Data from: The large-X effect in plants: increased species divergence and reduced gene flow on the Silene X-chromosome

The disproportionately large involvement of the X-chromosome in the isolation of closely related species (the large-X effect) has been reported for many animals, where X-linked genes are mostly hemizygous in the heterogametic sex. The expression of deleterious recessive mutations is thought to drive the frequent involvement of the X-chromosome in hybrid sterility, as well as to reduce interspecific gene flow for X-linked genes. Here, we evaluate the role of the X-chromosome in the speciation of two closely related plant species – the white and red campions (Silene latifolia and S. dioica) – that hybridize widely across Europe. The two species evolved separate sexes and sex chromosomes relatively recently (~107 years), and unlike most animal species, most X-linked genes have intact Y-linked homologs. We demonstrate that the X-linked genes show a very small and insignificant amount of interspecific gene flow, while gene flow involving autosomal loci is significant and sufficient to homogenise the gene pools of the two species. These findings are consistent with the hypothesis of the large-X effect in Silene and comprise the first report of this effect in plants. Non-hemizygosity of many X-linked genes in Silene males indicates that exposure of recessive mutations to selection may not be essential for the occurrence of the large-X effect. Several possible causes of the large-X effect in Silene are discussed.

X染色体在近缘物种生殖隔离中的异常突出作用（large-X effect，大X效应）已在多种动物类群中被报道：此类动物的X连锁基因（X-linked genes）大多在异配性别（heterogametic sex）中呈半合子状态。有害隐性突变的表达被认为是推动X染色体频繁参与杂交不育（hybrid sterility）、并降低X连锁基因种间基因流（interspecific gene flow）的核心机制。本研究针对两种在欧洲范围内广泛发生杂交的近缘植物物种——白剪秋萝（Silene latifolia）与红剪秋萝（S. dioica），评估X染色体在其物种形成过程中的作用。这两个物种相对较近（约10^7年前）才演化出雌雄异株与性染色体（sex chromosomes），且与多数动物类群不同，其多数X连锁基因拥有完整的Y连锁同源拷贝（Y-linked homologs）。本研究证实：X连锁基因的种间基因流程度极低且无统计学显著性，而常染色体位点（autosomal loci）的基因流则显著且足以使两个物种的基因库（gene pools）发生同质化。上述发现与剪秋萝属（Silene）的大X效应假说相符，同时也是该效应在植物类群中的首次报道。剪秋萝雄性个体内多数X连锁基因并非半合子状态，这表明隐性突变暴露于选择压力下或许并非大X效应发生的必要条件。本研究最后讨论了剪秋萝属中大X效应产生的数种潜在诱因。