*Department of Organismic and Evolutionary Biology, Museum of Comparative Zoology, Harvard University, Cambridge, MA 02138, USA.
Integr Comp Biol. 2008 Oct;48(4):512-9. doi: 10.1093/icb/icn041. Epub 2008 May 17.
Reptiles (sauropsids) represent the sister group to mammals, and the basal members of Reptilia may provide a good model for the condition of the common ancestor of both groups. Sex-determining mechanisms (SDM) and organizations of sex chromosomes among genotypically sex-determining (GSD) species vary widely across reptiles. Birds and snakes, for example, are entirely GSD whereas other reptiles, like all crocodilians, exhibit temperature-dependent sex determination (TSD). Here we explore the evolution of sex chromosomes and SDM within reptiles, using family-level analyses of character evolution and applying parsimony, likelihood, Bayesian, and stochastic methods. We find support for the common ancestor of amphisbaenians and whiptail lizards (Laterata) possessing the XY (male heterogametic) GSD mechanism, while the ancestors of Testudines and Crocodylia, as well as the larger group Archosauromorpha (here containing turtles) are inferred to have exhibited TSD. We also find evidence consistent with the hypothesis that the XY system is more labile and evolves faster than does the ZW (female heterogametic) system. Phylogenetic-based speciation tests do not support an association between GSD and speciation, and reject the hypothesis that the presence of the XY system is associated with speciation in reptiles.
爬行动物（蜥形纲）是哺乳动物的姐妹群，而爬行动物的基干类群可能为这两类群的共同祖先的状态提供了一个很好的模型。性别决定机制（SDM）和性染色体的组织在基因型性别决定（GSD）物种中在爬行动物中差异很大。例如，鸟类和蛇类完全是 GSD，而其他爬行动物，如所有鳄鱼类，表现出温度依赖的性别决定（TSD）。在这里，我们使用特征进化的家族水平分析，并应用简约法、似然法、贝叶斯法和随机法，探索了性染色体和 SDM 在爬行动物中的进化。我们支持蚓蜥和鞭尾蜥（Laterata）的共同祖先具有 XY（雄性异型配子）GSD 机制，而龟鳖目和鳄形目，以及更大的主龙形类（这里包含海龟）的祖先则被推断为表现出 TSD。我们还发现了与以下假说一致的证据，即 XY 系统比 ZW（雌性异型配子）系统更不稳定，进化速度也更快。基于系统发育的物种形成测试不支持 GSD 与物种形成之间存在关联，并且拒绝了 XY 系统的存在与爬行动物物种形成相关的假说。
