Sex determination in the chicken embryo

The chicken embryo represents a suitable model for studying vertebrate sex determination and gonadal sex differentiation. While the basic mechanism of sex determination in birds is still unknown, gonadal morphogenesis is very similar to that in mammals, and most of the genes implicated in mammalian sex determination have avian homologues. However, in the chicken embryo, th ese genes show some interesting differences in structure or expression patt erns to their mammalian counterparts, broadening our understanding of their functions. The novel candidate testis-determining gene in mammals, DMRT1, is also present in the chicken, and is expressed specifically in the embryo nic gonads. In chicken embryos, DMRT1 is more highly expressed in the gonad s and Mullerian ducts of male embryos than in those of females. Meanwhile, expression of the orphan nuclear receptor, Steroidogenic Factor 1 (SF1) is up-regulated during ovarian differentiation in the chicken embryo. This con trasts with the expression pattern of SF1 in mouse embryos, in which expres sion is down-regulated during female differentiation. Another orphan recept or initially implicated in mammalian sex determination, DAX1, is poorly con served in the chicken. A chicken DAX1 homologue isolated from a urogenital ridge library lacked the unusual DNA-binding motif seen in mammals. Chicken DAX1 is autosomal, and is expressed in the embryonic gonads, showing somew hat higher expression in female compared to male gonads, as in mammals. How ever, expression is not down-regulated at the onset of testicular different iation in chicken embryos, as occurs in mice. These comparative data shed l ight on vertebrate sex determination in general. J. Exp. Zool. 290:691-699, 2001. (C) 2001 Wiley-Liss, Inc.