White oaks phylogeography in the Iberian Peninsula

Abstract

The geographic distribution of maternally inherited chloroplast DNA polymorphisms was studied to determine the phylogeographic structure of white oaks in the Iberian Peninsula. Almost 1000 mature trees from nearly 200 populations were sampled in the distribution areas of the six white oak species that are recognised in Iberia. The analysed trees roughly represent the species distribution and their respective abundance. The PCR–RFLP procedure used by all project partners revealed up to 14 cpDNA haplotypes. Seven of them had not been previously described and have not been found elsewhere in Europe. Phylogenetic relationships among the haplotypes support the existence of four maternal lineages, one of them being restricted in Europe to the Mediterranean Spain and Corsica. Genetic diversity analysis confirmed the previously described extensive sharing of haplotypes among species, but also detected some species effect in intra- and inter-specific cytoplasmic gene flow. Geographic structuration of genotypes was studied by means of autocorrelation analysis. Autocorrelation was significant for ordered and unordered alleles both when considering all populations and when analysis was restricted to lineage B. However, in both instances there was not significant autocorrelation for the contribution of the genetic divergence between haplotypes to the total coefficient of differentiation. Autocorrelograms reflect the existence of a patchy distribution of haplotypes at different scales when all lineages are considered and when only lineage B is taken into account. Finally, discussion centres on the possible role played by the Iberian Peninsula as the European westernmost refugia for the white oaks during the last glacial period. We present several lines of evidence that make us to favour the persistence of white oaks in small northern refugia during the full glacial period.