Glacial relict Sphagnum fuscum at the Pyrenean rear-edge: genetically depauperate and highly differentiated populations with evidence of a shift to monoicy

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 aim to reveal its regional glacial legacy and reproductive adaptations to extreme fragmentation by studying its distribution, demography, genetic structure, and reproductive biology. Based on these findings, we aim to inform conservation strategies. We genotyped 109 shoots from seven Pyrenean populations using 16 nuclear microsatellite markers. Genetic patterns were analyzed using AMOVA, STRUCTURE, PCoA and UPGMA dendrograms based on Cavalli-Sforza and Edwards’ chord distances. 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 central populations. Sporophyte production was widespread, although many populations were monoclonal, consistent with regional monoicy. Our results support that S. fuscum is a glacial relict in the Pyrenees. Post-glacial contraction explains its current genetic structure. A shift toward monoicy may reflect a local adaptation enhancing persistence under extreme fragmentation. 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.