9603555 Wilson Most species of animals breed seasonally, thus discontinuously. Seasonal reproduction is an adaptation to a seasonal environment and is timed to occur when environmental conditions are optimal for raising young. This project explores the physiological mechanisms that turn seasonal breeding on and off in American tree sparrows, a species whose reproductive cycle (1) is driven by the annual cycle in daylength, and (2) can be simulated in the laboratory simply by manipulating the daily ration of light and darkness. Long daylengths (as during spring) induce gonadal growth and, several weeks later, gonadal regression and postnuptial molt. Short daylengths (as during winter) restore sensitivity to long days, thereby permitting gonadal growth to begin anew once daylength exceeds a critical duration. Recent work on seasonally breeding birds and mammals has established that the transition from the breeding state to the nonbreeding state is thyroid dependent. In American tree sparrows, the thyroid is not only required for that transition, but plays an even more pervasive role in seasonality, influencing the initiation of breeding and the onset of molt as well. Indeed, as shown earlier, long days and thyroid hormones interact to program male tree sparrows for seasonal reproduction and molt. Conceptualization of "seasonality" as the expression of a suite of long day--thyroid hormone-dependent processes is a novel contribution to reproductive endocrinology. The proposed research seeks to validate the PI's conceptual model of seasonality by rigorously challenging it. If male tree sparrows are, in fact, programmed for seasonal reproduction and molt only when exposed simultaneously to long days and thyroid hormones, then withdrawing one or both of the programming stimuli (that is, withdrawing long days and/or thyroid hormones) after they have been programmed should not compromise later expression of programmed responses. In previous experiments, the PI tested this corollary by withdrawing thyroid hormones. Now the PI will test it by withdrawing, in separate experiments, long days and both long days and thyroid hormones. In an attempt to generalize the model's applicability, the PI will test it on photostimulated female tree sparrows. The goal is to ascertain whether seasonal reproduction and molt are programmed and thyroid dependent, as in males. The proposed research also seeks to identify the thyroid hormone that is instrumental in programming seasonality, and to show whether its actions are consistent with a genomic model or with a nongenomic model. Understanding the mechanisms that turn seasonal reproduction on and off may enhance humankind's ability to preserve endangered species (for example, in zoos), manage game and pest species, manipulate domestic species for economic gain, and minimize the effects of environmental toxins that mimic thyroid hormones. ***
