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习性，是被子植物中最为常见的进化转变之一。在十字花科（Brassicaceae）中，异交通常由孢子体自交不亲和（sporophytic self-incompatibility, SI）机制维持，但该类群也存在诸多自交亲和的物种。Leavenworthia属（Leavenworthia）包含兼具或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关联，且在识别自身花粉的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Leavenworthia属物种的共同祖先所具备的异交交配系统相符，同时暗示该类群中曾多次独立丢失自交不亲和性。相较于其他十字花科物种，Leavenworthia的绝大多数S等位基因序列与其他十字花科类群的序列差异极大，且核苷酸多样性水平相对较低——这一模式表明该属的共同祖先可能经历了强烈的种群瓶颈效应。瓶颈效应后S位点再多样化的假说，得到了相关检验的支持：检验结果显示，Leavenworthia的S等位基因所承受的正向选择强度显著高于其他十字花科类群的S等位基因。