Patterns of weed invasion: evidence from the spatial genetic structure of Raphanus raphanistrum

Knowledge of the pathways of colonization is critical for risk assessment and management of weeds. In this study we adopted a landscape genetics approach to assess the impact of human disturbances and large-scale environmental features on the colonization of a global agricultural weed, Raphanus raphanistrum. We used nuclear microsatellite and chloroplast DNA sequence data to quantify the pattern of genetic diversity in 336 plants collected from 13 sites throughout the Cape Floristic Region, South Africa, one of the world’s recognized global biodiversity hotspots. The lack of strong spatial genetic structure suggests that R. raphanistrum colonized throughout the Cape Floristic Region via both local diffusive spread and long-distance jump dispersal. Furthermore, 47 % of analyzed plants contained Raphanus sativus (cultivated radish) chloroplast genomes, indicating historical and/or contemporary gene flow between wild and cultivated radish populations. The prevalence of high genetic diversity and long-distance gene flow are discussed in the context of ecological risk assessment.