Genetic structure of natural populations of Castanea sativa in Turkey: evidence of a hybrid zone

This study points out the evidence of a hybrid zone between two groups of genetically differentiated populations of chestnut (Castanea sativa Mill.) in Turkey. Genetic structure, gene flow and introgression levels, based on 16 allozyme loci, were investigated on 34 population samples spanning the entire C. Sativa distribution area in this country. The occurrence of the hybrid zone, located in the Bithynian region, was inferred in a group of populations showing the following genetic characteristics: (i) enhanced genetic variability and intermediate allelic frequencies between those of the Western and Eastern groups of populations; (ii) shall, and concordant changes in allele frequencies; (iii) decreased gene flow with the Western and Eastern populations. Starting from the dine width estimated to be 324 km, strength of selection was evaluated from the gene Bow distance, as indicated from the degree of genetic structuring outside the hybrid zone. Evolutionary processes shaping the observed genetic differentiation and introgression are discussed on the basis of palynological data, palaeoclimatic events and evidence of hybridization found in other plant and animal species in the same region.