Speciation and subsequent secondary contact in two edaphic endemic primroses driven by Pleistocene climatic oscillation.

Climatic perturbation during the Pleistocene era has played a major role in plant evolutionary history by altering species distribution range. However, the relative roles of climatic and geographic factors in the distribution dynamics remain poorly understood; in particular, the edaphic endemics. In this paper, we examine the evolutionary history of two ultramafic primroses, Primula hidakana and Primula takedana. These species are ecologically and morphologically distinct with disjunct distributions on Hokkaido Island, Japan. Primula hidakana is found on various rocks in southern Hokkaido and P. takedana in serpentine areas in northern Hokkaido. We performed population genetics analyses on nuclear and chloroplast data sets and tested alternative phylogenetic models of divergence using approximate Bayesian computation (ABC) analyses. Nuclear microsatellite loci clearly distinguished the two sister taxa. In contrast, chloroplast sequence variations were shared between P. takedana and P. hidakana. ABC analyses based on nuclear data supported a secondary contact scenario involving asymmetrical gene flow from P. hidakana to P. takedana. Paleodistribution modeling also supported the divergence model, and predicted their latitudinal range shifts leading to past secondary contact. Our findings highlight the importance of the distribution dynamics during the Pleistocene climatic oscillations in the evolution of serpentine plants, and demonstrate that tight species cohesion between serpentine and nonserpentine sister taxa has been maintained despite past interspecific gene flow across soil boundaries.