Intraspecific genetic divergence of the subalpine shrubby variety Quercus crispula var. horikawae from the mountain tree variety Q. crispula var. crispula in Japan

Ecotypic divergence in tree taxa often occurs in subalpine habitats, where environmental conditions are more stressful than those in lower elevations. In the white oak species in Japan Quercus crispula, the subalpine shrubby variety Q. crispula var. horikawae (Qch) has been recognized in central and northern Honshu. Although Qch has different phenotypes from Q. crispula var. crispula (Qcc), genetic divergence between Qcc and Qch has not been examined yet. Pairs of Qcc and Qch populations in eight locations and additional Qcc and Qch populations around these locations were investigated. Leaf size of Qch was smaller than that of Qcc. Chloroplast DNA haplotypes were shared between the Qcc and Qch populations. In genotypes at 29 nuclear microsatellite loci, genetic diversity did not differ between the Qcc and Qch populations. Principal component analysis and a neighbor-joining tree of populations based on the genotypes demonstrated that 13 Qcc populations and eight Qch populations were grouped separately, except for three Qch populations that were grouped to Qcc. Climatic conditions in the eight Qch populations were characterized by lower temperature and heavier snowfall than those in the 16 populations of the genetic group of Qcc. These results suggest genetic divergence between Qcc and Qch associated with subalpine climatic conditions, irrespective of leaf size. The origin of the subalpine Qch lineage and the history of ecotypic divergence should be investigated in future genomic studies.