Fig. 1

Brain sex differentiation. The steroid hormones synthesized in the gonads act on the brains to differentiate it into male and female types. In mammals, it is also indicated that the gene expression in developing brains is sexually dimorphic prior to the action of gonadal hormones. This suggests that the early brain could signal the gonads, or it could pre-condition the brain so that testosterone (T) and estradiol (E2) act differentially in male and female brains. The brain sex differentiation process in fish species, including zebrafish is largely unknown. Unlike mammals, fish are born with a developed brain, which helps them to forage and escape predators. This could indicate that the developing fish brain signals the gonad to develop into either a testis or ovary and later the steroid hormones synthesized by the gonads further enhances the sex-specific differences in the brain