Glacial relict Sphagnum fuscum at the Pyrenean rear-edge: Genetically depauperate and highly differentiated populations

Cold-adapted species often persist at their rear-edge in isolated refugia, where they are especially vulnerable to climate change. These populations frequently harbour unique genetic lineages and local adaptations, making them critical for biodiversity conservation. However, fine-scale intraspecific genetic and ecological data are rarely integrated into conservation planning. For the locally threatened peat moss Sphagnum fuscum at its southern rear-edge in the Pyrenees, we aimed to reveal its regional demographic status and genetic structure in order to inform conservation strategies. We genotyped 104 shoots from seven Pyrenean populations using 16 nuclear microsatellite markers. Genetic patterns were analysed using AMOVA, STRUCTURE, PCoA, MSN, and UPGMA and Complete Linkage dendrograms based on Bruvo’s distance. We also estimated population sizes and surveyed sporophyte production in the field. Pyrenean populations exhibited extremely low within-population diversity and complete genetic distinctiveness among them. Pyrenean genotypes shared a private allele distinctive from the populations of Canada and Norway. Sporophyte production was widespread, although many populations were monoclonal, probably consistent with regional monoicy. Pyrenean populations likely constitute an evolutionary significant unit, which should be prioritised in conservation planning. Our results provide a basis for designing a genetic rescue by promoting gene flow among these isolated populations.