Drought and waterlogging impede tree growth and may even lead to tree death. With climate change, these environmental factors are a growing source of concern, particularly for temperate forests. Oaks, an emblematic group of tree species, have evolved a range of adaptations to cope with these constraints. The two most widely distributed European species — pedunculate oak (PO) and sessile oaks (SO) — have overlapping ranges, but are highly constrained locally by soil water content variation. These differences in local ecological requirements provide a powerful biological model for studying the role of ecological barriers in speciation. We used an experimental set-up mimicking the ecological preferences of these species, in which seedlings were subjected to waterlogging and drought. We studied gene expression in roots by RNA-seq and identified genes differentially expressed between treatments with different outcomes depending on species. These “species x environment”-responsive genes revealed adaptive molecular strategies involving adventitious and lateral root formation, aerenchyma formation in PO, and osmoregulation and ABA regulation in SO. With this experimental design, we also identified genes with expression profiles presenting a “species” effect regardless of imposed constraints with important roles in intrinsic reproductive barriers. Finally, we compared our findings with those for a genome scan of species divergence and found that the candidate genes were enriched in highly differentiated SNPs. This suggests that many of the genes involved in the contrasting transcriptomic responses are subject to natural selection and that gene regulation helps to maintain these two different oak species in sympatry.

中文翻译：

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两种同域橡树种之间与水分胁迫相关的隔离屏障

干旱和内涝会阻碍树木的生长，甚至可能导致树木死亡。随着气候变化，这些环境因素越来越受到关注，尤其是温带森林。橡树是一个象征性的树种群体，已经进化出一系列适应性来应对这些限制。两种分布最广的欧洲物种——有花梗橡树 (PO) 和无梗橡树 (SO)——具有重叠的范围，但在当地受到土壤含水量变化的高度限制。当地生态要求的这些差异为研究生态屏障在物种形成中的作用提供了强大的生物学模型。我们使用了一个模拟这些物种生态偏好的实验装置，其中幼苗遭受了涝渍和干旱。我们通过 RNA-seq 研究了根中的基因表达，并确定了根据物种不同结果在不同处理之间差异表达的基因。这些“物种 x 环境”响应基因揭示了适应性分子策略，包括不定根和侧根形成、PO 中的通气组织形成以及 SO 中的渗透调节和 ABA 调节。通过这种实验设计，我们还确定了具有表达谱的基因，这些基因呈现出“物种”效应，而不管在固有生殖障碍中具有重要作用的强加限制。最后，我们将我们的发现与物种差异基因组扫描的结果进行了比较，发现候选基因富含高度分化的 SNP。