Phylogeographic history of white spruce during the last glacial maximum: Uncovering cryptic refugia

Although a recent study of white spruce using chloroplast DNA uncovered the presence of a glacial refuge in Alaska, chloroplast failed to provide information on the number or specific localities of refugia. Recent studies have demonstrated the utility of nuclear microsatellites to refine insights into postglacial histories. The greater relative rate of mutation may allow finer scale resolution of historic dynamics, including the number, location, and sizes of refugia. Genetic data were acquired from screening 6 microsatellite loci on approximately 14 trees from each of 22 populations located across the central and western boreal forests of Canada and Alaska. Our studies combining microsatellites with Bayesian analyses of population structure in white spruce support the phylogeographic patterns uncovered using chloroplast, separating Alaskan from non-Alaskan regions. Results also support the idea that north-central Alaska served as a glacial refugium during the last glacial maximum. Additionally, the relationship between the degree of genetic differentiation and geographic distance indicated that gene flow played a more important role in structuring non-Alaskan populations, whereas drift played a more important role in structuring Alaskan populations (R_ST's for non-Alaskan populations 0.029 ± 0.007 and 0.083 ± 0.012 for Alaskan populations). Microsatellites also substantiate the bidirectional patterns of gene flow previously uncovered using chloroplast DNA but indicate much greater movement and mixing. Results from our Bayesian analyses also suggest the existence of additional cryptic refugia. However, the locations have been obscured by high gene flow (R_ST averaging 0.057 ± 0.004).