Role of selection and gene flow in population differentiation at the edge vs. interior of the species range differing in climatic conditions

Evaluating the relative importance of neutral and adaptive processes as determinants of population differentiation across environments is a central theme of evolutionary biology. We applied the QST-FST comparison flanked by a direct test for local adaptation to infer the role of climate-driven selection and gene flow in population differentiation of an annual grass Avena sterilis in two distinct parts of the species range, edge and interior, which represent two globally different climates, desert and Mediterranean. In a multi-year reciprocal transplant experiment the plants of desert and Mediterranean origin demonstrated home advantage, and population differentiation in several phenotypic traits related to reproduction exceeded neutral predictions, as determined by comparisons of QST values with theoretical FST distributions. Thus variation in these traits likely resulted from local adaptation to desert and Mediterranean environments. The two separate common garden experiments conducted with different experimental design revealed that two population comparisons are likely to detect population differences in virtually every trait, but many of these differences reflect effects of local rather than regional environment. We detected a general reduction in neutral (SSR) genetic variation but not in adaptive quantitative trait variation in peripheral desert as compared with Mediterranean core populations. On the other hand, the molecular data indicated intensive gene flow from the Mediterranean core towards desert periphery. Although species range position in our study (edge vs. interior) was confounded with climate (desert vs. Mediterranean), the results suggest that the gene flow from the species core does not have negative consequences for either performance of the peripheral plants or their adaptive potential.

探究中性与适应性过程作为种群跨环境分化的决定因素的相对重要性，是进化生物学的核心研究主题。我们采用辅以本地适应直接检验的QST-FST比较方法，以推断气候驱动的选择与基因流在一年生草本植物异颖燕麦（*Avena sterilis*）种群分化中的作用。该物种的分布区包含边缘区与核心区两个截然不同的区域，分别对应荒漠与地中海两种全球典型气候。在多年互易移植实验中，源自荒漠与地中海生境的植株均表现出本地优势；多项与繁殖相关的表型性状的种群分化程度超出中性预测范围，该结论通过QST值与理论FST分布的对比得以验证。因此，这些性状的变异大概率源于对荒漠与地中海生境的本地适应。两项采用不同实验设计的独立同质园实验结果显示，尽管两种种群比较几乎可检测出所有性状的种群差异，但其中多数差异反映的是局域生境而非区域气候的影响。相较于地中海核心种群，我们在边缘荒漠种群中检测到以简单序列重复（Simple Sequence Repeat，SSR）为标记的中性遗传变异普遍降低，但适应性数量性状变异未出现类似变化。另一方面，分子数据显示存在从地中海核心种群向荒漠边缘种群的高强度基因流。尽管本研究中物种分布区位置（边缘区与核心区）与气候类型（荒漠与地中海）存在混淆，但结果表明，来自物种核心区的基因流并未对边缘种群植株的适合度表现或其适应性潜力产生负面影响。