Turning off certain neurons in adult male fruit flies makes them lose interest in females and express more than a little sexual curiosity about other males, a new study shows. The work is a step toward dissecting the portions of the central nervous system that control sexual behavior.
The sexual orientation of fruit flies is apparently influenced by a handful of genes expressed in the brain. Mutating these genes, researchers have found, prompts male flies to prefer other males over females. However, because the flies carried the mutated genes from birth, the preferences researchers saw in these studies could have been due to abnormal brain development rather than an interruption of the genes' action in the adult nervous system.
To investigate how the normally developed adult brain settles on sexual preference, neuroscientist Toshihiro Kitamoto of the City of Hope Beckman Research Institute in Duarte, California, devised a way to turn off certain neurons on demand in the brains of adults. First, he engineered flies to produce a mutant version of a gene called shibire-ts in different subsets of brain cells. Shibire-ts is a protein that normally recycles neurotransmitters. The mutant protein works properly at 19°C, but at 30°C, it malfunctions and the neurotransmitters run out, silencing the neurons.
When Kitamoto turned up the heat on a strain called C309, males that had been interested in female flies suddenly--within a few minutes--started to mostly woo other males. They engaged in courting and mating behavior, such as tapping another's abdomen, licking his genitals, and thrusting of the abdomen. When Kitamoto cooled the flies back down, the previously hot-to-trot males changed their minds and walked away, he reports online this week in the Proceedings of the National Academy of Sciences.
Kitamoto says the findings suggest that certain neurons normally inhibit male-male sexual interactions and that disrupting those neurons in the adult fly is enough to uncover the suppressed behavior. He isn't sure yet exactly which neurons were shut down in the C309 flies, but additional experiments ruled out the mushroom bodies, a brain region previously suspected to be involved in courtship because it receives a lot of sensory information.
Homing in on the neurons in this way furnishes important preliminary clues about how the brain directs courtship and mating, says neurogeneticist Jeffrey Hall of Brandeis University in Waltham, Massachusetts. More importantly, he says, the work "provides a tactical approach" for dissecting sexual behavior and might ultimately help illuminate sexual behavior in other animals, including humans.