Morphological diversity and phylogeography of the Georgian durmast oak (Q. petraea subsp. iberica) and related Caucasian oak species in Georgia (South Caucasus)

Abstract

The Caucasus region is one of the 25 global biodiversity hotspots and constitutes a shelter area for Neogene relict species as well as a center of ongoing radiation. In order to elucidate the taxonomic identity, divergence patterns, and evolutionary history of the largely widespread Georgian durmast oak (Quercus petraea subsp. iberica), we examined leaf morphology and chloroplast DNA (cpDNA) (trnH-psbA, trnK-matK) sequence variation across its South Caucasian range. Six other oak taxa distributed throughout Georgia were included in the dataset and used for comparison. Evidence for differentiation in both sets of traits was found. Populations represented by different taxa from ecologically equivalent areas showed common morphological features and genetic structures. Molecular analysis clearly indicated the presence of two major haplotype lineages (West Caucasian vs. East Caucasian zonation type) and suggested a maternal lineage diversification of Q. petraea subsp. iberica in the Late Miocene, as a likely result of complex patterns associated with major orogenic and climatic changes. The Quaternary glacial oscillations resulted in a number of less common, derived haplotypes. Based on mismatch distribution analysis and neutrality tests, we found no evidence of demographic expansion for the populations from the West and East Caucasian zonation types. The two Caucasian provinces therefore acted as important shelter/diversification areas and as a lineage crossroad for the Georgian oaks. Close intra- and interspecific cpDNA relationships shared with other oaks from bordering countries support the relevant role played by the Colchis region as a primary refugium for the European temperate forest species.