The changing climate of the Tertiary and Pleistocene led to changes in the distribution of plant species, leading to instances of divergence and speciation. Later, some of these species came into secondary contact and formed hybrids. The Japanese archipelago is home to many examples of these complex evolutionary patterns between Japanese species and their continental counterparts, a phenomenon reinforced by the repeated formation of land bridges between the Korean Peninsula and West Japan during the late Tertiary and Pleistocene.
The East Asian shrub Magnolia sieboldii is one such species, with two subspecies: japonica, which is found in East China and West Japan, and sieboldii, which is primarily found in the Korean peninsula. The proximity of their ranges suggests either recent divergence followed by rapid speciation, or secondary contact with post-divergence hybridization.
In a recent article published in Annals of Botany, authors Satoshi Kikuchi and Yoko Osone attempted to uncover the evolutionary history of the two subspecies, including when the two diverged, whether secondary hybridization took place, and how each shifted their distribution to survive the Pleistocene. The researchers used low-copy nuclear genes, chloroplast DNA, and microsatellites, as well as ecological niche modelling.
The genetic analyses turned up a major east-west genetic subdivision, but with the boundaries of the gene pools incongruent between different genes. Only the nuclear low-copy genes delineated the actual subspecies. This incongruence is usually the result of either ancient hybridization or incomplete lineage sorting, though the analyses could not determine which in this case. “[T]o identify the evolutionary scenario that led to the phylogenetic incongruence in M. sieboldii, more detailed genomic information and more efficient simulation methods, such as forward-time simulations, are required in a future study,” write the authors.
Subspecies divergence was estimated at circa four million years ago, during the Pliocene. Ecological niche modelling showed significant niche differentiation, with winter precipitation a major factor in that differentiation. Subspecies japonica requires more humid environments with higher winter precipitation, while subsp. sieboldii lives in drier, more continental surroundings. “M. sieboldii subspecies typically grow in vegetation zones ranging from upper cool-temperate to lower cold-temperate coniferous forests,” write the authors. “During the last glacial cycle, both subspecies were predicted to persist in stable latitudinal distributions by migrating to lower altitudes.”