Avian sex determination: a chicken egg conundrum.

Background Primary sex-determination is the developmental process that results in the sex determination of the gonads. Vertebrate sex determination is generally considered to follow the model based on the mammalian system, where a sex-specific master regulatory gene activates one of the two different gene networks that underlie testis and ovary differentiation. Summary It is now known that, while many of the molecular components of these pathways are conserved across different vertebrates, a wide variety of different trigger factors are utilised to initiate primary sex determination. In birds, the male is the homogametic sex (ZZ), and significant differences exist between the avian system of sex determination and that of mammals. For example, DMRT1, FOXL2 and estrogen are key factors in gonadogenesis in birds, but none are essential for primary sex determination in mammals. Key Message Gonadal sex determination in birds is thought to depend on a dosage-based mechanism involving expression of the Z-linked DMRT1 gene, and it may be that this 'mechanism' is simply an extension of the cell autonomous sex identity (CASI) associated with avian tissues, with no sex-specific trigger required.