Fruit flies are among the most sexually proficient creatures on earth. Their ability to produce a new generation in two weeks has made them the darlings of genetics researchers for nearly a century. Put a male fruit fly into a bottle with a female, and he doesn't waste any time before getting down to business.
So it's a bit bewildering to watch the behavior of certain fruit flies at the National Institutes of Health in Bethesda, Maryland. There, in the laboratories of biologists Ward Odenwald and Shang-Ding Zhang, strange things are happening inside the gallon-size culture jars. In some experiments, the female flies are cowering in groups at the top and bottom of the jars. The males, meanwhile, are having a party--no, an orgy -- among themselves.
With a frenzy usually reserved for chasing females, the males link up end-to-end in big circles or in long, winding rows that look like winged conga lines. As the buzz of the characteristic fruit fly "love song" fills the air, the males repeatedly lurch forward and rub genitals with the next ones in line.
What's going on?
Without a wink or a chuckle, Odenwald claims that these male fruit flies are gay -- and that he and Zhang made them that way. The scientists say they transplanted a single gene into the flies that caused them to display homosexual behavior. And that's very interesting, they assert, because a related gene exists in human beings, although there is no evidence yet that the human gene has an effect on sexual preference.
A report of Odenwald and Zhang's findings, to be published this week in Proceedings of the National Academy of Sciences, adds to the mounting evidence that homosexuality has genetic origins, and is sure to produce new fireworks in the contentious debate over what it means to be gay. The two scientists are not foolhardy enough to claim that a single gene can make a person homosexual. But they think their studies may yield important new insights into how genetic makeup, through a complex series of biochemical reactions, influences sexual orientation.
Such work stirs mixed emotions in the gay community. To some extent, gays and lesbians welcome the research because it supports what most of them have long felt: that homosexuality is an innate characteristic, like skin color, rather than a perverse life-style choice, as conservative moralists contend. And if that is true, then gays deserve legal protection similar to the laws that prohibit racial discrimination. "On a political level, genetic research does seem to move the debate along a certain path," says Denny Lee of the Lambda Legal Defense and Education Fund, a gay advocacy group in New York City. "When people understand that being gay or lesbian is an integral characteristic, they are more open-minded about equality for gay Americans."
On the other hand, many gays are wary of the genetic hypothesis. It could, they fear, help promote the notion that gayness is a "defect" in need of "fixing." "Any finding will be used and twisted for homophobic purposes," says Martin Duberman, head of the Center for Lesbian and Gay Studies at the City University of New York. "If it does turn out that for some people, there is a genetic or hormonal component, the cry will then arise to take care of that." Indeed, the cry is already rising.
The Rev. Louis P. Sheldon, president of the Traditional Values Coalition in Anaheim, California, says that if a biological cause of homosexuality is found, then "we would have to come up with some reparative therapy to correct that genetic defect."
No matter how people feel about the issue, it is increasingly hard to argue that genes play no role in homosexuality. The evidence began to pile up in 1991, when studies showed that identical twins were more likely to have the same sexual orientation than other pairs of siblings. That same year, a California scientist reported slight brain differences between gay and straight men, although the conclusion is disputed. And in 1993, an NIH researcher found a stretch of DNA on the X chromosome that seemed to harbor one or more genes affecting sexual orientation. But no one has proved that a particular gene promotes gayness or has offered any convincing theory of how genes could influence a person's choice of sleeping partners.
Odenwald and Zhang do not pretend to have any easy answers. In fact the type of gene they've been studying in fruit flies could not begin to account for the complex variations in human homosexual behavior. For one thing, the gene does not cause flies to renounce heterosexuality altogether. If a "gay" fly is surrounded by females instead of males, he'll fertilize the lady flies. So strictly speaking, the NIH flies are not homosexual but bisexual. And the gene produces no unusual behavior when transplanted into females: the scientists have produced no lesbian fruit flies.
Yet the way the gene works is intriguing, and may offer some clues to the biochemical roots of gayness. Surprisingly, the swatch of DNA in question was discovered long ago, and is one of the most thoroughly studied of all fruit-fly genes. It is called the "white" gene because, among many effects, it influences eye color, and a particular mutation in the gene causes a fly's normally red eyes to be white.
The gene's specific job is to produce a protein that enables cells to utilize an essential amino acid called tryptophan. If fruit flies are unable to process tryptophan properly, then they cannot manufacture red eye pigment.
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.
