Dissecting the continuum and unravelling the phylogeographic knot of plastid DNA in European white oaks (Quercus sect. Quercus): ancient signatures and multiple diversity reservoirs

Available knowledge on the European white oaks (Quercus, sect. Quercus) plastome still exhibits large gaps, mostly in respect of detailed phylogeny and information from southern Europe and adjacent Near East. We investigated DNA sequence polymorphism at two plastid loci on 270 individuals representing 13 white oak species/taxa and retrieved several hundred sequences of the same two DNA regions from GenBank to fill the above gaps, and contribute to a better understanding of white oak diversity and evolution. The modern European sect. Quercus species exhibits a poorly differentiated plastid DNA, despite its current highly diversified taxonomy. Twenty-nine haplotypes were identified across the Euro-Mediterranean and Near East regions, including two ancestral variants that are westerly and south-easterly partitioned. Other high- and lowfrequency haplotypes also showed a well-structured geographic distribution consistent with the current phytogeographic framework of the European continent. The climatic and geological events that characterized the end of the Tertiary and Quaternary periods played a crucial role in triggering haplotype mixing, isolation, and in offering niche opportunities for more recent diversification. Euro-Mediterranean southern territories host a high and as yet poorly studied genetic variation; the role of the Italian Peninsula as a crossroad and threshold for haplotype diversity and distribution clearly emerges from this study.