Patterns of seed dispersal in the wild sea beet (Beta vulgaris ssp. maritima) are predicted to be influenced by marine currents because populations are widely distributed along the European Atlantic coast. We investigated the potential influence of marine currents on the pattern of spatial genetic structuring in natural populations of sea beet. Populations were located along the French coasts of the Anglo-Norman gulf that features peculiar marine currents in the Channel. Thirty-three populations were sampled, among which 23 were continental and 10 were insular populations located in Jersey, Guernsey and Chausey, for a total of 1224 plants genotyped. To validate the coastal topography influence and the possibility of marine current orientated gene flow on the genetic features of sea beet populations, we assessed patterns of genetic structuring of cytoplasmic and nuclear diversity by: (i) searching for an isolation-by-distance (IBD) pattern using spatial autocorrelation tools; (ii) using the Monmonier algorithm to identify genetic boundaries in the area studied; and (iii) performing assignment tests that are based on multilocus genotype information to ascertain population membership of individuals. Our results showed a highly contrasted cytoplasmic and nuclear genetic differentiation and highlighted the peculiar situation of island populations. Beyond a classical isolation-by-distance due to short-range dispersal, genetic barriers fitting the orientation of marine currents were clearly identified. This suggests the occurrence of long-distance seed dispersal events and an asymmetrical gene flow separating the eastern and western part of the Anglo-Norman gulf.

Gene flow and introgression from cultivated to wild plant populations have important evolutionary and ecological consequences and require detailed investigations for risk assessments of transgene escape into natural ecosystems. Sugar beets (Beta vulgaris ssp. vulgaris) are of particular concern because: (i) they are cross-compatible with their wild relatives (the sea beet, B. vulgaris ssp. maritima); (ii) crop-to-wild gene flow is likely to occur via weedy lineages resulting from hybridization events and locally infesting fields. Using a chloroplastic marker and a set of nuclear microsatellite loci, the occurrence of crop-to-wild gene flow was investigated in the French sugar beet production area within a 'contact-zone' in between coastal wild populations and sugar beet fields. The results did not reveal large pollen dispersal from weed to wild beets. However, several pieces of evidence clearly show an escape of weedy lineages from fields via seed flow. Since most studies involving the assessment of transgene escape from crops to wild outcrossing relatives generally focused only on pollen dispersal, this last result was unexpected: it points out the key role of a long-lived seed bank and highlights support for transgene escape via man-mediated long-distance dispersal events.