Guioli, S. | bioRxiv Science

bioRxiv2020

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 gene DMRT1 (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 the DMRT1 gene. The resulting chromosomally male (ZZ) chicken with a single functional copy of DMRT1 developed 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 lacked DMRT1, 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.

developmental biology↗

bioRxiv2019

In the chick embryo, estrogen can induce chromosomally male ZZ left gonad epithelial cells to form an ovarian cortex which supports oogenesis

In chickens, as in most amniotes, the first morphogenetic event in ovary differentiation is the formation of two distinct domains: a steroidogenic core, the medulla, overlain by the germ cell niche, the cortex. This process normally starts immediately after sex determination in the ZW embryos, substantially before the progression of germ cells into meiosis. In order to address the extent to which the cortical domain depends on intrinsic or extrinsic factors, we generated models of gonadal intersex by mixing ZW (female) and ZZ (male) cells in gonadal chimeras, or by altering estrogen levels of ZZ and ZW embryos in ovo. We found that both chomosomally female and male cells can be induced to form the cortical domain and that this can form relatively independently of the phenotypic sex of the medulla as long as estrogen is provided. We also show that the cortex promoting activity of estrogen signalling is mediated via Estrogen Receptor alpha within the left gonad epithelium. Therefore, either a ZW or ZZ cortical domain can provide an adequate niche to allow the germ cells to progress into meiosis. However, the presence of a medulla with an \"intersex\" or male phenotype may compromise this progression, causing cortical germ cells to remain in an immature state in the embryo.

developmental biology↗