Data from: Fine-scale assessment of genetic diversity of trembling aspen in northwestern North America

Background
In North America, the last ice age is the most recent event with severe consequences on boreal species’ ranges. Phylogeographic patterns of range expansion in trembling aspen (Populus tremuloides) suggested that Beringia is likely to be a refugium and the “ice-free corridor” in Alberta may represent a region where small populations persisted during the last glacial maximum (LGM). The purpose of this study was to ascertain whether the origins of trembling aspen in western North America are reflected in the patterns of neutral genetic diversity and population structure. A total of 28 sites were sampled covering the northwestern part of aspen’s distribution, from Saskatchewan to Alaska. Twelve microsatellite markers were used to describe patterns of genetic diversity. The genetic structure of trembling aspen populations was assessed by using multivariate analyses, Mantel correlograms, neighbor-joining trees and Bayesian analysis.
Results
Microsatellite markers revealed little to no neutral genetic structure of P. tremuloides populations in northwestern North America. Low differentiation among populations and small isolation by distance (IBD) were observed. The most probable number of clusters detected by STRUCTURE was K = 3 (∆K = 5.9). The individuals in the populations of the 3 clusters share a common gene pool and showed a high level of admixture. No evidence was found that either Beringia or the “ice-free corridor” were refugia. Highest allelic richness (AR) and lowest heterozygosity (Ho) were observed in Alberta foothills of the Rocky Mountains.
Conclusions
Contrary to our hypothesis, our results showed that microsatellite markers revealed little to no genetic structure in P. tremuloides populations. Consequently, no divergent populations were observed near supposed refugia. The lack of detectable refugia in Beringia and in the “ice-free corridor” was due to high levels of gene flow between trembling apsen populations. More favorable environmental conditions for sexual reproduction and successful trembling aspen seedling establishment may have contributed to increase allelic richness through recombination in populations from the Albertan foothills of the Rocky Mountains.

背景
在北美地区，末次盛冰期（Last Glacial Maximum, LGM）是对寒温带（boreal）物种种群分布范围造成严重影响的最近一次重大事件。美洲山杨（Populus tremuloides）的种群扩张谱系地理（phylogeographic）格局显示，白令陆桥（Beringia）大概率为其冰期避难所，而阿尔伯塔省的“无冰走廊”或许是末次盛冰期期间小型种群存续的区域。本研究旨在明确北美西部美洲山杨的起源是否能通过中性遗传多样性与种群结构格局得以反映。研究共布设28个采样点，覆盖山杨分布区的西北部区域，范围从萨斯喀彻温省延伸至阿拉斯加。采用12个微卫星标记（microsatellite markers）来阐释遗传多样性格局。通过多元分析、Mantel相关图、邻接系统发育树以及贝叶斯分析（Bayesian analysis），对美洲山杨种群的遗传结构进行评估。
结果
微卫星标记结果显示，北美西北部美洲山杨种群几乎不存在中性遗传结构。观测到种群间分化程度较低，且距离隔离（isolation by distance, IBD）效应微弱。通过STRUCTURE软件检测到的最可能聚类数为K=3（ΔK=5.9）。该3个聚类的种群内个体共享同一基因库，且表现出高度的遗传混合。未发现白令陆桥或“无冰走廊”作为冰期避难所的相关证据。在落基山脉的阿尔伯塔山麓区域，观测到最高的等位基因丰富度（allelic richness, AR）与最低的观测杂合度（observed heterozygosity, Ho）。
结论
与本研究的初始假设相悖，结果表明微卫星标记未检测到美洲山杨种群存在显著的遗传结构。因此，在推测的避难所附近未观测到分化程度较高的种群。白令陆桥与“无冰走廊”中未发现可检测到的避难所信号，这一现象可归因于美洲山杨种群间极高的基因流水平。落基山脉阿尔伯塔山麓种群的等位基因丰富度通过重组得以提升，这或许得益于该区域更适宜的有性繁殖环境以及成功的山杨幼苗定植条件。