Seasonal shifts in sex ratios are mediated by maternal effects and fluctuating incubation temperatures

Sex-specific maternal effects can be adaptive sources of phenotypic plasticity. Reptiles with temperature-dependent sex determination (TSD) are a powerful system to investigate such maternal effects because offspring phenotype, including sex, can be sensitive to maternal influences such as oestrogens and incubation temperatures.
In red-eared slider turtles (Trachemys scripta), concentrations of maternally derived oestrogens and incubation temperatures increase across the nesting season; we wanted to determine if sex ratios shift in a seasonally concordant manner, creating the potential for sex-specific maternal effects, and to define the sex ratio reaction norms under fluctuating temperatures across the nesting season.
Eggs from early and late season clutches were incubated under a range of thermally fluctuating temperatures, maternally derived oestradiol concentrations were quantified via radioimmunoassay, and hatchling sex was identified. We found that late-season eggs had higher maternal oestrogen concentrations and were more likely to produce female hatchlings. The sex ratio reaction norm curves systematically varied with season, such that with even a slight increase in temperature (0·5 °C), late-season eggs produced up to 49% more females than early-season eggs.
We found a seasonal shift in sex ratios which creates the potential for sex-specific phenotypic matches across the nesting season driven by maternal effects. We also describe, for the first time, systematic variation in the sex ratio reaction norm curve within a single population in a species with TSD.

性别特异性母体效应可作为表型可塑性的适应性来源。具有温度依赖型性别决定（temperature-dependent sex determination, TSD）的爬行动物，是探究此类母体效应的经典模型系统——因包括性别在内的后代表型，可受到雌激素、孵化温度等母体因素的显著调控。在红耳滑龟（Trachemys scripta）中，母体源性雌激素浓度与孵化温度均随繁殖季推进而升高。本研究旨在明确两点：其一，性别比例是否会以与季节同步的方式发生偏移，进而产生性别特异性母体效应的潜在可能；其二，明确繁殖季内波动温度条件下的性别比例反应规范。本研究将繁殖季早、晚期产出的窝卵，置于一系列热波动温度下进行孵化；通过放射免疫分析法定量检测母体源性雌二醇浓度，并鉴定孵化幼龟的性别。结果显示，繁殖季晚期的窝卵具有更高的母体雌激素水平，且更易发育为雌性幼体。性别比例反应规范曲线随季节呈现系统性变化：即便温度仅升高0.5℃，繁殖季晚期窝卵所产生的雌性个体比例，最高可比早期窝卵高出49%。本研究揭示了性别比例随季节的动态变化，这为繁殖季内由母体效应驱动的性别特异性表型匹配提供了潜在条件。同时，本研究首次在具有温度依赖型性别决定的物种种群中，报道了单一种群内性别比例反应规范曲线的系统性变异。