Molecular mechanisms of sex determination in birds

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Since sex determination and differentiation mechanisms remain unsolved in birds, this project was conducted to elucidate the molecular mechanisms of sexual differentiation of the chicken gonad. The testis produces anti-Mullerian hormone (AMH) and androgen, whereas the ovary produces little AMH but produces estrogen predominantly. Our in situ hybridization study demonstrated that AMH mRNA was expressed as early as day 5 of incubation followed by a sharp increase on day 6 of incubation in males, which was subsequently maintained. In contrast, the female showed minimum levels of expression. Conversely, aromatase mRNA was expressed in the gonad as early as day 6 of incubation in female but little in the male. Possible transcription factors affecting the AMH and aromatase genes include Sox 9, Sf-1, Dax-1, and Dmrt-1. Sox 9 mRNA is predominantly expressed in male gonad, although it is expressed after the expression of AMH.The Sf-1 mRNA expression is sexually dimorphic after day 7.5 of incubation and expression is higher in the female gonad than in the male gonad. In the chicken Dax-1 mRNA is expressed in the chicken gonad in both sexes. Chicken Dmrt-1 gene on the Z chromosome was expressed in the genital ridge and Wolffian duct prior to sexual differentiation and is expressed at higher levels in male than in female embryos. Furthermore, this is expressed prior to AMH expression in male. This sex- and time-specific expression makes this gene an especially good candidate to be involved in avian sex determination.