Proximate Mechanisms of Sexual Size Dimorphism in Lizards

两性尺寸二态性（SSD）是一种普遍存在的生物学现象，其中一种性别的成年人比另一种性别的成年人具有更大的体型特征。根据物种的不同，雄性可能比雌性大，反之亦然。人们通常认为，SSD的进化是由于体型的差异选择，一些适应性假设引发了男性或女性偏好的SSD的优势，这些假设已经被广泛讨论。然而，SSD的遗传基础和进化潜力几乎没有被完全描述，而导致绝大多数物种体型性别差异的生理和其他近似机制则知之甚少。因此，关注SSD的最终原因而不是直接原因还为时过早，这是该领域先前研究的普遍倾向。本项目将重点研究一种蜥蜴（Sceloporus undulatus）的SSD的生理和生态机制，雌性在幼年时生长得更快，成年后比雄性大10%左右。该项目将解决以下问题：1)睾丸激素是否对两性二态生长速率和女性偏向性SSD的发展负有根本责任？2) SSD在多大程度上受环境变化的影响？3)增长/活动权衡所施加的约束如何影响SSD的发生？为了回答这个项目中提出的问题，我们将进行两组研究。1)描述性研究将描述在其原生栖息地活跃的幼蜥的生长速度、行为和性类固醇水平。2)涉及激素操作的实验研究将明确解决睾酮在导致女性偏向性SSD中的潜在作用。本项目完成的工作将为性别生长差异如何导致SSD这一一般性问题提供重要的新信息。这一工作可能为今后比较研究蜥毛纲这一具有良好系统发育史的大家族的生长调控打开大门。未来的比较分子研究可以为生长促进基因表达调控的进化提供详细的了解，并有助于解决遗传和环境因素对生长和SSD的影响。因此，毛虫蜥蜴可能成为研究生长调控的重要新模型系统。

Sexual size dimorphism (SSD) is a widespread biological phenomenon in which adults of one sex attain characteristically larger body sizes than the other sex. Depending on the species, males might be larger than females, or vice versa. It is often assumed that SSD evolves due to differential selection on body size, and several adaptive hypotheses invoking advantages for male- or female-biased SSD have been widely discussed. However, the genetic basis and thus the potential for evolution of SSD is almost completely undescribed, and relatively little is known about physiological and other proximate mechanisms that lead to sex differences in body size in the vast majority of species. It is therefore premature to focus on ultimate as opposed to proximate causation of SSD, as has been the general tilt of previous studies in this area. The present project will focus on physiological and ecological mechanisms of proximate causation of SSD in a lizard (Sceloporus undulatus) in which females grow faster as juveniles to become about 10% larger than males as adults. This project will address the following questions: 1) Is testosterone fundamentally responsible for sexually dimorphic growth rates and the development of female-biased SSD? 2) To what extent is SSD subject to environmental variation? 3) How do constraints imposed by growth/activity trade-offs affect the occurrence of SSD? To answer the questions posed in this project, two sets of studies will be done. 1) Descriptive studies will characterize growth rates, behavior, and sex steroid levels in juvenile lizards active in their native habitat. 2) Experimental studies involving hormone manipulations will resolve unambiguously the potential role of testosterone in leading to female-biased SSD. The work completed in this project will contribute important new information regarding the general question of how sex differences in growth lead to SSD. This work may open the door for future comparative studies on growth regulation in phrynosomatid lizards, a large family with a well-described phylogenetic history. Future comparative molecular studies in this group could provide a detailed understanding of the evolution of the regulation of growth-promoting gene expression and could help to resolve the contribution of genetic vs. environmental determinants of growth and SSD. As such, phrynosomatid lizards may prove to become a significant new model system for studies on growth regulation.