Sex determination involves synergistic action of SRY and SF1 on a specific Sox9 enhancer 
Sekido R, Lovell-Badge R. 
Nature. 2008 May 4. [Epub ahead of print] 

The mammalian Y chromosome acts as a dominant male determinant as a result of the action of a single gene, Sry, whose role in sex determination is to initiate testis rather than ovary development from early bipotential gonads. It does so by triggering the differentiation of Sertoli cells from supporting cell precursors, which would otherwise give follicle cells. The related autosomal gene Sox9 is also known from loss-of-function mutations in mice and humans to be essential for Sertoli cell differentiation; moreover, its abnormal expression in an XX gonad can lead to male development in the absence of Sry. These genetic data, together with the finding that Sox9 is upregulated in Sertoli cell precursors just after SRY expression begins, has led to the proposal that Sox9 could be directly regulated by SRY. However, the mechanism by which SRY action might affect Sox9 expression was not understood. Here we show that SRY binds to multiple elements within a Sox9 gonad-specific enhancer in mice, and that it does so along with steroidogenic factor 1 (SF1, encoded by the gene Nr5a1 (Sf1)), an orphan nuclear receptor. Mutation, co-transfection and sex-reversal studies all point to a feedforward, self-reinforcing pathway in which SF1 and SRY cooperatively upregulate Sox9 and then, together with SF1, SOX9 also binds to the enhancer to help maintain its own expression after that of SRY has ceased. Our results open up the field, permitting further characterization of the molecular mechanisms regulating sex determination and how they have evolved, as well as how they fail in cases of sex reversal.

Sex steroids are considered major regulators of sex change processes in fish. Estrogen depletion is shown to be crucial for female-male sex change initiation; however, its role in male-female sex change is largely unknown. In the present study, we examined the effects of estradiol-17beta (E2) treatments on testes of initial-phase (IP) males of the three-spot wrasse (Halichoeres trimaculatus), which naturally do not undergo male-female sex change. Sexually mature IP males were fed a diet containing E2 (low, 20 ug/g feed; high, 200 ug/g feed) for 6 or 12 weeks, and changes in gonadal structures were examined. Percentage of sex change varied with the dosage of E2 and the duration of treatment. All individuals treated with high-dose E2 for 6 weeks had ovaries with many immature oocytes; whereas 75% of individuals treated with low-dose of E2 for 6 weeks and sampled on the 12th week had ovaries with yolky oocytes and an ovarian cavity indicating a typical mature ovary. No testicular tissue was observed in sex-reversed gonads in both treatment groups. Contrary to the previous assumptions, present results suggest that IP male wrasses have the potential to undergo male-female sex change in response to exogenous estrogen. How the presence or absence of estrogen creates sexual plasticity in gonadal germ and somatic cells remains to be clarified.

"Sex Determination and Gametogenesis in Fish: Current Status and Future Challenges".