Extremely low genetic variability and highly structured local populations of Arabidopsis thaliana at higher latitudes

The genetic diversity and population structure of Arabidopsis thaliana populations from Norway were studied and compared to a worldwide sample of A. thaliana in order to investigate the demographic history and elucidate possible colonization routes of populations at the northernmost species limit. We genotyped 282 individuals from 31 local populations using 149 single nucleotide polymorphism (SNP) markers. A high level of population subdivision (FST = 0.85 ± 0.007) was found indicating that A. thaliana is highly structured at the regional level. Significant relationships between genetic and geographic distances were found, suggesting an isolation by distance mode of evolution. Genetic diversity was much lower and the level of linkage disequilibrium (LD) higher in populations from the north (65–68oN) compared to populations from the south (59–62oN); this is consistent with a northward expansion pattern. A neighbor-joining (NJ) tree showed that populations from northern Norway form a separate cluster, while the remaining populations are distributed over a few minor clusters. Minimal gene flow seems to have occurred between populations in different regions, especially between the geographically distant northern and southern populations. Our data suggest that northern populations represent a homogenous group that may have been established from a few founders during northward expansions, while populations in the central part of Norway constitute an admixed group established by founders of different origins, most probably as a result of human-mediated gene flow. Moreover, Norwegian populations appeared to be homogenous and isolated compared to a worldwide sample of A. thaliana, but they are still grouped with Swedish populations, which may indicate common colonization histories.

本研究针对挪威境内的拟南芥（Arabidopsis thaliana）种群开展遗传多样性与群体结构分析，并与全球范围的拟南芥样本进行对比，旨在探究其种群的演化历史，阐明该物种分布最北限种群的潜在定殖路径。本研究采用149个单核苷酸多态性（single nucleotide polymorphism, SNP）标记，对来自31个本地种群的282个个体进行基因分型。研究发现群体分化程度极高（FST = 0.85 ± 0.007），表明拟南芥在区域尺度上具有高度结构化的群体特征。检测到遗传距离与地理距离之间存在显著相关关系，提示其演化符合距离隔离模式。相较于北纬59°~62°的南部种群，北纬65°~68°的北部种群遗传多样性显著更低，而连锁不平衡（linkage disequilibrium, LD）水平更高，这与北向扩张的演化模式相符。邻接法（neighbor-joining, NJ）系统发育树显示，挪威北部的种群形成独立的演化支簇，其余种群则分散于数个小型支簇之中。不同区域的种群间基因交流极少，尤其是地理距离较远的北部与南部种群之间几乎无基因流动。本研究数据表明，北部种群属于同质化类群，可能是在北向扩张过程中由少数奠基者建立；而挪威中部种群则为混合类群，其奠基者来源多样，这极有可能是人类介导的基因流所导致的结果。此外，相较于全球拟南芥样本，挪威种群整体表现出同质化与隔离特征，但仍与瑞典种群聚为一类，这暗示二者拥有共同的定殖历史。