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Maintenance of High Genetic Diversity during Invasion of Rhododendron ponticum

Adaptive processes in invasive plant populations may explain exotic range expansions and ecotypic shifts between native and invasive populations of the same species. Sufficient genetic variation in invasive populations occurring within the new range is a precondition for selection. While genetic diversity can be considerably reduced during invasions following founder effects and genetic drift, it can also be maintained via mechanisms of effective gene flow. Rhododendron ponticum L., an aggressive invader in the British Isles, is an appropriate test species for investigating changes in genetic diversity and differentiation. Six native populations in Georgia and Spain each and six invasive populations in Ireland were analyzed with amplified fragment length polymorphism markers to calculate genetic distances, genetic diversity, and differentiation within and among populations. While the highest level of genetic diversity was found within native Georgian populations, thus indicating a refugium, native Spanish and invasive Irish populations showed reduced levels of genetic diversity but displayed no further reduction in the invasive range. Genetic differentiation within countries was weak. Spanish and Irish populations showed close relationships supporting a probable Spanish provenance for the invasive Rhododendron. We suggest that the preservation of sufficient genetic variation in invasive R. ponticum populations in Ireland has been vital to the species’ adaptation to new environmental conditions.