Fruit fly study finds chemical link to sex attraction

Male fruit flies can be induced to court other males by blocking a little-known chemical messenger system in the brain, scientists reported Monday.

The study, reported in the San Francisco Chronicle on Tuesday, is the latest evidence suggesting that even the most complex behavior patterns can sometimes be tied to basic neurobiology. But how much light it sheds on sexual orientation in humans is very much an open question. Previous studies have shown that several genes can influence mating behaviors in the Drosophila fruit fly, a popular laboratory model for gene studies.

The new research appears to be the first to identify a particular "neuronal pathway"--a collection of brain cells, receptors, and chemical transmitters in the brain--capable of eliciting male-male courtship. The study, by scientists at the City of Hope Medical Center, is published in the latest Proceedings of the National Academy of Sciences.

Lead researcher Toshihiro Kitamoto introduced a temperature-sensitive mutant gene into certain brain cells of male flies. The effect was to disrupt the flies' normal response to pheromones, chemical signals secreted by one fly to influence behavior in another.
Male fruit flies generally respond powerfully to female pheromones and rarely pay any heed to sex signals of fellow males. But the male flies with the mutant gene, called "shibire," when activated by warm temperatures, instantly switched their attentions to other males, Kitamoto found.

The modified flies formed long "courtship chains," the researchers noted, one fly vigorously courting the next in a kind of X-rated Drosophila line dance, exhibiting such behaviors as curling of the abdomen to achieve genital-genital contact, "tapping the partner's abdomen with the forelegs, unilateral wing vibration, and licking of the partner's genitalia."

Researchers said the study suggests male-male courtship is ordinarily suppressed by the activity of so-called cholinergic neurons, a system seemingly designed to make male flies responsive to "antiaphrodisiac sex hormones" to keep their sexual attentions focused on mating with females. By switching off that set of neurons, the result was "high levels of intermale interactions," the researchers found.

Scientists familiar with the new study said it should be interpreted cautiously as far as any hint of the neurobiological underpinnings of human behavior is concerned. They noted, however, that the same chemical messengers are found in flies and people. "Almost all the important signaling molecules in humans are there in fruit flies," said neurobiologist Wendi Neckameyer of St. Louis University Medical School, who also studies genetics and behavior in Drosophila.

The fundamental point, she said, is that biology and brain chemistry, along with environmental cues, are important in explaining behaviors. "What this says is, there are real neurochemical bases for behavior, which is not to say that everything is neurogenetically inherited either," she told the Chronicle. "Flies have a lot of hardwired behaviors, like reproduction, but these can be modified by environmental conditions."