The influence of contemporary and historic landscape features on the genetic structure of the sand dune endemic, Cirsium pitcheri (Asteraceae)

Narrow endemics are at risk from climate change due to their restricted habitat preferences, lower colonization ability and dispersal distances. Landscape genetics combines new tools and analyses that allow us to test how both past and present landscape features have facilitated or hindered previous range expansion and local migration patterns, and thereby identifying potential limitations to future range shifts. We have compared current and historic habitat corridors in Cirsium pitcheri, an endemic of the linear dune ecosystem of the Great Lakes, to determine the relative contributions of contemporary migration and post-glacial range expansion on genetic structure. We used seven microsatellite loci to characterize the genetic structure for 24 populations of Cirsium pitcheri, spanning the center to periphery of the range. We tested genetic distance against different measures of geographic distance and landscape permeability, based on contemporary and historic landscape features. We found moderate genetic structure (ave Fst =0.14), and a north -south pattern to the distribution of genetic diversity and inbreeding, with northern populations having the highest diversity and lowest levels of inbreeding. High allelic diversity, small average pairwise distances and mixed genetic clusters identified in Structure suggest populations in the center of the range represent the point of entry to the Lake Michigan and a refugia of diversity for this species. A strong association between genetic distances and lake level changes suggests that historic lake fluctuations best explain the broad geographic patterns, and sandy habitat best explain local patterns or movement.

狭域特有种（narrow endemics）因生境偏好受限、定殖能力薄弱以及扩散距离有限，正面临气候变化带来的生存风险。景观遗传学（Landscape genetics）整合了新型研究工具与分析方法，可用于探究历史与当前的景观特征如何促进或阻碍了物种既往的分布范围扩张与局地迁移模式，进而明确未来分布范围变动可能面临的限制因素。本研究以五大湖线性沙丘生态系统特有种皮氏蓟（Cirsium pitcheri）为研究对象，对比了其当前与历史时期的生境廊道，以解析当代迁移与冰期后分布范围扩张对其遗传结构的相对贡献。本研究利用7个微卫星位点（microsatellite loci），对覆盖该物种分布范围中心至边缘的24个皮氏蓟种群的遗传结构进行了表征。基于当代与历史时期的景观特征，本研究针对遗传距离（genetic distance）与不同维度的地理距离、景观通透性（landscape permeability）指标进行了相关性检验。研究结果显示该物种存在中等程度的遗传结构（平均Fst=0.14），且遗传多样性与近交程度的分布呈现南北梯度格局：北部种群拥有最高的遗传多样性，同时近交水平最低。较高的等位基因多样性、较小的平均成对遗传距离，以及Structure软件鉴定出的混合遗传聚类结果表明，分布范围中心的种群是该物种进入密歇根湖（Lake Michigan）的入侵点，同时也是其遗传多样性的避难所。遗传距离与湖泊水位变化之间存在显著关联，这表明历史时期的湖泊水位波动可较好解释该物种的大尺度地理分布格局，而沙生生境则可较好解释其局地迁移模式与扩散过程。