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.