Polyploidy, or whole-genome duplication, is a common speciation mechanism in plants. An important barrier to polyploid establishment is a lack of compatible mates. Because self-compatibility alleviates this problem, it has long been hypothesized that there should be an association between polyploidy and self-compatibility (SC), but empirical support for this prediction is mixed. Here, we investigate whether the molecular makeup of the Brassicaceae self-incompatibility (SI) system, and specifically dominance relationships among S-haplotypes mediated by small RNAs, could facilitate loss of SI in allopolyploid crucifers. We focus on the allotetraploid species Capsella bursa-pastoris, which formed ~300 kya by hybridization and whole-genome duplication involving progenitors from the lineages of Capsella orientalis and Capsella grandiflora. We conduct targeted long-read sequencing to assemble and analyze eight full-length S-locus haplotypes, representing both homeologous subgenomes of C. bursa-pastoris. We further analyze small RNA (sRNA) sequencing data from flower buds to identify candidate dominance modifiers. We find that C. orientalis-derived S-haplotypes of C. bursa-pastoris harbor truncated versions of the male SI specificity gene SCR and express a conserved sRNA-based candidate dominance modifier with a target in the C. grandiflora-derived S-haplotype. These results suggest that pollen-level dominance may have facilitated loss of SI in C. bursa-pastoris. Finally, we demonstrate that spontaneous somatic tetraploidization after a wide cross between C. orientalis and C. grandiflora can result in production of self-compatible tetraploid offspring. We discuss the implications of this finding on the mode of formation of this widespread weed.

多倍体或全基因组复制是植物中常见的物种形成机制。多倍体建立的一个重要障碍是缺乏相容的配偶。由于自亲和性缓解了这个问题，长期以来一直假设多倍体和自亲性 (SC) 之间应该存在关联，但对这一预测的实证支持喜忧参半。在这里，我们研究了十字花科植物自交不亲和性 (SI) 系统的分子构成，以及由小 RNA 介导的S-单倍型之间的显性关系，是否会促进异源多倍体十字花科植物中 SI 的丢失。我们专注于异源四倍体物种Capsella bursa-pastoris，通过杂交和全基因组复制形成约 300 kya，涉及来自东方荠和大花荠谱系的祖细胞。我们进行了靶向长读长测序，以组装和分析八个全长S -locus 单倍型，它们代表了毕氏酵母的两个同源亚基因组。我们进一步分析来自花蕾的小 RNA (sRNA) 测序数据，以确定候选优势修饰符。我们发现C. orientalis衍生的C. bursa-pastoris的S-单倍型具有雄性 SI 特异性基因SCR的截短版本并表达一个保守的基于 sRNA 的候选优势修饰符，其目标是C. grandiflora衍生的S单倍型。这些结果表明，花粉水平优势可能促进了毕氏酵母中 SI 的损失。最后，我们证明在侧柏和大花菊广泛杂交后自发的体细胞四倍体化可以产生自交相容的四倍体后代。我们讨论了这一发现对这种广泛分布的杂草形成模式的影响。