Data_Sheet_5_Sex-Biased Mortality and Sex Reversal Shape Wild Frog Sex Ratios.CSV

Population sex ratio is a key demographic factor that influences population dynamics and persistence. Sex ratios can vary across ontogeny from embryogenesis to death and yet the conditions that shape changes in sex ratio across ontogeny are poorly understood. Here, we address this issue in amphibians, a clade for which sex ratios are generally understudied in wild populations. Ontogenetic sex ratio variation in amphibians is additionally complicated by the ability of individual tadpoles to develop a phenotypic (gonadal) sex opposite their genotypic sex. Because of sex reversal, the genotypic and phenotypic sex ratios of entire cohorts and populations may also contrast. Understanding proximate mechanisms underlying phenotypic sex ratio variation in amphibians is important given the role they play in population biology research and as model species in eco-toxicological research addressing toxicant impacts on sex ratios. While researchers have presumed that departures from a 50:50 sex ratio are due to sex reversal, sex-biased mortality is an alternative explanation that deserves consideration. Here, we use a molecular sexing approach to track genotypic sex ratio changes from egg mass to metamorphosis in two independent green frog (Rana clamitans) populations by assessing the genotypic sex ratios of multiple developmental stages at each breeding pond. Our findings imply that genotypic sex-biased mortality during tadpole development affects phenotypic sex ratio variation at metamorphosis. We also identified sex reversal in metamorphosing cohorts. However, sex reversal plays a relatively minor and inconsistent role in shaping phenotypic sex ratios across the populations we studied. Although we found that sex-biased mortality influences sex ratios within a population, our study cannot say at this time whether sex-biased mortality is responsible for sex ratio variation across populations. Our results illustrate how multiple processes shape sex ratio variation in wild populations and the value of testing assumptions underlying how we understand sex in wild animal populations.

种群性比（population sex ratio）是影响种群动态与存续的核心人口统计学因子。性比可随个体发育（ontogeny）进程发生变化，跨度从胚胎发生（embryogenesis）直至个体死亡，但目前学界对调控个体发育过程中性比变化的具体条件仍知之甚少。本研究以两栖动物为研究对象——该演化支（clade）的野生种群性比研究普遍存在不足。此外，两栖动物的个体发育性比变异还因单只蝌蚪可发育出与其基因型性别相悖的表型（性腺）性别而更为复杂。由于性反转（sex reversal）现象的存在，整个同生群乃至种群的基因型与表型性别比也可能出现差异。

鉴于两栖动物在种群生物学研究中的重要作用，以及其作为生态毒理学（eco-toxicological）研究模型物种、用于探究有毒物质（toxicant）对性比的影响，阐明两栖动物表型性别比变异的近因机制具有重要意义。此前研究者普遍认为，偏离1:1的性比是由性反转导致的，但性偏倚死亡率同样是值得考量的替代性解释。

本研究采用分子性别鉴定（molecular sexing）技术，通过对两个独立的美洲绿蛙（Rana clamitans）种群的各繁殖池塘内多个发育阶段的个体进行基因型性别比检测，追踪了从卵块至变态发育（metamorphosis）阶段的基因型性别比变化。研究结果表明，蝌蚪发育过程中的基因型性偏倚死亡率会影响变态个体的表型性别比变异。我们还在变态发育的同生群中发现了性反转现象，但在本次研究的种群中，性反转在塑造表型性别比的过程中仅发挥了相对次要且不稳定的作用。

尽管我们发现种群内部的性偏倚死亡率会影响性比，但本研究目前尚无法阐明性偏倚死亡率是否是造成种群间性比差异的原因。本研究揭示了多种过程如何共同塑造野生种群的性比变异，同时也印证了检验野生动物种群性别认知背后预设假设的重要价值。