Abstract
BackgroundIn 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.ResultsMicrosatellite 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.ConclusionsContrary 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0810-1) contains supplementary material, which is available to authorized users.
Highlights
In North America, the last ice age is the most recent event with severe consequences on boreal species’ ranges
Variation of genetic diversity Over the entire population, the number of alleles that were observed per locus ranged from 10 (PTR2 and WPMS16) to 30 (PMGC2571; Additional file 1: Table S1)
Significant M-ratio values were obtained for Discussion The purpose of this study was to ascertain whether the origin of trembling aspen in northwestern North America is reflected in the patterns of genetic diversity and population structure
Summary
In North America, the last ice age is the most recent event with severe consequences on boreal species’ ranges. 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. Earth has experienced several episodes of severe climatic variation that have led to a succession of ice ages during the Quaternary (2.58 Mya until present) [1] During this period, many species have experienced a succession of range expansions and contractions that have affected their genetic structure and diversity [1,2,3]. In North America, the last ice age is unquestionably the most recent event to have had severe consequences for boreal species genetic diversity [5]. Molecular studies have shown evidence of genetic structure for several North American plant species [9,10,11,12,13,14]
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