Endocrine Control of Fish Reproduction

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Development and maturation of oocytes (eggs) within the ovary are complex events that require the coordination of signaling molecules originating from the brain, pituitary, and ovary. We have been using zebrafish as an experimental model to investigate how the ovarian-derived signaling molecules, particularly members of the transforming growth factor-beta (TGF-beta) family and microRNAs (miRNAs), exert regulatory effects within the ovary to control the development and maturation of eggs. In the next five years, we will extend these studies to investigate the role of Nodal and several specific miRNAs: Aim 1. Physiological roles and signaling mechanisms of Nodal in zebrafish ovary Nodal is a member of the TGF-beta family known to play critical roles in embryonic development. However, its role in ovarian physiology is not known. There are three Nodal related (ndr) genes in zebrafish (ndr1, ndr2, and ndr3). We are using genetic tools to delete these genes individually or in combination and will use these knockout lines to study how these genes are involved in regulating reproduction in female zebrafish. Specifically, we will compare the production of steroid hormones, oocyte maturation, ovulation, spawning, and embryo production between wild type fish and ndr mutants. We will also determine how Nodal exerts the regulatory effects in zebrafish ovary by investigating various signaling pathways induced by Nodal. Aim 2. Role of microRNAs in zebrafish follicle growth and oocyte maturation miRNAs are small non-coding RNAs that play important roles in gene regulation. The role of miRNAs in ovarian development and oocyte maturation is largely unknown. We recently identified many miRNAs that are differentially expressed between stage IIIa (small vitellogenic follicles that are unresponsive to maturation-inducing hormones) and stage IIIb (large vitellogenic follicles that are maturationally competent) follicular cells. We will select four miRNAs that are most strongly decreased in stage IIIb follicular cells to investigate their functions and mode of actions in the ovary. We will develop knockout lines to study the consequences of loss of these miRNAs on follicle development, oocyte maturation, ovulation, spawning, and embryo production. These miRNAs are predicted to target Nodal receptors and signaling components for key hormones involved in oocyte maturation. We will characterize the target genes of these miRNAs to understand how these miRNAs play a role in reproduction. This research program will provide novel insights into how Nodal and miRNAs are involved in the regulation of ovarian physiology in fish and may have practical applications into aquaculture. It will also further enhance our understanding on the reproduction and fertility control of other vertebrates, including human.