Proximate Mechanisms of Sexual Size Dimorphism in Lizards

性别大小二态(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获得更大的特征体型。根据物种的不同，雄性可能比雌性更大，反之亦然。人们通常认为，SSD的进化是由于对身体大小的差异选择，几种适应性假设援引了男性或女性偏向SSD的优势，这一假设得到了广泛的讨论。然而，SSD的遗传基础和进化潜力几乎完全没有描述，对导致绝大多数物种身体大小性别差异的生理和其他邻近机制知之甚少。因此，现在关注SSD的最终原因还为时过早，而不是像这一领域以往研究的普遍倾向那样，关注SSD的近因。本项目将集中在一种蜥蜴(Scelopus Undulatus)中SSD的直接原因的生理和生态机制上，在这种情况下，雌性在幼年生长得更快，成年后比雄性大10%左右。该项目将解决以下问题：1)睾丸素是否从根本上导致了性二态增长率和女性偏向SSD的发展？2)SSD在多大程度上受到环境变化的影响？3)生长/活动权衡施加的限制如何影响SSD的发生？为了回答这个项目中提出的问题，将进行两组研究。1)描述性研究将描述活动在其原生栖息地的幼年蜥蜴的生长速度、行为和性类固醇水平。2)涉及激素调控的实验研究将明确解决睾酮在导致女性偏见的SSD中的潜在作用。该项目完成的工作将对性别差异如何导致SSD这一普遍问题提供重要的新信息。这项工作可能会为未来对文体蜥蜴生长调控的比较研究打开大门，文体蜥蜴是一个有着良好系统发育历史的大家族。这一组未来的比较分子研究可以提供对促进生长基因表达调控进化的详细了解，并有助于解决生长和SSD的遗传与环境决定因素的贡献。因此，硬体蜥蜴可能会成为研究生长调控的一个重要的新模式系统。

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.