Phylogeography of Oak Species in the Caucasus Based on Results of Chloroplast DNA Analysis

In order to study the biogeography and history of temperate biota of the Caucasian region, one of the centers of biological diversity of Eurasia, the phylogeographic structure of the chloroplast DNA of roburoid oaks ( Quercus L., section Quercus , Fagaceae) was studied. We analyzed 926 trees of five species ( Quercus robur, Q. petraea, Q. pubescens, Q. hartwissiana, and Q. macranthera ) from 70 populations located in different parts of the Caucasus. Sequencing of five fragments with a total length of more than 10 000 bp revealed eight haplotypes, which were typed using chloroplast microsatellite loci (cpSSRs), sequencing, and restriction analysis. The Quercus phylogenetic tree, which includes 34 chloroplast haplotypes of West Eurasian and East Asian roburoid species, confirmed the monophyly of roburoid oaks, which form several lineages that do not have supported topological relationships with each other. The haplotypes identified in the Caucasus belong to two divergent Western Eurasian lineages. These haplotypes are endemic for the Eastern Black Sea and Caucasian region and do not have related haplotypes beyond its borders, which indicates the long-term presence of oaks in the Caucasus and the historical links of the oaks of the Caucasian region with the Eastern Crimea and Eastern Turkey in the absence of chloroplast DNA exchanges with the Eastern European part of the range. The results of the present study show that Caucasian populations could not significantly contribute to the colonization of northern territories and were not influenced by more northern populations, at least through seed transfer. The distributions of the Caucasian haplotypes almost do not intersect with each other, occupying certain geographical areas, which is probably caused by the settlement from individual glacial refugia and genetic drift. The geographical structure of chloroplast DNA variability indicates a long-term presence of oak, in addition to Western Transcaucasia (Colchis refugium), in the regions of the Northwestern, Central, Eastern Caucasus, and Lesser Caucasus. The coincident composition of the haplotypes of the studied Quercus species within the geographic regions indicates the long-term coexistence of different species in different parts of the Caucasus.