Data from: Mating system contributes only slightly to female maintenance in gynodioecious Geranium maculatum (Geraniaceae)

Gynodioecy, the co-occurrence of female and hermaphroditic individuals within a population, is an important intermediate in the evolution of separate sexes. The first step, female maintenance, requires females to have higher seed fitness compared with hermaphrodites. A common mechanism thought to increase relative female fitness is inbreeding depression avoidance, the magnitude of which depends on hermaphroditic selfing rates and the strength of inbreeding depression. Less well studied is the effect of biparental inbreeding on female fitness. Biparental inbreeding can affect relative female fitness only if its consequence or frequency differs between sexes, which could occur if sex structure and genetic structure both occur within populations. To determine whether inbreeding avoidance and/or biparental inbreeding can account for female persistence in Geranium maculatum, we measured selfing and biparental inbreeding rates in four populations and the spatial genetic structure in six populations. Selfing rates of hermaphrodites were low and did not differ significantly from zero in any population, leading to females gaining at most a 1–14% increase in seed fitness from inbreeding avoidance. Additionally, although significant spatial genetic structure was found in all populations, biparental inbreeding rates were low and only differed between sexes in one population, thereby having little influence on female fitness. A review of the literature revealed few sexual differences in biparental inbreeding among other gynodioecious species. Our results show that mating system differences may not fully account for female maintenance in this species, suggesting other mechanisms may be involved.

雌全同株（Gynodioecy）指种群内同时存在雌性个体与雌雄同体个体的繁殖系统，是雌雄异体演化路径中的关键过渡阶段。该过程的首要步骤为雌性维持，即要求雌性个体相较雌雄同体个体具备更高的种子适合度。学界普遍认为，规避近交衰退（inbreeding depression）是提升雌性相对适合度的常见机制，其效应强度取决于雌雄同体的自交率与近交衰退的严重程度。而双亲近交（biparental inbreeding）对雌性适合度的影响则较少得到研究关注。仅当双亲近交的后果或发生频率在两性间存在差异时，它才会影响雌性相对适合度——这种差异可能出现在同时兼具性别结构与遗传结构的种群中。为探明近交规避和/或双亲近交能否解释斑点老鹳草（Geranium maculatum）中雌性个体的存续问题，我们对4个种群测定了自交与双亲近交率，并对6个种群开展了空间遗传结构（spatial genetic structure）分析。结果显示，雌雄同体的自交率普遍较低，且在所有种群中均与零无显著统计学差异，这使得雌性个体通过近交规避获得的种子适合度提升至多仅为1%~14%。此外，尽管所有种群均检测到显著的空间遗传结构，但双亲近交率整体偏低，且仅在1个种群中出现两性间的显著差异，因此其对雌性适合度的影响微乎其微。文献综述结果显示，在其他雌全同株物种中，双亲近交的两性差异同样极为少见。本研究结果表明，交配系统差异或许无法完全解释该物种的雌性维持机制，暗示可能存在其他尚未被揭示的作用机制。