New Haven, CT, USA. Human males and females often behave in markedly different ways, due in part to significant differences between their brains. The findings from experiments with model organisms have led to a wide variety of confirmation studies involving humans that confirm and refine the differences. It seems reasonable to expect that the brains of the two sexes have a fundamentally different architecture. But is that the case or do the differing brains exploit common structures?

 

It is well-established that females are the default sex for humans, with males the exception — a common pattern in nature and a development found fortuitous in many men and women. How we get to (the general condition of) sexual dimorphism is a matter under intense investigation by scientists. Having passed beyond simple social analysis, scientists have turned their attention to the underlying mechanisms.

 

Sex-Specific Control and Tuning of the Pattern Generator for Courtship Song in Drosophila. J. Dylan Clyne and Gero Miesenböck. Cell, Vol 133, 354-363, 18 April 2008.

 

 

 

Nature favors the principle of parsimony, a preference for the least complex explanation for an observation. Just as the same foundation for a house can accomodate either a living room or a kitchen, the basic architecture and building blocks of the human brain can mostly accomodate either female or male elaboration. Females provide the basic

 

A new study suggests that males and females brains indeed are equipped with some structures in common. One of the key model organisms for researchers is Drosophila. By artificially triggering the neurons responsible for singing — normally a male only activity — the researchers made female flies play their first tune. The findings appear in the journal Cell.

 

Gero Miesenböck says there appears to be a "… a few critical switches that make the difference between male and female behavior." Miesenböck, now with the University of Oxford, was formerly associated with Yale University.

 

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. They used  light-activated ion channels to stimulate the appropriate neurons (located in the flies' thoracic-abdominal ganglia). This enabled direct comparisons of comparable circuitry between sexes.

 

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.

 

The findings in flies suggest that dormant circuits in the females are available for activation in males. 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." Moreover, the artificial stimulation of the female neurons expressed did not result in identical male behaviors but an approximation. The investigation demonstrated the general mechanism but did not thoroughly define the specifics.

 

The differences between sexes can be crucial for socialization and survival. Further investigation is needed to determine if more behaviors can be altered on the neuronal level to better assess how the underlying mechanisms actually work in complex behaviors.

 

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. The researchers have set their next goal to find those controls.

 

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

 

Sex-Specific Control and Tuning of the Pattern Generator for Courtship Song in Drosophila. J. Dylan Clyne and Gero Miesenböck. Cell, Vol 133, 354-363, 18 April 2008.

Summary. The differentially spliced transcription factors encoded by the fruitless (fru) gene are key determinants of sexual behavior in Drosophila. They are expressed in a minority of neurons with limited dimorphisms and regulate neural processes that remain largely unknown. Here, we use light-activated ion channels to stimulate fru-expressing neurons in the thoracic-abdominal ganglia, enabling direct functional comparisons of homologous circuitry between sexes. Optical stimulation of males or females initiates the unilateral wing vibrations that normally generate the male courtship song. The pattern-generating circuit operates differently in the two sexes, producing wing movement and sound in both but authentic songs only in males and in females expressing male fru product. A song-like motor program is thus present in females but lies dormant because the neural commands required for song initiation are absent. Supplying such commands artificially reveals fru-specific differences in the internal dynamics of the song generator and sets the stage for exploring their physiological basis.

 

Introduction (excerpt). The males and females of most animal species exhibit profound differences in behavior. Male fruit flies, for instance, court females with a ritual whose central element is a unilateral wing vibration (Sturtevant, 1915; Bastock and Manning, 1955) that produces near-field sound in two modes (reviewed by Tauber and Eberl, 2003). One mode, sine song, is an 140–170 Hz hum (von Schilcher, 1976b), whereas the other mode, pulse song, consists of brief, repetitive amplitude modulations of an 150–300 Hz carrier wave (Shorey, 1962; Bennet-Clark and Ewing, 1967; Ewing and Bennet-Clark, 1968). Only male flies sing, and only females respond to song by allowing copulation.

 

These sex-specific differences in behavior are considered instinctive or innate (Bastock, 1956; Baker et al., 2001); they are thought to reflect genetically determined variation in the structure and/or function of the underlying neural circuits. In Drosophila, the expression of many aspects of male or female reproductive behavior results from the action of a single regulatory
gene termed fruitless (fru) (Baker et al., 2001; Demir and Dickson, 2005).

 

 

Name

Email

Title

Comment

smaller | bigger

I have read and agree to the Terms of Use.

Write the displayed characters

Add Comment