BACKGROUND The sex of many reptiles is determined by the temperature an embryo experiences during its development. Three patterns of temperature-dependent sex determination (TSD) have been defined, but one pattern where only males are produced above an upper temperature threshold (Type IB) is controversial. Here we report new data on the relationship between constant temperature incubation and sexual phenotype in two species of tuatara (Sphenodon), archaic reptiles of enormous zoological significance as the sole representatives of a once widespread reptilian order. RESULTS In both species, the pattern observed with constant incubation temperatures from 18 to 23 degrees C (or 24 degrees C) supported a female-->male (FM or Type IB) pattern of TSD: in Sphenodon guntheri males were produced above a pivotal temperature of 21.6 degrees C, and in S. punctatus (unnamed subspecies on Stephens Island, Cook Strait), males were produced above a pivotal temperature of 22.0 degrees C. The pivotal temperatures and scaling parameters differed between species (p < 0.001). The thermosensitive period (TSP), where temperature influences gonad morphogenesis, occurs between 0.25 and 0.55 of embryonic development. While it is possible that the more common female-->male-->female (FMF or Type II) pattern exists, with a second pivotal temperature above 23-24 degrees C, we review several lines of evidence to the contrary. Most notably, we show that in S. punctatus, the warmest natural nests during the TSP produce predominantly males. CONCLUSION An FM pattern of TSD could be currently adaptive in promoting sexual size dimorphism in tuatara. However, an FM pattern has particularly serious consequences for S. guntheri because current patterns of global warming could exacerbate the male bias already present in the relic population.

中文翻译：

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支持古代爬行动物中一种罕见的依赖温度的性别决定模式：来自两种蜥蜴（Sphenodon）的证据。

背景技术许多爬行动物的性别是由胚胎在其发育过程中经历的温度决定的。温度依赖性性别决定 (TSD) 的三种模式已被定义，但其中一种仅在高于温度阈值上限（IB 型）时产生雄性的模式存在争议。在这里，我们报告了两种蜥蜴（Sphenodon）恒温孵化与性表型之间关系的新数据，这两种古老的爬行动物具有巨大的动物学意义，是曾经广泛传播的爬行动物目的唯一代表。结果 在这两个物种中，在 18 至 23 摄氏度（或 24 摄氏度）的恒定孵化温度下观察到的模式支持 TSD 的雌性->雄性（FM 或 IB 型）模式：在 Sphenodon Guntheri 中，雄性是在关键温度之上产生的。关键温度为 21.6 摄氏度，而在 S. punctatus（库克海峡斯蒂芬斯岛的未命名亚种）中，雄性是在 22.0 摄氏度的关键温度之上产生的。不同物种之间的关键温度和缩放参数有所不同 (p < 0.001)。温度影响性腺形态发生的热敏感期 (TSP) 发生在胚胎发育的 0.25 至 0.55 之间。虽然可能存在更常见的女性-->男性-->女性（FMF 或 II 型）模式，且第二个关键温度高于 23-24 摄氏度，但我们回顾了一些相反的证据。最值得注意的是，我们发现，在 TSP 期间最温暖的自然巢穴中，S. punctatus 主要产生雄性。结论 TSD 的 FM 模式目前可能适用于促进斑蜥的性别大小二态性。然而，FM模式对S. Guntheri造成特别严重的后果，因为当前的全球变暖模式可能会加剧遗迹种群中已经存在的雄性偏见。