East Asian origin of the widespread alpine snow-bed herb, Primula cuneifolia (Primulaceae), in the northern Pacific region

Aim: The evolutionary importance of southern mountain ranges has been noted for alpine as well as arctic–alpine plants. However, it remains unclear whether these isolated marginal populations of cold-adapted species have contributed to the establishment of their current widespread distribution. We aim to explore the molecular evidence for the recent northward migration of alpine snow-bed species in the northern Pacific region. Location: Japanese Archipelago, Kamchatka Peninsula, Alaska. Taxon: Primula cuneifolia Ledeb., the wedgeleaf primrose, distributed in high mountains across the northern Pacific region and its sister species, Primula nipponica Yatabe, which is endemic to northern Japan. Methods: The phylogenetic relationships and genetic structure of P. cuneifolia and its sister species were estimated using multiple loci from genotyping by random amplicon sequencing-direct (GRAS-Di) analyses. The temporal framework of genetic divergence was estimated using the isolation-with-migration model. The post-glacial range shift was inferred using approximate Bayesian computation and ecological niche modelling. Results: Phylogenetic analyses revealed that P. cuneifolia diverged into four clades, among which the southernmost clade in Japan diverged earlier than the northern clades. The Japanese clade likely diverged during the last glacial period, followed by further post-glacial divergence of clades in the Kamchatka Peninsula and Alaska. Main conclusion: Our study demonstrated that P. cuneifolia originated in Japan and spread northward into the northern Pacific region during and after the last glacial period. This finding provides the first evidence suggesting that population persistence and genetic divergence as well as speciation in the less glaciated mountains in Japan play important roles in the diversity and distribution of alpine plants in the northern Pacific region.