Wide-range analysis of genetic structure of Betula maximowicziana, a long-lived pioneer tree species and noble hardwood in the cool temperate zone of Japan

Betula maximowicziana is a long-lived pioneer tree species in Japanese cool temperate forests that plays an important role in maintenance of the forest ecosystem and has high economic value. Here we assess the wide-range genetic structure of 23 natural populations of B. maximowicziana using 11 simple sequence repeat (SSR) loci. Genetic diversity within populations was relatively low in all populations (mean H(E), 0.361; mean allelic richness, 2.80; mean rare allelic richness, 1.02). The population differentiation was also relatively low (F(ST), 0.062). Genetic distance-based and Bayesian clustering analysis revealed that the populations examined here could be divided into a southern group and a northern group. Analysis of rare allelic richness and Bayesian clustering revealed evidence for both southern and northern refugia during the last glacial period. Furthermore, a comparison of regional genetic diversity revealed significant clines in allelic richness. In spatial genetic structure evaluation, significant isolation by distance (IBD) was detected among the 23 populations, but not within regions. Moreover, significant population bottlenecks were found in all populations under infinite allele model (IAM) assumptions. These unusual, significant bottlenecks might be because of the processes of postglacial colonization and the species' characters and/or life history as a long-lived pioneer tree species. The wide-range, regional genetic structure found in this study provides an important baseline for conservation and forest management, including the identification of evolutionarily significant units (ESUs) and/or management units (MUs) of B. maximowicziana.