Understanding the evolution of mammalian sex determination using nuclease-dependent genome editing

Male and females differ in their sex chromosome complement: males have one X and one Y chromosome (XY), while females have two X chromosomes (XX).

Sex chromosomes control diverse aspects of human biology, but one of their best known roles is in sex determination. In eutherian mammals (e.g. humans), the Y chromosome directs the development of the testis and of all male secondary sexual characteristics. Intriguingly, in the second largest class of mammals, the marsupials (metatherians), the formation of the testis is also dependent on the Y chromosome, but the identity of the Y-linked testis-determination gene is unknown. Furthermore, in metatherians secondary sexual characteristics are governed not by the Y chromosome but by the number of X chromosomes, with one X driving male development and two Xs, female development (O, 1988). Candidate metatherian X and Y-sex determination genes have been identified, including the transcription factor Sry and the chromatin remodelling proteins Atry and Atrx, mutations of which cause gonadal defects in humans. However, the function of these genes is unclear because gene targeting has never been performed in metatherians mammals.

In this project, the candidate will apply next generation sequencing (e.g. RNA-Seq) and cell biological approaches (e.g. RNA FISH) to assess the candidacy of Sry, Atry and Atrx, and other X/Y genes in metatherian sex determination. They will then apply the recently developed CRISPR/Cas9 genome editing technique to metatherians in order to investigate the role these genes in sexual development. The results will provide unique insight into mammalian sex chromosome evolution and the aetiology of human sex differentiation disorders. The host lab has extensive experience in metatherian sex chromosome biology (Grant, 2012; Mahadevaiah, 2009) and has optimised metatherian genome editing in vitro. The project offers an excellent opportunity to gain experience in a wide range of subjects, including evolutionary and developmental biology, genomics, transcriptomics and bioinformatics.