Population structure over a broad spatial scale driven by non-anthropogenic factors in a wide-ranging migratory mammal, Alaskan caribou

Wide-ranging mammals face significant conservation threats, and knowledge of the spatial scale of population structure and its drivers is needed to understand processes that maintain diversity in these species. We analysed DNA from 655 Alaskan caribou (Rangifer tarandus granti) from 20 herds that vary in population size, used 19 microsatellite loci to document genetic diversity and differentiation in Alaskan caribou, and examined the extent to which genetic differentiation was associated with hypothesized drivers of population subdivision including landscape features, population size and ecotype. We found that Alaskan caribou are subdivided into two hierarchically structured clusters: one group on the Alaska Peninsula containing discrete herds and one large group on the Mainland lacking differentiation between many herds. Population size, geographic distance, migratory ecotype and the Kvichak River at the nexus of the Alaska Peninsula were associated with genetic differentiation. Contrary to previous hypotheses, small Mainland herds were often differentiated genetically from large interconnected herds nearby, and genetic drift coupled with reduced gene flow may explain this pattern. Our results raise the possibility that behaviour helps to maintain genetic differentiation between some herds of different ecotypes. Alaskan caribou show remarkably high diversity and low differentiation over a broad geographic scale. These results increase information for the conservation of caribou and other migratory mammals threatened by population reductions and landscape barriers and may be broadly applicable to understanding the spatial scale and ecological drivers of population structure in widespread species.

广布型哺乳动物面临严峻的保护威胁，要阐明维持这类物种种内多样性的演化过程，亟需明晰其种群结构的空间尺度及其驱动因素。本研究采集了20个种群规模各异的阿拉斯加驯鹿（*Rangifer tarandus granti*）共655份个体的DNA，利用19个微卫星位点解析了阿拉斯加驯鹿的遗传多样性与遗传分化水平，并探究了遗传分化与种群分化的假说驱动因素（包括景观特征、种群规模与生态型）之间的关联程度。研究结果显示，阿拉斯加驯鹿可划分为两个具有层级结构的遗传聚类群：一类分布于阿拉斯加半岛，包含多个分化显著的种群；另一类分布于阿拉斯加大陆主体，多数种群间未呈现明显遗传分化。种群规模、地理距离、迁徙生态型以及位于阿拉斯加半岛枢纽地带的基维克查克河，均与遗传分化显著相关。与既往假说相悖的是，阿拉斯加大陆主体上的小型种群往往与其周边存在基因交流的大型种群之间存在显著遗传分化，遗传漂变伴随基因流降低或可解释这一现象。本研究结果提示，行为模式或有助于维持不同生态型的部分种群间的遗传分化。阿拉斯加驯鹿在广阔的地理范围内展现出极高的遗传多样性与较低的种群分化水平。本研究结果可为受种群缩减与景观阻隔威胁的驯鹿及其他迁徙哺乳动物的保护工作提供更丰富的科学依据，同时也可为解析广布物种种群结构的空间尺度与生态驱动因素提供普适性参考。