Data from: Maintenance of species boundaries in a Neotropical radiation of Begonia

A major goal of evolutionary biology is to determine the mechanisms generating biodiversity. In Begonia, one of the largest plant genera (1900+ species), it has been postulated that the high number of endemic species is a by-product of low gene flow among populations, which predisposes the group to speciation. However, this model of divergence requires that reproductive barriers accumulate rapidly among diverging species that overlap in their geographic ranges, otherwise speciation will be opposed by homogenizing gene flow in zones of secondary contact. Here, we test the outcomes of secondary contact in Begonia by genotyping multiple sympatric sites with 12 nuclear and 7 plastid loci. We show that three sites of secondary contact between B. heracleifolia and B. nelumbiifolia are highly structured, mostly containing parental genotypes, with few F1 hybrids. A sympatric site between B. heracleifolia and B. sericoneura contains a higher proportion of F1s, but little evidence of introgression. The lack of later-generation hybrids contrasts with that documented in many other plant taxa, where introgression is extensive. Our results, in conjunction with previous genetic work, show that Begonia demonstrate properties making them exceptionally prone to speciation, at multiple stages along the divergence continuum. Not only are populations weakly connected by gene flow, promoting allopatric speciation, but species often show strong reproductive barriers in secondary contact. Whether similar mechanisms contribute to diversification in other large genera remains to be tested.进化生物学的核心目标之一，是阐明驱动生物多样性产生的内在机制。作为全球物种数量最多的植物属之一（逾1900种），秋海棠属（Begonia）中曾有假说提出：其丰富的特有物种数量，是种群间基因流（gene flow）水平较低的副产物，而这一特征会促使该类群发生物种形成。不过，这一分化模型要求：在地理分布重叠的分化物种间，生殖隔离（reproductive barrier）需快速累积；否则在次生接触带（secondary contact zone）中，同质化的基因流会抵消物种形成过程。本研究通过对12个核基因座（nuclear locus）与7个质体基因座（plastid locus）进行基因分型，对秋海棠属的多个同域分布位点的次生接触结果进行验证。研究结果显示，厚叶秋海棠（Begonia heracleifolia）与莲叶秋海棠（Begonia nelumbiifolia）之间的3处次生接触位点具有高度的遗传结构，种群中大多为亲本基因型，仅存在极少量F1代杂种。而厚叶秋海棠（B. heracleifolia）与丝毛秋海棠（Begonia sericoneura）的一处同域分布位点中，F1代杂种占比更高，但未检测到明显的基因渐渗（introgression）现象。该类群缺乏后续世代杂种的现象，与诸多已报道的存在广泛基因渐渗的其他植物类群形成鲜明对比。结合既往遗传学研究，本研究结果表明：秋海棠属在物种分化连续体的多个阶段，均展现出极易发生物种形成的特性。其种群间不仅因基因流微弱而连通性差，促进了异域物种形成（allopatric speciation），且不同物种在次生接触带中往往存在极强的生殖隔离。其他大型植物属是否也存在类似的物种分化机制，仍有待后续研究验证。