Genetic diversity within populations of an arctic–alpine species declines with decreasing latitude across the Northern Hemisphere

Aim: We identified and evaluated general latitudinal trends in genetic diversity within populations of a widespread arctic–alpine plant, Dryas octopetala, to examine the applicability to this species of the dominant hypothesis that intraspecific genetic diversity is highest in the tropics and declines towards the poles. Location: The circumpolar Arctic and northern temperate alpine ranges, with a focus on high altitude mountains at the species lowest latitudinal margin in the Japanese archipelago. Methods: Within-population genetic diversity was assessed using genotypes determined at nine microsatellite loci (n = 489), chloroplast DNA sequences (atpB-rbcL and trnV-ndhC spacers, n = 181) and a nuclear gene sequence (LEAFY, n = 173) of 18 populations, as well as a previously published amplified fragment length polymorphism data-set for 26 populations, across the distributional range of the species. The latitudinal pattern of intra-population genetic diversity was modelled at hemispheric scale to discriminate linear latitudinal and quadratic central-marginal trends in genetic diversity. Population genetic structure was assessed by Bayesian clustering analyses. Results: At hemispheric scale, we identified two interacting effects: a general latitudinal decline in genetic diversity towards the equator and a central–marginal effect, whereby genetic diversity decreases towards the margins of a species' range. This decrease was more marked in low-latitude marginal populations than in high-latitude marginal populations. Populations at the lowest latitudes in the Japanese archipelago showed the lowest level of genetic diversity but exhibited distinctive genetic variation. Main conclusion: The latitudinal decline in genetic diversity within populations of this arctic–alpine plant across its range was opposite to the commonly observed trend. A significant part of the equator-ward latitudinal decline in genetic diversity in this arctic–alpine species may be attributable to a "sky island" effect, which played a greater role at low latitudes.