Parallel rapid phenotypic differentiation along climatic gradients in multiple introduced lineages of an alien plant species

Widespread plant species are faced with various climatic gradients and undergo clinal differentiation. Alien species which have been introduced repeatedly can be used to investigate the parallel processes of clinal differentiation during their range expansions. Cardamine hirsuta L. has been unintentionally introduced to Japan at least three times and has become widely distributed across Japan. To elucidate the processes of clinal differentiation, we conducted a common garden experiment using three lineages of C. hirsuta (North, East, and West). We tested for phenotypic differentiation among and within the lineages, and for a common pattern of clinal variations in phenotypic traits. We detected differentiations of flowering phenology and body size within and among the lineages. The East lineage showed delayed flowering phenology and larger mass, whereas the West lineage showed rapid flowering phenology and smaller mass. In addition, three patterns of variation between climate factors in seed source habitats and phenotypic traits were detected in all three lineages: higher temperature was related to earlier flowering and lower leaf number at the start of flowering, and more hours of sunshine were related to shorter flowering period. These patterns imply independent and parallel differentiations in the three lineages. These results suggest that analyses of distinct lineages allow repeated observations of differentiation processes and rapid responses of species during successful distribution in new environments.