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female. Females that have already mated, and hence are more likely to reject the courtship advances of subsequent males, produce a different odor than virgin females or other males. But the male also needs a gene to enable him to detect those chemicals. Flies don't have separate taste and smell organs; they detect both with sensory cells on their feet (which is why they often walk on things before deciding whether they are food, and why they can spread germs so easily), and work by researchers at Duke University published in 2008 showed that a pheromone detection gene was critical to the mating game. This one is unimaginatively dubbed Gr68a, and males with a mutation in it will court already-mated females as well as other males, and they will even go beyond the tapping and singing behavior to try and actually mate with the males. What is more, the signals that it receives bypass the rest of the nervous system and go directly to the brain. Such an express route is unusual for a smell or taste receptor, which underlines the essential nature of Gr68a to the mating process.

Other flies that exhibit male-male courtship have alterations in genes called dissatisfaction, prospero, and quick-to-court. What's more, the neurochemical dopamine, which is important in a wide variety of physiological activities, including learning, movement, and the brain's processing of painful or pleasurable stimuli, also turns out to feature in same-sex courtship in Drosophila. Dopamine is found in many animals, both vertebrate and invertebrate, including humans, but if you increase it in the flies, males are more likely to court other males, although they don't change how they react to virgin females or to odor cues in general. And if news about dopamine alone leaves you cold, further research in this area demonstrated that when flies genetically altered to be unable to release dopamine at normal temperature were exposed to ethanol, the type of alcohol in beer, vodka, and other adult beverages, they too exhibited same-sex courtship. The male-male courtship became more pronounced with repeated exposure to the alcohol; the experimental arena where the scientists placed the flies was quickly named the "Flypub," and the inevitable news coverage trumpeted, "Fruit Flies Prove That Alcohol Makes People Gay."

Better Sex through Chemistry

MOST of this research used flies that exhibited the altered behavior permanently. But one of the most exciting new developments in the genetics of Drosophila sexual behavior showed that the tendency to court males or females could be switched on or off within minutes.

Dave Featherstone at the University of Illinois in Chicago said in an email to me that he was envious of what I do, because he "got into biology because I imagined myself traveling all over the world living in the wilds watching animals. Somehow I ended up studying bizarre minutia in a lab. I might as well be an accountant." As someone who does watch animals in nature for a living, at least some of the time, I was flattered by his comment, but his modesty underplays the significance of his work, which is hardly bizarre or trivial. Featherstone is interested in how the cells of the nervous system send and receive messages, particularly across the gaps between them, called synapses. His laboratory focuses on a nervous system chemical called glutamate, which his website describes as follows: "Glutamate is the voice by which brain cells speak to each other. Glutamate receptors are the ears by which they hear."

Information—whether about sex, food, or anything else—does not simply slosh from one nerve cell to another, making its way haphazardly to the brain. Instead, the receptors regulate which memories are retained, which behaviors are executed, and which signals are recognized as important. So Featherstone studies glutamate and its role in brain messages using Drosophila, which show many of the same patterns of glutamate use as humans but are obviously much easier to manipulate.

For the work on the chemical courtship switch, Featherstone and his colleagues