Neurogeneticist Ward Odenwald was studying mutant drosophilas when he noticed that the males were acting out: instead of just courting females with their alluring dances and love songs, they were also courting one another. Last summer, after a year of research, Odenwald and his colleagues at the National Institutes of Health identified the mutant gene responsible for their flies’ bisexuality--a red-eye-color gene, similar to a human gene with an unknown function. The popular press has anthropomorphized what we’ve done, trying to draw a straight line between our research and human sexuality, says Odenwald. Although you can’t do that, many basic mechanisms in life have been highly conserved from day one right on up to man. By understanding molecular mechanisms in the fly, scientists are gaining new insights into similar mechanisms that occur in man.
All fruit flies have the red-eye-color gene, but generally it’s turned on only in pigment-making cells and has no bisexual side effects. By injecting his fly embryos with both the gene and a piece of DNA that would turn on the gene in response to heat, Odenwald activated it all over his flies. Not only did they turn out red-eyed, but when heated for about an hour, the males of the group formed single-sex courtship chains and circles, each fly vamping the fly ahead.
The red-eye-color gene codes for a protein that is involved in the production of both the pigment and the neurotransmitter serotonin. In rabbits and rats a low serotonin level causes homosexual mounting behavior. I think it’s more than a coincidence, says Odenwald. The next thing to ask is, are serotonin levels being changed in the fly due to misexpression of the gene? It’s a highly speculative hypothesis--but a testable one.