Nunatak survival vs. tabula rasa in the Central Pyrenees:: a study on the endemic plant species Borderea pyrenaica (Dioscoreaceae)

Aim Borderea pyrenaica (Dioscoreaceae) is a Tertiary relict plant endemic to the Central Pyrenees. Because of its narrow distribution in a small geographical area and the fact that it is restricted to high alpine habitats, it constitutes an ideal model species for inferring the historical dynamics of population survival and migration during and after Quaternary glaciations in the Pyrenees. Location Central Pyrenees and pre-Pyrenees, Spain-France. Methods Eleven primer pairs were used to amplify 18 microsatellite loci in this allotetraploid species in a sample of 804 individuals from 15 populations, revealing a total of 77 alleles. Genotypic data of individuals and populations were analysed using clustering and Bayesian methods of analysis of population structure. Results A higher number of private alleles and a significantly higher allelic richness (A*) were found in the southern area (21, A* = 2.295) than in the northern area (5, A* = 1.791). Furthermore, the allelic composition of the northern area represented a subset of that from the southern area. Main conclusion The hypothesis of in situ survival in northern Pyrenean nunataks was rejected, while peripheral refugia were considered to be restricted to the southern Pyrenees and pre-Pyrenees, where historical geographical fragmentation probably caused the divergence among southern Pyrenean populations. Molecular evidence indicates that these refugial populations probably colonized the northern area after sheet-ice retreat. Borderea pyrenaica lineages followed two migratory pathways in their northward colonization, suggesting several founder events for the populations that eventually reached the territory of the Gavarnie cirque.