Chloroplast DNA phylogeography of the hornbeam in Europe: Evidence for a bottleneck at the outset of postglacial colonization

The hornbeam, a shade tolerant tree species, has recolonised Europe very late during the Holocene. In order to investigate whether this postglacial recolonisation had led to a loss of chloroplast (cp) DNA diversity, as already described for other tree species, and especially the beech, another late-successional species, we have studied the phylogeography of the hornbeam using cpDNA. Three types of cpDNA markers were used (PCR-RFLP, microsatellites, sequences) to analyse 36 European populations of C. betulus and five populations of C. orientalis. Six haplotypes specific to C. betulus were detected, one of them completely fixed in all populations from northern and western Europe, whereas the others were restricted to the eastern European countries and to southern Italy. A remarkably high value of differentiation among populations was found (G(ST) = 0.972). Two different haplotypes specific to C. orientalis were detected, suggesting no ongoing gene flow between these two species, and similar refugia in Italy and in the Balkans for both Carpinus species. Pollen data are congruent with genetic data: during the last ice age, the hornbeam was restricted to the Balkans and to southern Italy and recolonised Europe northwards and westwards. Conservation priorities should target several populations from these regions that are hotspots of diversity and that represent a small part of the range of the species.