Nose goes, gender bends
Knocking out pheromone sensor makes female mice act male.
Flipping one genetic switch in the brains of female mice makes them behave like sex-crazed males. The finding implies that females' brains have the same circuit that governs sexual behaviour in male mice — and that it's simple to convert one to the other.
It also goes against the prevailing idea that sex hormones, such as testosterone, are crucial regulators of sexual behaviour in animals.
Catherine Dulac and her colleagues at Harvard University genetically engineered female mice to lack a gene called TRPC2. This gene is essential for the functioning of a pheromone-sensing organ in the nose called the vomeronasal organ.
Without the gene, female mice acted exactly like males — even towards male mice — complete with mounting, pelvic thrusts and the ultrasound calls that males use to attract a mate. The results are reported in Nature1.
Left in a large semi-natural enclosure for a month, female mice lacking TRPC2 still chased males and attempted to mount them. Many of the females gave birth to pups, but they were bad at nursing them and protecting them from other mice in the enclosure.
"What's startling about the finding is that you make a tiny change and full-blown male behaviour comes jumping out," says Marc Breedlove, a neuroscientist at Michigan State University in East Lansing.
Dulac thinks it makes total sense: "Instead of building a male brain and then a female brain, you build a mouse brain, and then there's a sensory switch that makes sure that the animal behaves appropriately according to its gender."
Nor was the mounting behaviour just a show of dominance, as is sometimes seen in female lab rats, says Dulac. Mice lacking TRPC2 behaved in every way like a male trying to attract and breed with a mate.
It isn't clear how these findings might translate to other species. Many researchers think that the human vomeronasal organ is defunct, and the human TRPC2 gene is functionless.
"Different species use different sensory strategies to understand the world," says Dulac. She notes that whereas rodents use pheromones as an important trigger for sexual behaviour, primates and humans are more visual creatures.
- Kimchi, T. et al. Nature doi:10.1038/nature06089 (2007).