Sex Determination and Gonadal Development in Birds

The mechanisms regulating sex determination and gonadal development in birds and mammals share many common morphological and molecular features. However, these mechanisms have evolved independently and there are elements of these processes that are absent in placental mammals but are present in most avian species. These include, a left-right gonadal asymmetry, the direct involvement of estrogen in primary sex determination, a strong cellular sex identity in non-reproductive tissues, and the type of dosage compensation mechanism employed. In terms of genetic control, the avian testis-determining pathway is thought to be regulated by expression of the Z-linked gene DMRT1 (sex chromosomes are ZZ in males and ZW in females), while the autosomal FOXL2 gene product is thought to play a major role in ovarian development. As our understanding of this process improves, it is likely that genetic and hormonal elements will merge into one interacting network that regulates sex determination and the sexual phenotype in birds.