BioEd Online

Researchers have finally pinned down a physical difference between male flies that are engineered to behave homosexually and those that are not: the tweaked variety is missing a small cluster of nerve cells in the brain.

Genetically altered flies that are designed to court members of their own sex, or no one at all, have made headlines in recent months (see ' Fruitflies tap in to their gay side '). But no one knew exactly what those genes were doing, or how the flies differed physically from heterosexual ones. Now Japanese researchers have pinpointed one difference in the brain.

Scientists caution that fly mating behaviour is very different from that of humans, as are our brains, so these results cannot be extrapolated to people. "No homologue of the fruitless gene is found in mammals and humans," points out Ken-Ichi Kimura of the Hokkaido University of Education in Iwamizawa, Japan.

But such work does help researchers to work out the complex genetic and environmental factors that help animals to choose their mates. "This finding will provide insight for understanding how a sexual behaviour is constructed in the circuitry of the brain through a function of single gene," adds Kimura.

The straight story

The sexual behaviour of insects has been tweaked before by altering a gene called fruitless. Male fruitflies (Drosophila melanogaster) that have mutations in this gene often fail to make a protein called Fru. These flies do not engage in the normal mating ritual of tapping and tilting movements that attract females.

If male flies have a milder mutation in the fruitless gene, resulting in production of small amounts Fru protein, they court male and female flies alike.

On the flipside, females engineered to carry the male-specific fruitless gene suddenly begin wooing other females with the tapping and tilting routine.

Although these behavioural switches intrigued researchers, any anatomical difference in the flies remained elusive.

Sex on the brain

To investigate the effect of the Fru protein, Kimura and his colleagues dissected the brains of Drosophila with and without the fruitless mutation.

They found that normal males possess a cluster of neurons in the brains that female flies lack. Further investigation revealed that the Fru protein keeps these cells alive during the early development of males.

The Japanese team show that homosexual females designed to produce the Fru protein develop an intact nerve-cell cluster. And males with mutations in the fruitless gene lack these nerve cells. The results of the study, which appear in Nature, suggest that the presence of this network in the insects' brains determines which partners they woo¹.

"The idea that differences in a highly complex behaviour such as courtship can be caused by a small number of cells is very interesting," says Toshihiro Kitamoto, a researcher at the University of Iowa in Iowa City who has studied same-sex courtship in flies. Kimura and his co-authors speculate that the nerve network controlled by the Fru protein relays information about chemical cues that allow flies to recognize the sex of a potential mate.

But experts caution that there could be many other, subtle changes in the brain and elsewhere that together account for the behavioural differences. "The much bigger and far more interesting question, I think, is whether it is this difference that matters," says Barry Dickson, a researcher at the Austrian Academy of Sciences in Vienna, who has worked with the fruitless mutation in flies.