Proximate Mechanisms of Sexual Size Dimorphism in Lizards

性别大小二态性（SSD）是一种普遍的生物学现象，其中一种性别的成年人获得比另一种性别更大的身体尺寸。 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的发展，由于身体大小的差异选择，和几个自适应假说调用男性或女性偏向SSD的优势已被广泛讨论。 然而，SSD的遗传基础和进化潜力几乎完全未被描述，并且对导致绝大多数物种体型性别差异的生理和其他近似机制知之甚少。 因此，它是不成熟的，而不是直接的原因，专注于最终的SSD，因为已经在这一领域的以往研究的一般倾斜。 本项目将集中在SSD的近因在蜥蜴（Sceloporus undulatus），其中女性成长更快的少年成为约10%的成年男性大的生理和生态机制。 本项目将解决以下问题：1）睾酮是否从根本上负责性二态生长率和女性偏见的SSD的发展？ 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？ 为了回答本项目提出的问题，将进行两组研究。 1)描述性研究将描述生长速度，行为和性类固醇水平在幼年蜥蜴活跃在他们的原生栖息地。 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SSD中的潜在作用。 在这个项目中完成的工作将有助于重要的新信息有关的一般性问题，如何性别差异的增长导致SSD。 这项工作可能会打开大门，为未来的比较研究的生长调节在phrynosomatid蜥蜴，一个大家庭与一个良好的描述系统发育史。在这个组中的未来比较分子研究可以提供一个详细的了解的进化的调节生长促进基因的表达，并可能有助于解决的贡献，遗传与环境因素的增长和SSD。 因此，棘体蜥蜴可能成为研究生长调节的重要新模式系统。

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.