By transplanting a single gene into male fruit flies, researchers have been able to induce homosexual behavior - adding to the growing body of evidence that there may be a genetic component to sexual orientation.
But environment also may influence sexual orientation, at least among male fruit flies. In a related experiment, the researchers found that when a group of “heterosexual” male fruit flies was mixed with a larger group of genetically altered “homosexual” male fruit flies, the straights began to act gay.
To complicate the picture, transplanting the same gene into female fruit flies did not produce “lesbians.”
Communication among courting fruit flies involves an elaborate repertoire of gender-specific activities, many of which sound like giant fly orgies. The researchers - Shang-Ding Zhang and Ward F. Odenwald at the National Institutes of Health - found that male flies with the transplanted gene formed courtship chains of five or more individuals, “none of which displayed courtship-repelling signals (wing flicking, face kicking and/or running away).”
When female fruit flies were added to the mix, male suitors “rarely abandoned their partners to court nearby females,” researchers wrote.
The main purpose of the transplanted gene - which also can cause fruit flies’ eyes to be white instead of red in a mutated version - is to produce a protein that enables cells to use an amino acid called tryptophan. The gene normally is active only in certain cells, including brain cells, and has nothing to do with sexual behavior.
But in the experiment, the scientists inserted a normal version of the gene into embryonic flies in such a way that a copy of the gene was activated in every cell of the flies’ bodies.
That apparently produced something of an overload effect, with every cell in an affected fly’s body extracting tryptophan from the blood. Scientists theorize that caused a shortage of tryptophan in the brain with a resulting decline in serotonin, one of the neuro-transmitters that carry messages between nerve cells.
Depletion of tryptophan in rats and rabbits has been shown to lower serotonin levels and trigger male sexual mounting. And lowering serotonin levels in cats also has been shown to induce male homosexual activity, they noted.
The study, reported in the June issue of the Proceedings of the National Academy of Sciences, was released Sunday.