Data from: Phylogeny of Leavenworthia S-alleles suggests unidirectional mating system evoution and enhanced positive selection following an ancient population bottleneck

The adoption of self-fertilization from an ancestral outcrossing state is one of the most common evolutionary transitions in the flowering plants. In the mustard family, outcrossing is typically enforced by sporophytic self-incompatibility (SI), but there are also many self-compatible species. The genus Leavenworthia contains taxa that either possess or lack SI. Here we present data showing that SI is associated with strict outcrossing and that there is widespread trans-specific sequence polymorphism at the locus involved in the recognition of self-pollen (the S-locus). This ancestral polymorphism is consistent with the presence of an outcrossing mating system in the common ancestor of Leavenworthia species, and suggests that there have been several independent losses of SI in the group. When compared with other mustard species, the bulk of Leavenworthia S-allele sequences are highly diverged from those found in other Brassicaceae and show relatively low levels of nucleotide diversity, a pattern that suggests the common ancestor of the genus likely underwent a strong population bottleneck. The hypothesis of post-bottleneck S-locus rediversification is supported by tests showing stronger positive selection acting on S-alleles from Leavenworthia than those found in other Brassicaceae.

从祖先异交状态演化出自交策略，是被子植物中最为常见的演化过渡事件之一。在十字花科（Brassicaceae）中，异交通常由孢子体自交不亲和性（sporophytic self-incompatibility, SI）维持，但该科亦存在大量自交亲和物种。莱文沃思荠属（Leavenworthia）涵盖了携带与缺失自交不亲和性的类群。本研究呈现的数据表明，自交不亲和性与严格异交存在显著关联，且在识别自身花粉的位点（S位点，S-locus）上存在广泛的跨物种序列多态性。这种祖先多态性与莱文沃思荠属物种共同祖先所具备的异交交配系统相契合，同时暗示该类群中曾发生过多次独立的自交不亲和性丢失事件。相较于其他十字花科物种，莱文沃思荠属的绝大多数S等位基因序列与其他十字花科类群的序列高度分化，且核苷酸多样性水平相对偏低——这一模式提示该属的共同祖先大概率经历了强烈的种群瓶颈效应。针对S等位基因的选择检验显示，莱文沃思荠属的S等位基因所受的正选择强度高于其他十字花科类群，这一结果为“瓶颈后S位点发生再多样化”的假说提供了支撑。