Background/Question/Methods
Molecular investigation of haploid organelles (chloroplast and mitochondria) can help assess vegetation response to shifting Quaternary climates. For example, chloroplast DNA analysis of North American white spruce revealed genetic differentiation between populations inside and outside of Alaska, explained by isolation in separate refugia long before the last glacial maximum. However, intricate details of postglacial patterns difficult to uncover using plastid genomes since the chloroplast is widely dispersed through wind-born pollen in conifers, and little to no genetic variation has been found in the mitochondria of several conifer species. Recent studies have demonstrated the utility of nuclear markers (in particular, microsatellites) in gaining insights into postglacial histories, suggesting that nuclear markers may be an underutilized resource.

Results/Conclusions
Investigation of six nuclear microsatellite loci in white spruce revealed that genetic relationships support the broad-scale phylogeographic patterns uncovered using chloroplast markers, separating Alaskan from non-Alaskan regions. Microsatellites also indicate that there were multiple refugia (at least three), and a mix of large and small refugial populations. In addition, the relationship between the degree of genetic differentiation and geographic distance within the two regions indicates that gene flow plays a more important role in structuring non-Alaskan populations while drift plays a more important role in structuring Alaskan populations. Microsatellite markers also substantiate the bi-directional patterns of gene flow previously uncovered using chloroplast markers but indicate much greater movement and mixing. As in several recent studies, these results indicate that microsatellites are useful markers for revealing patterns of postglacial histories that more conserved cytoplasmic markers cannot.