Data from: The evolution of intrinsic reproductive isolation in the genus Cakile (Brassicaceae)

In theory, adaptive divergence can increase intrinsic post-zygotic reproductive isolation (RI), either directly via selection on loci associated with RI, or indirectly via linkage of incompatibility loci with loci under selection. To test this hypothesis, we measured RI at five intrinsic post-zygotic reproductive barriers between 18 taxa from the genera <i>Cakile</i> and <i>Erucaria</i> (Brassicaceae). Using a comparative framework, we tested whether the magnitude of RI was associated with genetic distance, geographic distance, ecological divergence, and parental mating system. Early stages of post-zygotic RI related to F1 viability (i.e., initial seed set) tended to be stronger than later stages related to F1 fecundity (i.e., flower number, fruit number). Mating system significantly influenced early stages of RI, such that RI was lowest when the mother was selfing and father was outcrossing, consistent with an imbalance between sink strength and resistance to provisioning. We found little evidence that adaptive divergence accelerates the evolution of intrinsic post-zygotic RI, consistent with non-ecological model of evolution that predicts the non-linear accumulation of RI and RI asymmetry with time (i.e., genetic distance), irrespective of adaptive divergence. Thus, while certain aspects of ecological divergence do not appear to have contributed strongly to the evolution of RI in this system, divergence in mating system actually reduced RI, suggesting that mating-system evolution may play a significant role in speciation dynamics.

从理论上来说，适应性分化可通过两种途径提升内在合子后生殖隔离（intrinsic post-zygotic reproductive isolation, RI）：一是直接对与RI相关的基因座（loci）施加选择，二是通过不相容基因座（incompatibility loci）与受选择基因座之间的连锁产生间接效应。为验证这一假说，我们对十字花科（Brassicaceae）两节荠属（Cakile）和芝麻菜属（Erucaria）共18个类群（taxa）间的5种内在合子后生殖隔离屏障开展了RI水平检测。我们采用比较框架（comparative framework），检验了RI强度是否与遗传距离（genetic distance）、地理距离（geographic distance）、生态分化（ecological divergence）以及亲本交配系统（parental mating system）存在关联。研究发现，与F1存活力（F1 viability，即初始结籽率）相关的合子后RI早期阶段，其强度往往高于与F1繁殖力（F1 fecundity，即花数、果数）相关的后期阶段。交配系统对RI早期阶段存在显著影响：当母本为自交型、父本为异交型时，RI水平最低，这与库强度（sink strength）与养分供给（provisioning）抗性之间的失衡现象相一致。我们几乎未发现适应性分化可加速内在合子后RI演化的证据，这与非生态演化模型的预测相符——该模型认为，无论适应性分化与否，RI及生殖隔离不对称性（RI asymmetry）会随时间（即遗传距离）呈非线性积累（non-linear accumulation）。因此，尽管本研究体系中生态分化的某些维度并未对RI的演化产生显著贡献，但交配系统的分化反而降低了RI水平，这表明交配系统演化可能在物种形成动态（speciation dynamics）中发挥重要作用。