The line between male and female just got blurrier. New research with mice indicates that both sexes have male gender circuitry in their brains. What makes females act like females, according to the study, is the ability to turn off this circuitry.
Animals emit pheromones to attract mates, mark territory, and warn intruders. In mice, pheromone detection occurs within a region of the brain known as the vomeronasal organ (VNO) (ScienceNOW, 19 November 2002). Previous studies indicate that the organ plays a role in sexual identity. Male fruit flies indiscriminately court both male and female flies when a particular pheromone receptor is disrupted, for example, and male mice lacking the TRPC2 calcium channel--which is critical for VNO function--court and attempt to mate with other males.
To get a better sense of VNO's role in gender differences, biologist Catherine Dulac of Harvard University genetically modified female mice to lack the TRPC2 gene. When presented with another female, these mutants behaved like male mice, mounting the females and thrusting at them with their pelvises. They even communicated with normal females the way male mice do, in a high-frequency squeak. What's more, TRPC2 mutants spent significantly less time with their pups and, when lactating, displayed little aggression toward intruding males, the team reported online 5 August in Nature. When the researchers removed the VNO from normal female mice, the mice acted just like the TRPC2 mutants.
The findings challenge widely held theories about the nature of sexual identity. Many researchers believe that hormones shape the brain's gender circuitry during development. But the current study indicates that the female brain may be wired the same way as the male brain and that some mice disable this wiring to become female. Further work may shed more light on the timing of this switch and whether males similarly mask "female" regions of their own brains, say the researchers.
"It is a very elegant study," says biologist Georgy Koentges of the University of Warwick in the U.K. "This type of genetic study is the only way to untangle the complexities in the mammalian brain and yield findings that years of behavioral neuroscience could not have gathered."
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