Rear-edge, low-diversity, and haplotypic uniformity in cold-adapted Bupleurum euphorbioides interglacial refugia populations

The high genetic diversity of rear-edge refugia populations is predicted to have resulted from species repeatedly migrating to low latitudes during glacial periods over the course of Quaternary climate change. However, several recent empirical studies of cold tolerant plants revealed the opposite pattern. We investigated whether current habitats of the cold-adapted and range-restricted Bupleurum euphorbioides in the Baekdudaegan, South Korea and North Korea could be interglacial refugia, and documented how their rear-edge populations differ genetically from those of typical temperate species. Phylogeographic analysis and ecological niche modeling (ENM) were used. The genetic structure was analyzed using microsatellite markers and chloroplast DNA sequences. The congener B. longiradiatum was included as a typical temperate plant species. Despite having almost identical life history traits, these congeneric species exhibited contrasting patterns of genetic diversity. ENM revealed an apparent maximum range contraction during the last interglacial. In contrast, its range expanded northward to the Russian Far East (Primorsky) during the Last Glacial Maximum. Thus, we hypothesize that B. euphorbioides retreated to its current refugia during interglacial periods. Unlike populations in the central region, the rear-edge populations were genetically impoverished and uniform, both within populations and in pooled regional populations. The rear-edge B. euphorbioides survived at least one past interglacial, contributing to the species' genetic diversity. We believe that such genetic variation in the cold-adapted B. euphorbioides gives the species the necessary adaptations to survive an upcoming favorable environment (the next glacial), unless there is artificial environmental change.