Primary sex determination in chickens depends on DMRT1 dosage, but gonadal sex does not determine secondary sexual characteristics in adult birds

In birds, males are the homogametic sex (ZZ) and females the heterogametic sex (ZW), and primary sex determination is thought to depend on a sex chromosome gene dosage mechanism. Previous studies have suggested that the most likely sex-determinant is the Z chromosome geneDMRT1(Doublesex and Mab-3 Related Transcription factor 1). To clarify this issue, we used a CRISPR-Cas9 based mono-allelic targeting approach and sterile surrogate hosts to generate birds with targeted mutations in theDMRT1gene. The resulting chromosomally male (ZZ) chicken with a single functional copy ofDMRT1developed ovaries in place of testes, demonstrating the avian sex determining mechanism is based on DMRT1 dosage. These ZZ ovaries expressed typical female markers and showed clear evidence of follicular development. However, these ZZ adult birds with an ovary in place of testes were indistinguishable in appearance to wild type adult males, supporting the concept of cell-autonomous sex identity (CASI) in birds. In experiments where oestrogen synthesis was blocked in control ZW embryos, the resulting gonads developed as testes. In contrast, if oestrogen synthesis was blocked in ZW embryos that lackedDMRT1, the gonads invariably adopted an ovarian fate. Our analysis shows that DMRT1 is the key sex determination switch in birds and that it is essential for testis development, but that production of oestrogen is also a key factor in primary sex determination in chickens, and that this production is linked to DMRT1 expression.