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 Borderea 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.