It's A Unisex Brain With Specific Signals That Trigger 'Male' Behavior, At Least In Flies

By artificially triggering the neurons responsible for singing--normally a male only activity--the researchers made female flies play their first tune, as reported in the April 18th issue of the journal Cell, a Cell Press publication.

" You might expect that the brains of the two sexes would be built differently, but that does not seem to be the case," said Gero Miesenböck, formerly of Yale University and now at the University of Oxford. "Instead, it appears there is a largely bisexual or 'unisex brain' with a few critical switches that make the difference between male and female behavior."

Male flies have to work hard to convince females to mate with them, he explained. They do it by showing their talents--sticking out one wing and vibrating it to produce sound. Earlier studies had identified the neurons responsible for the male singing behavior. But it seemed that females had that circuit too, even though they don't sing.

" The mystery at the root of our study is the neuronal basis of differences in male and female behavior," said Miesenböck. "Anatomically, the differences are subtle. How is it that the neural equipment is so similar, but the sexes behave so differently?"

To answer that question in the flies, the researchers used a special technique they developed in which the singing circuit could be turned on in either males or females with a simple flash of light. First, they confirmed the connection between that circuit and the courting behavior in males.

When they triggered the same circuit in members of the opposite sex, the otherwise quiet female flies immediately began acting like males. "They just stuck out their wing and started singing," Miesenböck said. However, the rookie females did sing off key.

" If you look carefully, the females do sound different," he said. "They have a different pitch and rhythm and aren't as well controlled." He thinks those distinctions probably stem from real, if subtle, differences between the male and female brains, not from a lack of practice.

Nevertheless, the findings in flies suggest that the circuits for maleness are present but dormant in females.

In an earlier study in mice, other researchers found that females took on masculine behaviors when a particular pheromonal cue was blocked, suggesting that male behavior is actively repressed in the rodents.

" In flies, you don't see a spontaneous emergence of male behavior when you block pheromonal cues," he said. "Rather, it requires an artificial trigger. Female flies have the program, but they seem to lack the activating command. Either way, the principle is the same [in flies and mice]: males and females are not as different as you might think."

The new findings suggest that flies must harbor key nodes or "master switches" that set the whole system to the male or female mode, according to the researchers. Their next goal is to find those controls.

The researchers include J. Dylan Clyne, Yale University School of Medicine, New Haven, CT; and Gero Miesenbock, Yale University School of Medicine, New Haven, CT, University of Oxford, Oxford, UK.