Conditional strategies to study gene function during gonadal development in mammals.

Sexual development in mammals involves a complex cascade of genetic events. These begin with a cell fate decision, whether to make Sertoli or follicle cells, that gives rise to the development of a male or female gonad, which is controlled by the testis-determining gene Sry. Following the expression of Sry, genes involved in the male pathway act to reinforce and maintain testis-specific cell fate decisions, as well as to repress the female pathway. Sox9 becomes rapidly upregulated after the onset of Sry expression, and is expressed in Sertoli cells throughout life. From mutation studies, SOX9 is known to be essential for male development in humans and to initiate Sertoli cell differentiation in mice. However, the function of SOX9 after sex determination and the reason for its maintenance in Sertoli cells remains unknown. In order to understand the function of Sox9 in the fetal and adult mouse testis, new tools have been generated to control gene activity in a conditional manner. This thesis mainly describes strategies to control either deletion of misexpression of Sox9. To make the tools useful at different stages, the tamoxifen-inducible Cre/loxP system was employed. This involves the establishment of two elements: a "Cre-driver" and a ' Sox9-responder". Cre-driver transgenes were made under the control of several gonadal-specific regulatory elements, as well as a strong, ubiquitous promoter. Responder mice allow Cre activated conditional misexpression or deletion of Sox9. Analyses on gonad morphologies and gene expression levels were compared between animals that have altered Sox9 expression and those that have not. The results reveal that Sox9 is necessary and sufficient for the expression of Sfl in the Sertoli cells, and suggest that Sox9 is antagonistic to the ovarian- specific gene Foxl2. The newly established Cre-drivers can also be applied in functional studies involving other genes implicated in sexual development