Genetic structure of wild sweet chestnut (Castanea sativa Mill.) populations in northwest of Spain and their differences with other European stands

The genetic structure of 15 wild populations of Castanea sativa located in Galicia (northwest of Spain) were compared with another 10 Spanish, 2 Italian, and 2 Greek populations using 9 nuclear microsatellites. Additionally, 41 Castanea crenata, 19 Castanea mollissima and 54 accessions of varieties were used to detect the presence of Asiatic germplasm and grafted varieties in wild stands. Bayesian methods detected two main clusters in the European populations of sweet chestnut: a Northern Iberian Peninsula cluster included the northern and northwestern Spanish populations and a Mediterranean cluster contained central and southern Spanish, Italian and Greek populations. Further subdivisions were also detected. The Northern Iberian Peninsula cluster was divided into Cantabrian and Atlantic clusters and the Mediterranean cluster was divided into western and eastern Mediterranean clusters. Several populations were highly affected by domestication because they were classified as admixed among the four defined groups. Negative values for the fixation index were detected in northern and northwestern Spain showing an excess of heterozygotes that could be explained by the introduction of grafted varieties and its introgression with local populations or by natural selection in favor of heterozygotes produced by ink disease. The combined analysis with the Asiatic species also indicated their presence and possible introgression in the wild populations of northwestern Spain. The results of this study suggest that the current genetic structure of sweet chestnut can be explained by existence of several refugia during Pleistocene and also by human activity that changes the structure of the populations.