Genetic and morphological variation in the circumpolar distribution range of Sphagnum warnstorfii: indications of vicariant divergence in a common peatmoss

Abstract The Quaternary climatic oscillations caused pronounced changes in the distribution of the genetic variation among populations as well as phenotypic diversification worldwide. However, how important these processes have been in plants with high gene flow potential has been less studied. Sphagnum warnstorfii is a peatmoss species with a wide circumpolar distribution range exhibiting considerable morphological variation. In particular, many Arctic plants differ morphologically from plants in the rest of its distribution range. We used single nucleotide polymorphism (SNP) genotyping to investigate the patterns of genetic diversity in 112 plants from 105 localities sampled throughout the species distribution range and explored how this was correlated with phenotypic variation. Genetic cluster analysis identified two main genetic lineages with an average FST of 0.21 between them. The first cluster is restricted to the Arctic region, whereas the second has a wider distribution range covering the Arctic, boreal and boreo-nemoral regions of Eurasia and North America. We show that morphological variation is mostly concordant with patterns of genetic differentiation, and possibly representing adaptation to local environments. Based on approximate Bayesian computation simulations, we find that the two lineages probably diverged from each other before the Last Glacial Maximum (LGM). Our results show that vicariance due to glacial oscillations probably played a role for current patterns of diversification in a common peatmoss exhibiting a high gene flow potential.