Abstract
As a result of recent or past evolutionary processes, a single species might consist of distinct Evolutionary Significant Units (ESUs), even corresponding to cryptic species. Determining the underlying mechanisms of range shifts and the processes at work in the build-up of divergent ESUs requires elucidating the factors that contribute to population genetic divergence across a species' range. We investigated the large-scale patterns of genetic structure in the perennial herbaceous plant species Silene nutans (Caryophyllaceae) in Western Europe. We sampled and genotyped 111 populations using 13 nuclear microsatellite loci and 6 plastid single-nucleotide polymorphisms. Broad-scale spatial population genetic structure was examined using Bayesian clustering, spatial multivariate analyses and measures of hierarchical genetic differentiation. The genotypic structure of S. nutans was typical of a predominantly allogamous mating system. We also identified plastid lineages with no intra-population polymorphism, mirroring two genetically differentiated nuclear lineages. No evidence of admixture was found. Spatial trends in genetic diversity further suggested independent leading-edge expansion associated with founding events and subsequent genetic erosion. Overall, our findings suggested speciation processes in S. nutans and highlighted striking patterns of distinct stepwise recolonisation of Western Europe shaped by Quaternary climate oscillations. Two main potential ESUs can be defined in Western Europe, corresponding to Eastern and Western nuclear-plastid lineages. In situ preservation of populations and genetic rescue implying ex situ conservation techniques should take the lineage identity into account. This is particularly true in Great Britain, northern France and Belgium, where S. nutans is rare and where distinct lineages co-occur in close contact.