Morphological and genetic divergence between two lineages of Magnolia salicifolia (Magnoliaceae) in Japan

Abstract

Uncovering how populations of a species differ genetically and ecologically is important for understanding evolutionary processes. We investigated genetic structure using nuclear microsatellites and chloroplast DNA sequences and geographical variation in leaf morphological traits among Magnolia salicifolia populations across its entire range. Two distinct lineages, northern and southern, were detected genetically, and both lineages had substructure among populations. The width/length ratio and area of leaves showed latitudinal gradients, while the position of maximum leaf width exhibited a discontinuous change between the lineages. Approximate Bayesian computation detected exponential population growth and stable population size from the past to the present in the northern and southern lineages, respectively. Small migrations between the lineages were inferred. Divergence time between the lineages was estimated to be the early to middle Pleistocene. Ecological niche modelling revealed a single large potential area of distribution on the Sea of Japan side and multiple intermittent areas on the Pacific Ocean side during the Last Glacial Maximum. We suggest that these distinct evolutionary histories of the northern and southern lineages after diversification have influenced not only neutral markers but also genes controlling leaf morphological traits.