Data from: Immigrant and extrinsic hybrid seed inviability contribute to reproductive isolation between forest and dune ecotypes of Epipactis helleborine (Orchidaceae)

Reproductive isolation caused by divergent natural selection arising from differences between ecological environments or ecological interactions represents a major mechanism contributing to speciation, but its relative importance is poorly known. In this study, controlled reciprocal crossings and seed germination experiments were combined with genetic and morphometric analyses to test the hypothesis that previously described differences in mycorrhizal communities between forest and dune ecotypes of Epipactis helleborine were sufficiently strong to create complete reproductive isolation between the two ecotypes. The results showed that immigrant seeds had a significantly lower probability of protocorm formation than native seeds, indicating strong immigrant inviability. Although both ecotypes were able to cross easily and to produce a large number of viable seeds, hybrid seeds showed significantly lower protocorm formation than pure seeds, further contributing to reproductive isolation. Molecular analyses using 770 SNP markers showed that the two ecotypes were genetically distinct and that populations of the dune ecotype were genetically impoverished compared to populations of the forest ecotype. Morphologically, the two ecotypes were also significantly different, with plants of the dune ecotype generally being smaller than plants of the forest ecotype. Overall, these results indicate that interfertile populations of a widespread orchid adapting to contrasting environments diverge as a consequence of concurrent selection acting against immigrants and hybrids and suggest that mycorrhizal fungi can play a role in the early stages of plant speciation.

由生态环境差异或生态相互作用差异引发的趋异自然选择所导致的生殖隔离，是促成物种形成的核心机制之一，但其相对重要性迄今尚未被充分阐明。本研究通过将受控正反交、种子萌发实验与遗传及形态计量分析相结合，旨在验证如下假说：此前已报道的火烧兰（Epipactis helleborine）森林生态型与沙丘生态型之间的菌根群落（mycorrhizal communities）差异，足以使两类生态型之间产生完全的生殖隔离。研究结果显示，外来种子的原球茎（protocorm）形成概率显著低于本土种子，表明存在强烈的外来个体存活力缺陷。尽管两类生态型均可轻松杂交并产生大量可育种子，但杂交种子的原球茎形成率显著低于纯合种子，进一步强化了生殖隔离。采用770个单核苷酸多态性（SNP）标记开展的分子分析显示，两类生态型在遗传上存在显著分化；且相较于森林生态型种群，沙丘生态型种群的遗传多样性更为匮乏。形态学层面，两类生态型同样存在显著差异：沙丘生态型植株整体尺寸普遍小于森林生态型植株。综合来看，本研究结果表明：一类广布兰花的可互育种群在适应迥异环境的过程中，由于针对外来个体与杂交个体的协同选择作用而发生遗传分化；同时也提示，菌根真菌（mycorrhizal fungi）可在植物物种形成的早期阶段发挥重要作用。