Monumental chestnut trees: source of genetic diversity, cultural and landscape value

Abstract

The monumental trees are unique individuals of venerable age and considerable size, which represent a heritage of inestimable historical, cultural, landscape, and scientific value for the territtory. They also constitute a source of genetic diversity which confers them longevity and ability to adapt to climate and environmental changes. In this context, studies on centennial trees can be useful for interpretatiton of species history as migration events, selection and anthropogenic actiton. The aim of this research was to evaluate the genetic variability of ancient Castanea sativa trees and relate them to actual natural/naturalized populatitons and varieties in order to enhance our knowledge about the demography, cultivatiton processes and the impact of these giant trees on the genetic diversity of the species. We selected a total of 182 ancient trees from Spain and Central - Southern Italy. For each tree, more than one sample was collected to test for genetic integrity and grafing. The samples were genotyped by means of nuclear microsatellite markers and the variability of plastid DNA regitons (trnH-psbA and trnK/matK) was also tested. Using the sofware GeneALex and HPrare, we evaluated observed (Hto) and expected (He) heterozygosity, allelic richness (Ar) private allelic richness (pAr). A Bayesian analysis was performed using the sofware STRUCTURE to identify the different gene pools and gentotypes. The obtained genetic data were compared with those of natural populations and cultivars collected in the same geographic areas. Higher values of allelic richness were observed in the ancient chestnut trees, a genetic similarity of these individual trees to the natural populations was highlighted. A phylogetographic structure of plastid diversity was alsto established. Eleven genotypes were coincident with 11 cultivars in the EU database. Based on the putative age of giant trees we can hyptothesize that the grafing practice occurred in the Iberian peninsula in the 15th century and in the 17th century in Italy. This work provides new knowledge about the history and domesticatiton tof European chestnut, the results are relevant for the conservatiton and management of Castanea sativa genetic resources.