Development of Chloroplast Markers for Studying the Population Structure of Birches in the North Caucasus

Previous phylogeographic studies of Betula species in Northern Eurasia have been largely limited to the latitudinal forest zone, while the southern mountainous areas where birch is widespread, including the Caucasus region, have remained unexplored. The aim of this study is to investigate the structure of chloroplast DNA variation in North Caucasus birch species, including Betula pendula Roth, Betula litwinowii Doluch., and Betula raddeana Trautv., and compare it with variation in the main, boreal, part of the birch distribution. A total of 258 samples were analyzed: 208 samples of three birch species from the Caucasus and 50 B. pendula samples from boreal Russia (Siberia, the Urals, and Kaliningrad oblast). Two chloroplast markers, B896 and B553, were developed and demonstrated good variability. Six Caucasian haplotypes and six haplotypes common in the northern part of the B. pendula range were identified. Relatively weak population differentiation was found for all studied species (NST = 0.460), along with population differentiation within regions (NSC = 0.343) and between regions (NCT = 0.178), reflecting the large effective size of these populations, significant gene flow (due to pollen and seed volatiles), and the relatively recent separation of these two regions. An endemic haplotype was discovered in the central part of the North Caucasus, possibly indicating a local glacial refugium.