Species Boundaries and Population Divergence in the Pyrenean Endemic Relict Genus Borderea (Dioscoreaceae) as Revealed by Microsatellite (SSR) and Other Hypervariable Markers

Microsatellite alleles were used to delimit the genetic boundaries and divergence of the two relictual endemic Pyrenean taxa Borde rea chouardii and B. pyrenaica (Dioscoreaceae ), and to infer the different life histories followed by each species. Our study was conducted on the same populations previously analyzed with allozymes and RAPD markers. The three studied data sets were congruent in the inference of a single evolutionary scenario for the split of the two Borderea taxa from a common Tertiary ancestor in the Prepyrenees, thus supporting their taxonomic treatment as separate species. However, the more variable SSR and RAPD data provided better resolution for a stepping-stone model of local colonization of B. pyrenaica populations from southern Prepyrenean refugia to the northern Pyrenees. SSR markers proved to be more robust than RAPD markers in assessing the genetic structure of recently diverged populations of B. pyrenaica and thus qualified as the best molecular markers for fine-scale evolutionary investigations of Dioscoreaceae. Furthermore, microsatellites rendered unique clues to decipher the mechanisms involved in the origin of these relictual species and their genetic background. Borderea was shown to be a tetraploid genus of hybrid origin with a chromosome base number of x = 6. Phylogenetic data, karyological evidence, and our present knowledge based on microsatellite analyses allowed us to speculate that the Pyrenean endemic genus Borderea and its sister taxon, the Mediterranean genus Tamus, represent some of the oldest paleopolyploid lineages of the mostly pantropical yam family.