Spatial genetic structure and reproductive success in fragmented and continuous populations of Primula vulgaris

In Flanders (northern Belgium), Primula vulgaris, a self-incompatible long-lived perennial herb, is rare and consists of a network of fragmented populations in the intensively used agricultural landscape. We investigated genetic variation and structure using 27 allozyme loci in 41 populations, and reproductive success to assess the effect of fragmentation on gene flow and the influence of the nearest neighbouring (large and/or highly genetically diverse) population on within-population genetic variation and reproductive success. Isolation by distance was found among and within populations. Smaller and more isolated populations showed a slight loss of allelic variation, but maintained high levels of observed heterozygosity. They were not more differentiated from each other than large populations. No significant difference in the regression slopes of the spatial autocorrelation analysis was found between two continuous populations and two groups of fragmented populations With similar distance classes. Multiple regression showed that population allelic richness and reproductive success were higher when the nearest neighbouring population was genetically more diverse. These results suggest moderate current gene flow within and among populations rather than historical gene flow. We conclude that small and isolated populations of P. vulgaris should be considered not only as remnants of previously larger populations, but also as potential stepping stones insuring gene flow processes. For conservation, all highly variable and flowering populations should be considered, irrespective of their size or their isolation from large and continuous populations.