Candidate gene SNP variation in floodplain populations of pedunculate oak (Quercus robur L.) near the species' southern range margin: weak differentiation yet distinct associations with water availability

Populations residing near species' low-latitude range margins (LLM) often occur in warmer and drier environments than those in the core range. Thus, their genetic composition could be shaped by climatic drivers that differ from those occurring at higher latitudes, resulting in potentially adaptive variants of conservation value. Such variants could facilitate the adaptation of populations from other portions of the geographic range to similar future conditions anticipated under ongoing climate change. However, very few studies have assessed standing genetic variation at potentially adaptive loci in natural LLM populations. We investigated standing genetic variation at SNPs located within 117 candidate genes and its links to putative climatic selection pressures across 19 pedunculate oak (Quercus robur L.) populations distributed along a regional climatic gradient near the species' southern range margin in southeastern Europe. These populations are restricted to floodplain forests along large lowland rivers, whose hydric regime is undergoing significant shifts under modern rapid climate change. The populations showed very weak geographic structure, suggesting extensive genetic connectivity and gene flow or shared ancestry. We identified eight (6.2%) positive FST-outlier loci, and genotype-environment association analyses revealed consistent associations between SNP allele frequencies and several climatic variables linked to water availability. A total of 61 associations involving 37 SNPs (28.5%) from 35 annotated genes provided important insights into putative functional mechanisms in our system. Our findings provide empirical support for the role of LLM populations as sources of potentially adaptive variation that could enhance species’ resilience to climate change-related pressures.

栖息于物种低纬度分布边缘（low-latitude range margins, LLM）的种群，相较于分布于物种核心分布区的种群，通常所处环境更为温暖干旱。因此，这些种群的遗传组成可能受到与高纬度地区不同的气候驱动因子影响，进而产生具有保育价值的潜在适应性变异。此类变异或可帮助地理分布区内其他区域的种群，适应未来气候变化背景下预期的类似环境条件。然而，目前极少有研究针对自然LLM种群中潜在适应性位点的现存遗传变异展开评估。本研究针对分布于欧洲东南部物种南部分布边缘附近、沿区域气候梯度分布的19个夏栎（*Quercus robur* L.）种群，对117个候选基因内的单核苷酸多态性（single nucleotide polymorphism, SNP）位点的现存遗传变异，及其与推定气候选择压力的关联展开了调查。这些种群仅分布于大型低地河流沿岸的泛滥平原森林中，而该类森林的水文格局正随着当代快速气候变化发生显著改变。种群间的地理结构极弱，这表明其存在广泛的遗传连通性、基因流或共同祖先。本研究共鉴定出8个（占比6.2%）显著FST异常位点，基因型-环境关联分析结果显示，SNP等位基因频率与多个与水分可利用性相关的气候变量间存在稳定关联。总计61个关联涉及35个已注释基因中的37个SNP位点（占比28.5%），为研究体系中的推定功能机制提供了重要见解。本研究结果为LLM种群作为潜在适应性变异来源、可提升物种应对气候变化相关压力的抗逆能力这一观点提供了实证支持。