Sex is determined by chromosomes in mammals but it can be influenced by the environment in many worms, crustaceans, and vertebrates. Despite this, there is little understanding of the relationship between ecology and the evolution of sexual systems. The nematode Auanema freiburgensis has a unique sex determination system in which individuals carrying one X chromosome develop into males while XX individuals develop into females in stress‐free environments and self‐fertile hermaphrodites in stressful environments. Theory predicts that trioecious populations with coexisting males, females, and hermaphrodites should be unstable intermediates in evolutionary transitions between mating systems. In this article, we study a mathematical model of reproductive evolution based on the unique life history and sex determination of A. freiburgensis. We develop the model in two scenarios, one where the relative production of hermaphrodites and females is entirely dependent on the environment and one based on empirical measurements of a population that displays incomplete, “leaky” environmental dependence. In the first scenario environmental conditions can push the population along an evolutionary continuum and result in the stable maintenance of multiple reproductive systems. The second “leaky” scenario results in the maintenance of three sexes for all environmental conditions. Theoretical investigations of reproductive system transitions have focused on the evolutionary costs and benefits of sex. Here, we show that the flexible sex determination system of A. freiburgensis may contribute to population‐level resilience in the microscopic nematode's patchy, ephemeral natural habitat. Our results demonstrate that life history, ecology, and environment may play defining roles in the evolution of sexual systems.

中文翻译：

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环境压力维持线虫蠕虫的三重性

性别由哺乳动物的染色体决定，但它会受到许多蠕虫、甲壳类动物和脊椎动物的环境的影响。尽管如此，人们对生态与性系统进化之间的关系知之甚少。弗莱堡线虫 Auanema freiburgensis 具有独特的性别决定系统，其中携带一条 X 染色体的个体在无压力环境中发育为雄性，而 XX 个体在无压力环境中发育为雌性，而在压力环境中则为自交雌雄同体。理论预测，雄性、雌性和雌雄同体共存的三雌性种群应该是交配系统之间进化过渡的不稳定中间体。在本文中，我们基于弗莱堡 A. 独特的生活史和性别决定研究了生殖进化的数学模型。我们在两种情况下开发模型，一种是雌雄同体的相对产量完全取决于环境，另一种是基于对显示不完整、“泄漏”环境依赖性的种群的经验测量。在第一种情况下，环境条件可以推动种群沿着进化连续体前进，并导致多个生殖系统的稳定维持。第二种“泄漏”情景导致在所有环境条件下都保持三种性别。生殖系统转变的理论研究集中在性的进化成本和收益上。在这里，我们展示了 A. freiburgensis 灵活的性别决定系统可能有助于微观线虫斑驳的、短暂的自然栖息地的种群水平恢复力。