Mechanism of Female Fate Determination and Maintenance in Zebrafish

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The zebrafish has been an important model organism for biomedical research for over 30 years. Surprisingly, it is still not known how zebrafish determine which gender they will become. In humans, gender is determined by the specific chromosomes that are inherited from the parents, were XY individual are male and XX individuals are female. However, in zebrafish no such specific chromosomes have been identified. Recent studies from several labs, including the Draper lab, have found that oocytes, the precursor to eggs, are required for female development and for maintenance of the female fate. Zebrafish that cannot produce oocytes develop and males, and adult female fish that lose their oocytes revert to males. These studies have concluded that oocytes produce a signal that functions to promote and maintain female development. The Draper lab has identified two candidate oocyte-produced signaling molecules and they have strong preliminary data that indicates that one functions to initiate female development, while the other functions to maintain female development in adults. With this award they will further determine how these signals regulate the choice between male and female development. Completion of these studies will provide fundamental knowledge about how zebrafish gender is determined, which will be important for other scientist working with zebrafish, but will also provide insights into this process in other fish species. Finally, understanding how these signals influence gender in zebrafish may lead to the identification of previously unknown pathways by which environmental pollutants can affect gender ratios and function in wild fish populations. This is of major importance as fish are a vital food source for much of the world's population and wild fish stocks are under intense and increasing pressures that are due to over-fishing, pollution and habitat destruction. In addition to these benefits, this study will also provide mentorship and meaningful scientific research experience to underrepresented and/or disadvantaged graduate, undergraduate and high school students. The zebrafish is an ideal model organism for introducing students to cutting edge biological research.The zebrafish is a major model organism that is used by hundreds of labs around the world. Yet, the mechanism of zebrafish gender determination remains unknown. Several recent studies have determined that no single genetic locus is responsible for gender determination, and that the ability to produce oocytes is a prerequisite for female determination. While zebrafish do not switch gender as adults, the Draper lab has recently discovered that maintenance of the adult female phenotype is an active process that requires continuous signaling from oocytes to the somatic gonad. Together these studies suggest that an oocyte-derived signal(s) influences the fate of the somatic gonad, which in turn regulates the overt phenotype of the animal through production of hormones. Several key questions remain to be resolved: 1) what is the nature of the oocyte-derived signal? 2) What is the function of the oocyte-derived signal? 3) Is there a mechanistic link between female primary determination and maintenance of the adult female phenotype? The working hypothesis is that the oocyte signal promotes estrogen production by the somatic gonad, and that a threshold level of estrogen is required for female primary determination and maintenance of the adult phenotype. Absent this, the somatic gonad defaults to a male developmental program, resulting in male primary determination in larvae, or female-to-male reversal in adults. These predictions will be tested in three aims using a combination of gene knockout and transgenic technologies: Specific Aim 1 will test if estrogen production by the somatic gonad (specifically by granulosa cells) is necessary and sufficient to promote female primary determination and maintenance of the adult female phenotype. Specific Aim 2 will determine whether canonical Wnt signaling is necessary and sufficient for female primary determination. Specific Aim 3 will test the necessity of the oocyte-expressed cell-signaling molecule Bmp15 to promote granulosa cell development, estrogen production, and maintenance of female determination. It is anticipated that these studies will have significant impact on our understanding of the mechanism that allows 2% of teleost species to revert as adults. In addition, they should lead to methods that allow the regulation of gender ratios in zebrafish and perhaps other fish species. Finally, knowing the detailed mechanism of the germ cell-somatic gonad signaling circuitry in zebrafish may identify previously unknown pathways by which environmental pollutants can affect gender ratios and function in wild fish populations.