Data from: Asymmetric evolutionary responses to sex-specific selection in a hermaphrodite

Sex allocation theory predicts that simultaneous hermaphrodites evolve to an evolutionary stable resource allocation, whereby any increase in investment to male reproduction leads to a disproportionate cost on female reproduction and vice-versa. However, empirical evidence for sexual trade-offs in hermaphroditic animals is still limited. Here, we tested how male and female reproductive traits evolved under conditions of reduced selection on either male or female reproduction for 40 generations in a hermaphroditic snail. This selection favors a reinvestment of resources from the sex function under relaxed selection towards the other function. We found no such evolutionary response. Instead, juvenile survival and male reproductive success significantly decreased in lines where selection on the male function (i. e., sexual selection) was relaxed, while relaxing selection on the female function had no effect. Our results suggest that most polymorphisms under selection in these lines were not sex-antagonistic. Rather, they were deleterious mutations affecting juvenile survival (thus reducing both male and female fitness) with strong pleiotropic effects on male success in a sexual selection context. These mutations accumulated when sexual selection was relaxed, which supports the idea that sexual selection in hermaphrodites contributes to purge the mutation load from the genome as in separate-sex organisms.

性分配理论（Sex allocation theory）预测，雌雄同体生物（simultaneous hermaphrodites）将演化至进化稳定资源分配状态：对雄性生殖投入的任何增加，都会对雌性生殖造成不成比例的代价，反之亦然。然而，目前关于雌雄同体动物存在性权衡的实证证据仍较为有限。本研究以一种雌雄同体蜗牛为实验材料，通过连续40代分别施加针对雄性或雌性生殖的选择压力松弛处理，探究雌雄生殖性状的演化规律。这类选择压力松弛本应促使资源从受选择放松的性别功能，向另一性别功能重新分配，但我们并未观测到此类演化响应。相反，当雄性功能（即性选择（sexual selection））的选择压力被松弛时，实验品系的幼体存活率与雄性生殖成功率均显著下降；而松弛雌性功能的选择压力则未产生任何效应。研究结果表明，本实验品系中受选择作用的多数多态性并非性拮抗多态性，而是一类会影响幼体存活率（进而同时降低雌雄个体适合度）的有害突变（deleterious mutations），且在性选择情境下对雄性生殖成功率具有强烈的多效性效应（pleiotropic effects）。这类突变在性选择压力松弛时发生累积，这支持了如下观点：雌雄同体生物的性选择，如同雌雄异体生物（separate-sex organisms）一般，能够帮助清除基因组中的突变负荷（mutation load）。