Oaks of the Western Caucasus: Species Structure and Hybridization Investigated by Genetic Markers

The geographic distribution and hybridization patterns of six roburoid oak species growing in the Western Caucasus, as well as the ecological association of the species and their hybrids were studied using 18 nuclear microsatellite loci. The wide distribution of limestone oak Q. calcarea Troitsky, a species genetically and ecologically distinct from sessile oak Q. petraea and downy oak Q. pubescens, was confirmed in the studied region. Five species were found in a genetically pure form (Q. robur, Q. hartwissiana, Q. petraea, Q. calcarea, and Q. pubescens). Using the Bayesian clustering method, their individuals are grouped into separate clusters according to their taxonomic affiliation. These species grow sympatrically in the Western Caucasus, occupying different ecological niches. The morphological features that distinguish pure individuals are stable and consistent with the genetic data. Hybridization is observed in all species, but the number of hybrids between species varies significantly. The largest number of hybrids is formed between the short-peduncle oaks Q. pubescens, Q. calcarea and Q. petraea. According to the degree of hybridization, two zones are clearly distinguished in the Western Caucasus: the area of distribution of Q. pubescens, where hybrids accounted for more than half of the composition in all populations (64.3% in mean), and the area outside the range of Q. pubescens, where the proportion of hybrid samples was significantly lower (10.6%). In all populations involving Q. pubescens in the extreme west of the Caucasus, a high level of introgression is revealed between Q. pubescens and Q. calcarea, with a predominance of hybrid samples over pure individuals, with a continuum of transitional morphological forms. Significant heterogeneity is observed in the composition of species and hybrids depending on the location of the sampling points and habitat conditions. The proportion of the Q. pubescens cluster, identified using the Bayesian clustering STRUCTURE, is highest in the driest habitats near the seashore and decreases with distance from the sea and in more humid ecotopes, reflecting different ecological properties of Q. pubescens and Q. calcarea, as well as ecological properties of hybrids, depending on the contribution of genes of the parental species. The high-mountain species Q. macranthera, extremely rare in the Western Caucasus, was not observed in the studied populations in pure form, but was found in hybrids.