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go to a specific kind of rotting fruit or other plant matter to lay their eggs. When they are there, the males detect the presence of a potential mate by smelling the surface chemicals on the female's body, and then pursue the object of their affection, performing a stereotyped series of movements, attempting to lick the female, and vibrating their wings to produce a song that is audible to human ears only if it is greatly amplified. The details of the movements and the song vary among species, and males differ in how vigorously they perform the actions and in the way in which their advances are received. Often the female walks away or lashes out with her legs in response. If a male is successful, the female stops long enough to allow him to mount her and transfer his sperm. Both in the laboratory and in the wild, males will also attempt to court other males, particularly younger ones that have just become adult.

One can, of course, study the flies simply by observing their behavior using a magnifying glass to get at some of the finer details, but for many years scientists have been using extremely sophisticated genetic technology to understand exactly which genes control which aspect of the mating ritual, and how they interact. It is now possible to produce knockout strains of the flies, which lack a particular gene but are otherwise like normal, or wild-type, as they are called, Drosophila. Alternatively, scientists can manipulate individual genes so that they are still present but are inactivated; genes can also be inserted into places they wouldn't normally occur.

One of the most important genes regulating sexual behavior in the flies is called fruitless (many genes in model organisms have special names, some of which are quite fanciful, for example, sonic hedgehog). Flies with one kind of mutation in this gene will try to court females, but they do so incorrectly. It's still not clear where their problem lies, but it may be that they fail to fully extend their wings to sing, a deal breaker from the female's perspective. This defect applies only to courtship—the mutants can fly normally and can flick their wings dismissively when rejecting advances made by another male. Flies with other types of mutations of the fruitless gene court both males and females. When several of the mutant males are placed in a Petri dish, they form male-male courtship chains in which each male is simultaneously both courting and being courted. Female flies with the altered fruitless gene will court other females with the same stereotyped set of movements ordinarily used by males.

The fruitless gene affects many different parts of the fly brain, each of which is important in regulating sexual behavior. A Japanese researcher, Ken-Ichi Kimura, meticulously dissected the brains of Drosophila that did and did not have the mutation in fruitless. He and his coworkers found that a just a handful of nerve cells in the wild-type males are absent in the males with the mutation. In the females that court other females, the cluster is also present, although normal females lack it.

So is fruitless "the gay gene," or do the nerve cells themselves keep flies from being gay? Not so fast. Kimura and his colleagues also worked with mutations on another gene, called doublesex. They found that a nerve cell group that is affected by mutations on both of the genes simultaneously can turn on courtship behavior in females. Ordinarily, this cell cluster dies in females because of a feminizing protein in the brain, but if fruitless is present, the cluster survives. Both of the genes are needed to ensure that males court females and females don't.

Then are both genes "gay genes"? Once again, no. Just having the genes that control the courtship behavior itself isn't enough. A male also needs to distinguish that a female is out there in the first place, which means processing the sight, smell, and maybe sound of another fly, and yet more genes seem to be involved in that process. The male flies' behavior is triggered by pheromones, or odors that are emitted by the