Thyroxine and Seasonality in American Tree Sparrows

9603555威尔逊大多数动物的繁殖是季节性的，因此是非连续的。季节性繁殖是对季节性环境的一种适应，发生在环境条件最适合养育后代的时候。这个项目探索了美洲树麻雀季节性繁殖的生理机制，这种物种的繁殖周期(1)是由每年的白昼长度周期驱动的，(2)可以在实验室中简单地通过控制每天的光照和黑暗比例来模拟。白天过长（如春天）会导致性腺生长，几周后，性腺退化和婚后蜕皮。短的白昼（如冬季）恢复了对长白昼的敏感性，从而允许性腺生长在白昼长度超过临界持续时间后重新开始。最近对季节性繁殖的鸟类和哺乳动物的研究表明，从繁殖状态到非繁殖状态的转变依赖于甲状腺。在美洲树麻雀中，甲状腺不仅是这种转变所必需的，而且在季节性方面起着更普遍的作用，影响着繁殖的开始和蜕皮的开始。事实上，正如前面所示，漫长的白昼和甲状腺激素相互作用，使雄性树雀进行季节性繁殖和蜕皮。将“季节性”概念化为一套长时间的甲状腺激素依赖过程的表达是对生殖内分泌学的一项新贡献。拟议的研究试图通过严格挑战PI的季节性概念模型来验证它。事实上，如果雄性树麻雀只有在同时暴露于长白天和甲状腺激素的情况下才能进行季节性繁殖和蜕皮，那么在它们被编程后，撤回一种或两种编程刺激（即，撤回长白天和/或甲状腺激素）不应该损害编程反应的后期表达。在之前的实验中，PI通过提取甲状腺激素来验证这一推论。现在，PI将通过在单独的实验中提取长时间的白天和长时间的甲状腺激素来测试它。为了推广该模型的适用性，PI将在光刺激的雌性树麻雀上进行测试。目的是确定季节性繁殖和蜕皮是否像雄性一样是程序化的和甲状腺依赖的。拟议的研究还试图确定甲状腺激素在规划季节性方面的作用，并表明其作用是否与基因组模型或非基因组模型一致。了解季节性繁殖的开启和关闭机制可以提高人类保护濒危物种（例如，在动物园）、管理狩猎和害虫物种、操纵家养物种以获取经济利益的能力，并最大限度地减少模拟甲状腺激素的环境毒素的影响。＊＊＊

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. ***