Hybridization between species can affect the strength of the reproductive barriers that separate those species. Two extensions of this effect are (1) the expectation that asymmetric hybridization or gene flow will have asymmetric effects on reproductive barrier strength and (2) the expectation that local hybridization will affect only local reproductive barrier strength and could therefore alter within‐species compatibility. We tested these hypotheses in a pair of morning glory species that exhibit asymmetric gene flow from highly selfing Ipomoea lacunosa into mixed‐mating Ipomoea cordatotriloba in regions where they co‐occur. Because of the direction of this gene flow, we predicted that reproductive barrier strength would be more strongly affected in I. cordatotriloba than I. lacunosa. We also predicted that changes to reproductive barriers in sympatric I. cordatotriloba populations would affect compatibility with allopatric populations of that species. We tested these predictions by measuring the strength of a reproductive barrier to seed set across the species’ ranges. Consistent with our first prediction, we found that sympatric and allopatric I. lacunosa produce the same number of seeds in crosses with I. cordatotriloba, whereas crosses between sympatric I. cordatotriloba and I. lacunosa are more successful than crosses between allopatric I. cordatotriloba and I. lacunosa. This difference in compatibility appears to reflect an asymmetric decrease in the strength of the barrier to seed set in sympatric I. cordatotriloba, which could be caused by I. lacunosa alleles that have introgressed into I. cordatotriloba. We further demonstrated that changes to sympatric I. cordatotriloba have decreased its ability to produce seeds with allopatric populations of the same species, in line with our second prediction. Thus, in a manner analogous to cascade reinforcement, we suggest that introgression associated with hybridization not only influences between‐species isolation but can also contribute to isolation within a species.

中文翻译：

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牵牛花物种同时出现与生殖隔离不对称和级联有关

物种之间的杂交会影响分离这些物种的生殖屏障的强度。该效应的两个扩展是（1）预期不对称杂交或基因流将对生殖屏障强度产生不对称影响，以及（2）预期局部杂交仅影响局部生殖屏障强度并因此可能改变种内相容性。我们在一对牵牛花物种中检验了这些假设，这些物种表现出不对称的基因流动，从高度自交的Ipomoea lacunosa到它们同时发生的区域的交配的Ipomoea cordatotriloba。由于此基因流的方向，我们预测生殖屏障强度将在脐带三叶草中受到更大的影响。比lacunosa。我们还预测，同胞三线虫种群中生殖屏障的变化将影响与该物种的异源种群的相容性。我们通过测量跨物种范围的种子结实生殖屏障的强度来测试这些预测。我们的第一个预测相一致，我们发现，同域和异域一lacunosa产生交叉相同数量的种子与一cordatotriloba，而同域之间的杂交一cordatotriloba和一lacunosa是比较成功的不是异域的杂交I. cordatotriloba和I. lacunosa。在兼容性这种差异似乎反映在阻挡到种子集同域的强度的非对称降低I. cordatotriloba，这可能由下列原因造成I. lacunosa已经渐渗入等位基因I. cordatotriloba。我们进一步证明，对同胞三带虫的改变降低了其产生具有相同物种的异源种群种子的能力，这与我们的第二个预测一致。因此，以类似于级联增强的方式，我们建议与杂交相关的渗入不仅影响物种之间的隔离，而且还可能有助于物种内的隔离。