Glacial survival, phylogeography, and a comparison of microsatellite evolution models for resolving population structure in two species of dwarf yams (Borderea, Dioscoreaceae) endemic to the central Pyrenees

Abstract

Background: Borderea is a relict genus in the Pyrenees comprising only two extant allopolyploid species. The pre-Pyrenean, cliff-dweller B. chouardii is confined to a single critically endangered population, whereas the subalpine B. pyrenaica is more widely distributed in the mobile scree-habitats of the central Pyrenees and pre-Pyrenees. Aims: To determine the genetic structure and relationships among individuals and populations of both taxa to understand their origin. To compare two alternative evolutionary models, the Infinite Allele Model (IAM) and the Stepwise Mutation Model (SMM), in an examination of the primary factors responsible for population divergence during the Quaternary. Methods: A survey of nuclear microsatellite (Simple Sequence Repeat, SSR) variation was conducted to infer relationships among allelic phenotypes and genotypes and to evaluate the relative contributions of migration and mutation to population genetic structure. Results: The occurrence of shared ancestral alleles among populations of both Borderea species supported a presumably late-Tertiary divergence of the species in the low-altitudinal pre-Pyrenees. Borderea chouardii exhibited strong genetic differentiation between two sub-populations separated by only 150 m of vertical distance. In B. pyrenaica, populations from the pre-Pyrenees and southern Pyrenees showed greater genetic differentiation and more allelic diversity relative to populations from the northern Pyrenees. Conclusions: These results suggest that B. pyrenaica experienced glacial survival in warm southern pre-Pyrenean refugia followed by rapid postglacial migration towards the northern Pyrenees. Despite the a priori high suitability of the SMM for microsatellite markers, genotypic relationships among individuals and populations and genotypic assignments to predefined groups of B. pyrenaica were always better resolved by the IAM than by the SMM. This points towards the overwhelming influence of migration and genetic drift over mutation in the Quaternary history of the B. pyrenaica populations. However, for B. chouardii a similar pattern was shown by both evolutionary models, suggesting that sub-population divergence of this species could have also been favoured by mutation.