Figure 2. Sex determination and sexual differentiation of behavior.

(A) In mice the presence or absence of Sry drives the differentiation of the bipotential gonad into testes or ovaries, respectively. Sex hormones released into the circulation by the gonads act on their cognate receptors to organize the brain during development and to control the activation of sex-typical behaviors in the adult. In males, estrogen organizes the neural substrates for behavior neonatally, and both estrogen and testosterone activate these pathways for male-typical behavior in adults. In the absence of the neonatal organizational effect of estrogen, the default differentiation program of the brain is female although this hormone may be important in adolescence for maturation of the neural substrates underlying female sexual receptivity (not shown) (Bakker et al., 2002; Brock et al., 2011). Both estrogen and progesterone activate the neural circuit underlying this behavior in adult females.

(B) The sex of a fruit fly is determined in a cell autonomous manner, with the expression of sex lethal (Sxl) specifying a female differentiation program. Sex-specific splice forms of doublesex (Dsx) and fruitless direct the cell autonomous differentiation of neurons that control sex-typical behaviors. Ix, intersex, and Tra, transformer.