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

In birds, males are the homogametic sex (ZZ) and females the heterogametic sex (ZW). Primary sex determination is thought to depend on a sex chromosome gene dosage mechanism, and the most likely sex determinant is the Z chromosome gene Doublesex and Mab-3-Related Transcription factor 1 (DMRT1). To clarify this issue, we used a CRISPR-Cas9-based monoallelic targeting approach and sterile surrogate hosts to generate birds with targeted mutations in the DMRT1 gene.

Oestrogen in the chick embryo can induce chromosomally male ZZ left gonad epithelial cells to form an ovarian cortex that can support oogenesis.

In chickens, the embryonic ovary differentiates into two distinct domains before meiosis: a steroidogenic core (the female medulla), overlain by the germ cell niche (the cortex). The differentiation of the medulla is a cell-autonomous process based on chromosomal sex identity (CASI). In order to address the extent to which cortex differentiation 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 oestrogen levels of ZW and ZZ embryos.

RNA FISH, DNA FISH and Chromosome Painting of Chicken Oocytes.

Fluorescence in situ hybridization (FISH) is a molecular cytogenetic technique. It identifies the location of DNA loci and RNAs, including nascent RNAs in the process of being transcribed, within individual cells. Great advances in fluorescent dye technology and technique sensitivity, combined with developments in light microscopy and imaging software have made it widely accessible and have expanded the range of applications in basic research as well as in diagnostics.

Gonadal asymmetry and sex determination in birds.

Although vertebrates display a superficial bilateral symmetry, most internal organs develop and locate with a consistent left:right asymmetry. There is still considerable debate as to when this process actually begins, but it seems that, at least for some species, the initial steps occur at a very early stage of development. In recent years, a number of model systems, including the chick embryo, have been utilised to increase our understanding of the molecular basis of this complex developmental process.

Error-prone ZW pairing and no evidence for meiotic sex chromosome inactivation in the chicken germ line.

In the male mouse the X and Y chromosomes pair and recombine within the small pseudoautosomal region. Genes located on the unsynapsed segments of the X and Y are transcriptionally silenced at pachytene by Meiotic Sex Chromosome Inactivation (MSCI). The degree to which MSCI is conserved in other vertebrates is currently unclear.