Are road verges corridors for weed invasion? Insights from the fine-scale spatial genetic structure of Raphanus raphanistrum

Raphanus raphanistrum (Brassicaceae) is considered amongst the world's worst agricultural weeds. We address critical issues in its management by studying the pathway of colonisation at local scales. For this, we assessed the small-scale spatial genetic structure of 231 samples collected from three different sites across the Cape Floristic Region, South Africa, using 11 nuclear microsatellite markers. Although natural pollen and seed dispersal were expected to be restricted, we found no significant relationship between genetic and geographical distance within sites. Instead, our results suggest that R. raphanistrum had colonised new habitats via jump dispersal, rather than through natural diffusive dispersal at local scales. We did not find evidence for road verges as dispersal corridors, as evidenced by a lack of isolation-by-distance at local scales. Instead, the absence of spatial genetic structure suggests that R. raphanistrum had rapidly spread throughout its current range, possibly facilitated by human-mediated actions. Management plans addressing containment or suppression of the weedy species R. raphanistrum (and possibly other weedy species) should take the high degree of connectivity between distant geographical localities into account.