Lawson Lauren, Rollinson Njal
Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks St., Room 3055, Toronto ON, Canada M5S 3B2.
School of the Environment, University of Toronto, 33 Willcocks St., Suite 1016V, Toronto ON, Canada M5S 3E8.
Conserv Physiol. 2021 May 10;9(1):coab020. doi: 10.1093/conphys/coab020. eCollection 2021.
A common reptile conservation strategy involves artificial incubation of embryos and release of hatchlings or juveniles into wild populations. Temperature-dependent sex determination (TSD) occurs in most chelonians, permitting conservation managers to bias sex ratios towards females by incubating embryos at high temperatures, ultimately allowing the introduction of more egg-bearing individuals into populations. Here, we revisit classic sex allocation theory and hypothesize that TSD evolved in some reptile groups (specifically, chelonians and crocodilians) because male fitness is more sensitive to condition (general health, vigor) than female fitness. It follows that males benefit more than females from incubation environments that confer high-quality phenotypes, and hence high-condition individuals. We predict that female-producing temperatures, which comprise relatively high incubation temperatures in chelonians and crocodilians, are relatively stressful for embryos and subsequent life stages. We synthesize data from 28 studies to investigate how constant temperature incubation affects embryonic mortality in chelonians with TSD. We find several lines of evidence suggesting that warm, female-producing temperatures are more stressful than cool, male-producing temperatures. Further, we find some evidence that pivotal temperatures (TPiv, the temperature that produces a 1:1 sex ratio) may exhibit a correlated evolution with embryonic thermal tolerance. If patterns of temperature-sensitive embryonic mortality are also indicative of chronic thermal stress that occurs post-hatching, then conservation programs may benefit from incubating eggs close to species-specific TPivs, thus avoiding high-temperature incubation. Indeed, our models predict that, on average, a sex ratio of >75% females can generally be achieved by incubating eggs only 1°C above TPiv. Of equal importance, we provide insight into the enigmatic evolution of TSD in chelonians, by providing support to the hypothesis that TSD evolution is related to the quality of the phenotype conferred by incubation temperature, with males produced in high-quality incubation environments.
一种常见的爬行动物保护策略是对胚胎进行人工孵化，然后将幼体或幼崽放归野生种群。大多数龟鳖目动物存在温度依赖型性别决定（TSD），这使得保护管理人员能够通过在高温下孵化胚胎来使性别比例偏向雌性，最终让更多能产卵的个体进入种群。在此，我们重新审视经典的性别分配理论，并提出假说，即TSD在一些爬行动物类群（特别是龟鳖目和鳄目）中进化，是因为雄性适合度比雌性适合度对身体状况（总体健康、活力）更敏感。由此可知，雄性比雌性从能赋予高质量表型的孵化环境中受益更多，进而受益于高身体状况的个体。我们预测，在龟鳖目和鳄目动物中，产生雌性的温度（包括相对较高的孵化温度）对胚胎及后续生命阶段而言压力相对更大。我们综合了28项研究的数据，以探究恒温孵化如何影响具有TSD的龟鳖目动物的胚胎死亡率。我们发现几条证据表明，温暖的、产生雌性的温度比凉爽的、产生雄性的温度压力更大。此外，我们发现一些证据表明关键温度（TPiv，产生1:1性别比例的温度）可能与胚胎热耐受性呈现协同进化。如果温度敏感型胚胎死亡率的模式也表明孵化后会出现慢性热应激，那么保护计划可能会受益于将卵孵化在接近物种特异性TPiv的温度，从而避免高温孵化。实际上，我们的模型预测，平均而言，仅将卵孵化在比TPiv高1°C的温度下，通常就能实现超过75%的雌性性别比例。同样重要的是，我们通过支持TSD进化与孵化温度赋予的表型质量相关这一假说，为龟鳖目动物中TSD的神秘进化提供了见解，高质量孵化环境会产生雄性。
