Can gene flow among populations counteract the habitat loss of extremely fragile biotopes? An example from the population genetic structure in Salix daphnoides

Wild river gravel banks (RGB) represent an extremely fragile biotope that is significantly endangered by human activities due to its fragmentation over the past century. The consequences of such processes were studied using the endangered violet willow (Salix daphnoides) at the westernmost foothills of the Carpathian Mountains. We quantified population genetic characteristics for 14 ecologically and demographically characterised populations using simple sequence repeats (SSR) and amplified fragment length polymorphism (AFLP) markers. We found a significant correlation between the biotope, sex, age and genetic structure of the populations. The natural RGB populations revealed high genotypic variability when using SSR markers, in contrast to low genotypic variability of the populations of other biotopes of Cirsium wet meadows (CWM) and Ash-alder forests (AAF) that consisted of one to two clones at each site. High heterozygosity (H obs = 0.428–0.532) was similar across all natural (RGB) populations; however, these populations were deficient in heterozygotes (F IS/ρ IS > 0). All RGB populations exhibited moderate to very significant genetic differentiation for microsatellites, despite the fact that the AFLP data showed strong differentiation only between CWM and AAF populations. Division into clusters by Structure confirmed consistent geographic groups for the RGB populations. According to our results, the strong decrease in previously continuous and large natural habitats for the violet willow is slightly counterbalanced by among-population gene flow. However, the survival of natural populations is tightly linked to the presence of river gravel banks and bars. Strict protection of this habitat is therefore essential for the conservation of the species.