Researchers at the University of Illinois at Chicago have found that a mutation in a gene known as 'gender-blind' (GB), can make fruit flies homosexual, and that manipulating its activity can switch this trait on and off within hours.
David Featherstone, associate professor of biological sciences at UIC, and his colleagues found the gene interesting initially because it has the unusual ability to transport the neurotransmitter glutamate out of glial cells - cells that support and nourish nerve cells but do not fire like neurons do.
Prior studies showed that altering levels of glutamate outside cells can change the strength of nerve cell junctions, called synapses, which play a key role in human and animal behaviour.
However, the GB gene became even more interesting when post-doctoral researcher Yael Grosjean found that all male fruit flies with a mutation in their GB gene courted other males.
"It was very dramatic. The GB mutant males treated other males exactly the same way normal male flies would treat a female. They even attempted copulation," Nature quoted Featherstone, as saying.
Researchers said that other genes are known to alter sexual orientation, but most just control whether the brain develops as genetically male or female. It's not known why a male brain does male things and a female brain acts in female ways. The discovery of GB provided an opportunity to understand why males choose to mate with females.
"Based on our previous work, we reasoned that GB mutants might show homosexual behaviour because their glutamatergic synapses were altered in some way," Featherstone said.
"Homosexual courtship might be sort of an 'overreaction' to sexual stimuli," he added.
To test this, the researchers genetically altered synapse strength, independent of GB. They also gave flies drugs to alter synapse strength. As predicted, they were able to turn fly homosexuality on and off, within hours.
"It was amazing. I never thought we'd be able to do that sort of thing, because sexual orientation is supposed to be hard-wired," he said. "This fundamentally changes how we think about this behavior."
Researchers figured fly brains maintain two sensory circuits: one to trigger heterosexual behavior and one for homosexual. When GB suppresses glutamatergic synapses, the homosexual circuit is blocked.
Researchers' further study showed precisely how this happens - without GB to suppress synapse strength, the flies no longer interpreted smells the same way.
"Pheromones are powerful sexual stimuli. As it turns out, the GB mutant flies were perceiving pheromones differently. Specifically, the GB mutant males were no longer recognizing male pheromones as a repulsive stimulus," Featherstone said.
Featherstone said it might someday be possible to domesticate insects such as fruit flies and manipulate their sense of smell to turn them into useful pollinators rather than costly pests.
The study has appeared on line in Nature Neuroscience.