DMRT1 is required for Müllerian duct formation in the chicken embryo.

DMRT1 is a conserved transcription factor with a central role in gonadal sex differentiation. In all vertebrates studied, DMRT1 plays an essential function in testis development and/or maintenance. No studies have reported a role for DMRT1 outside the gonads. Here, we show that DMRT1 is expressed in the paired Müllerian ducts in the chicken embryo, where it is required for duct formation. DMRT1 mRNA and protein are expressed in the early forming Müllerian ridge, and in cells undergoing an epithelial to mesenchyme transition during duct morphogenesis. RNAi-mediated knockdown of DMRT1 in ovo causes a greatly reduced mesenchymal layer, which blocks caudal extension of the duct luminal epithelium. Critical markers of Müllerian duct formation in mammals, Pax2 in the duct epithelium and Wnt4 in the mesenchyme, are conserved in chicken and their expression disrupted in DMRT1 knockdown ducts. We conclude that DMRT1 is required for the early steps of Müllerian duct development. DMRT1 regulates Müllerian ridge and mesenchyme formation and its loss blocks caudal extension of the duct. While DMRT1 plays an important role during testis development and maintenance in many vertebrate species, this is the first report showing a requirement for DMRT1 in Müllerian duct development.

Avian sex determination: what, when and where?

Sex is determined genetically in all birds, but the underlying mechanism remains unknown. All species have a ZZ/ZW sex chromosome system characterised by female (ZW) heterogamety, but the chromosomes themselves can be heteromorphic (in most birds) or homomorphic (in the flightless ratites). Sex in birds might be determined by the dosage of a Z-linked gene (two in males, one in females) or by a dominant ovary-determining gene carried on the W sex chromosome, or both. Sex chromosome aneuploidy has not been conclusively documented in birds to differentiate between these possibilities. By definition, the sex chromosomes of birds must carry one or more sex-determining genes. In this review of avian sex determination, we ask what, when and where? What is the nature of the avian sex determinant? When should it be expressed in the developing embryo, and where is it expressed? The last two questions arise due to evidence suggesting that sex-determining genes in birds might be operating prior to overt sexual differentiation of the gonads into testes or ovaries, and in tissues other than the urogenital system.