrupt standard sexual behavior. In the NIH experiments, Odenwald and Zhang inserted a normal version of the gene into embryonic flies, but transplanted the gene in such a way that it was activated in every cell. That's what apparently played havoc with the flies' sex lives. With every cell sucking in tryptophan from the blood, a shortage of tryptophan developed in the brain, where it has important uses. Since tryptophan levels were altered, the researchers hypothesize, the brain was unable to make enough serotonin, one of the neurotransmitters that carry messages between nerve cells. Serotonin is a multi-purpose chemical, and abnormal levels of it in humans have been linked to everything from depression to violent behavior. In the case of the gay fruit flies, the scientists speculate, a shortfall of serotonin produced those all-male conga lines.
Though the idea seems far-fetched, it jibes with two decades of research suggesting that serotonin plays a role in regulating sexual behavior. One piece of evidence is the action of the drug Prozac, which relieves depression by lifting serotonin levels in the brain. At the same time, though, the serotonin boost tends to dampen sexual desire. In contrast, low serotonin levels can produce heightened sexual activity, at least in lab animals. In experiments done in the U.S. and Italy, scientists used drugs and special diets to suppress serotonin in rats, mice, cats and rabbits. The result was increased sex drive and, sometimes, homosexual couplings.
As intriguing as it sounds, the serotonin theory is still full of holes. Even if shortages of the chemical increase sexual activity, why would it often be homosexual rather than heterosexual? And if sexual orientation is genetically determined, then why do some identical twins differ in sexual preferences?
Getting the answers, if possible at all, will require much more research. Even harder will be knowing how to use any knowledge that emerges. Will children be given genetic tests to determine the odds of their becoming homosexual? Will prenatal tests lead to abortions of fetuses that might grow up to be gay?
Scientists caution against jumping to conclusions about the meaning of the NIH studies. To complicate the picture, some of the work shows that environment, along with genetics, influences sexual behavior. In one experiment, a small group of "straight" flies was mixed with a larger group of genetically altered "gay" flies. While the gays formed their conga lines, the straights stayed to the side -- but only temporarily. After a few hours, the straights joined in and, for the time being, acted gay.
In fruit flies, and certainly in humans, sexual orientation is just not a simple matter. And no amount of scientific research is going to change that fact of life.