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Researchers Induce Homosexual Behavior in Male Fruit Flies

June 4, 1995

WASHINGTON (AP) _ 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 may also 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 _ at least for awhile.

And to further complicate the picture, transplanting the same gene into female fruit flies did not produce ``lesbians.″

The study, reported in the June issue of the Proceedings of the National Academy of Sciences, was released Sunday.

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,″ the researchers wrote.

The gene that the scientists transplanted is called the ``white″ gene because, among its many influences, it can cause the flies’ normally red eyes to be white. The white gene produces a protein that enables cells to use tryptophan, an amino acid. Fruit flies that cannot process tryptophan properly don’t produce red eye pigment.

The researchers conceded that they don’t understand exactly what mechanism caused the homosexual sexual behavior among the genetically altered male flies. They theorized that the shortage of tryptophan in fly brains may lead to reduced amounts of serotonin, a neurotransmitter that carries messages between brain 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 has also been shown to induce male homosexual activity, they noted.

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