Large-scale patterns in genetic variation, gene flow and differentiation in five species of European Coenagrionid damselfly provide mixed support for the central-marginal hypothesis

Recently, an increased effort has been directed towards understanding the distribution of genetic variation within and between populations, particularly at central and marginal areas of a species' distribution. Much of this research is centred on the central-marginal hypothesis, which posits that populations at range margins are sparse, small and genetically diminished compared to those at the centre of a species' distribution range. We tested predictions derived from the central-marginal hypothesis for the distribution of genetic variation and population differentiation in five European Coenagrionid damselfly species. We screened genetic variation (microsatellites) in populations sampled in the centre and margins of the species' latitudinal ranges, assessed genetic diversity (HS) in the populations and the distribution of this genetic diversity between populations (FST). We further assessed genetic substructure and migration with Bayesian assignment methods, and tested for significant associations between genetic substructure and bioclimatic and spatial (altitude and latitude) variables, using general linearized models. We found no general adherence to the central-marginal hypothesis; instead we found that other factors such as historical or current ecological factors often better explain the patterns uncovered. This was illustrated in Coenagrion mercuriale whose colonisation history and behaviour most likely led to the observation of a high genetic diversity in the south and lower genetic diversity with increasing latitude, and in C. armatum and C. pulchellum whose patterns of low genetic diversity coupled with the weakest genetic differentiation at one of their range margins suggested, respectively, possible range shifts and recent, strong selection pressure.