Successful reproduction in all animals involves a complex environmental and endocrine signaling network that controls gametogenesis, regulates gamete maturation, and induces behavior that results in a spawning event. In seasonally breeding fishes, environmental signals (e.g., length of daylight) trigger brain production of gonadotropin releasing hormone (GnRH) that initiates a reproductive cascade leading to eggs in the female or sperm in the male. GnRH signals the pituitary gland to produce two gonadotropins (GTHs), follicle stimulating hormone and luteinizing hormone. Carried by the blood these hormones bind to specific gonadal receptors, which stimulates the gonads to synthesize the sex steroid hormones (estrogens, progestins, and androgens). The sex steroids act within the gonads for proper gametogenesis in each sex and provide feedback to the pituitary gland and hypothalamus to regulate GTH production and secretion. Underlying this complex network are key genes involved in the endocrine signaling and in the downstream events triggered by this hormonal cascade. To clarify relationships among this signaling system, gametogenesis, and associated gene activity, a hormone-signal-(GnRH, GTHs, sex steroids)-based, mathematical model will be developed in a salmonid fish, the female rainbow trout (Oncorhynchus mykiss) at 12oC. Novel mathematical techniques will be developed to characterize the impact of the persistence of circadian rhythms on the stability and sensitivity properties of the photoperiod-adjusted model. Gene array technology will enable the identification of activated and suppressed transport and signaling genes, whose qRT-PCR levels will be statistically associated with hormonal stimulation and response, and consequent ovarian development. The signaling system will be modulated by photoperiod-controlled advancement of annual spawn timing, both to evaluate the capacity of the model to accommodate changes induced by photoperiod and to better understand the sensitivity of the spawning cycle to this environmental cue.
Reproduction in the rainbow trout involves an annual cycle of egg development over several months in the female, followed by egg deposition and fertilization by sperm from the male. The cycle is controlled by a complicated signaling system that includes stimuli such as length of daylight and water temperature. These environmental cues operate through the brain to prompt the pituitary gland and the gonads to produce several hormones. The annual reproductive cycle and associated hormone signals are understood qualitatively, but not at the gene level where the hormone signals activate hundreds of genes to produce proteins that carry out the production of eggs and sperm. To better understand the reproductive cycle, the concentrations of the hormone signals and the activity of the genes that they turn on or off will be measured. A mathematical model of the hormone signals and gene activity will be constructed. Mathematical model development will provide insights into the complex reproductive hormone system of the rainbow trout, and the interplay of genes involved in this important signaling cascade. The resulting novel mathematical model and gene activity analyses will be generally applicable to other species. The model will aid exploration of environmental cues on reproduction in fishes. The model will be particularly relevant to salmonid fishes, but will also provide a template for other fish species and egg laying vertebrates in general. It will provide a framework for characterization of species differences in the reproductive endocrine system and associated gene activity. Finally, the model will provide knowledge useful for guiding the management of wild and cultured populations of fish.
