Genetic variation and evolutionary history of holly oak: a circum-Mediterranean species-complex [Quercus coccifera L./Q. calliprinos (Webb) Holmboe, Fagaceae]

Holly oak is the only evergreen oak possessing a circum-Mediterranean range; it has two predominant morphological forms, calliprinos and coccifera, described in the eastern and western Mediterranean Basin respectively. The concordance of allozyme and morphotype variation was analysed in the whole holly oak range, and the most plausible historical events responsible for the current geographic pattern of genetic variation were investigated. Individuals from 24 populations were scored for allozyme variation at seven polymorphic loci. Multi-locus genotypes were analysed by using a correspondence analysis (CA) and a Bayesian clustering approach. The relative positions of the populations were obtained from multi-dimensional scaling coupled with UPGMA treatment. A continuous genotype distribution was observed in the CA, and two groups were identified using the Bayesian approach. With a 0.95 threshold, 66 and 69% of the individuals showing the calliprinos and the coccifera morphotypes respectively were assigned to the corresponding genetic groups, which differed by private alleles. As compared to coccifera, the genetic group calliprinos was characterized by higher allelic richness and a strong geographical genetic structure. In agreement with fossil records, the most parsimonious explanation for lack of geographical structure in coccifera is a substantial regression of holly oak in the western Mediterranean Basin during the glaciations and a fast westward post-glacial expansion of coccifera populations, probably from Greece. Two population groups were obtained from the scaling/UPGMA treatment. One included all of the calliprinos populations and a Greek coccifera population, suggesting that the two morphotypes are closely related genetically and constitute two components of the same species.