Biocultural diversity of Castanea sativa (Mill.) across Eurasia

A biocultural diversity approach integrates plant biology and germplasm dispersal processes with human cultural diversity. In this study, we integrated archeological, linguistic, and genetic data to address the role of landscape and ethno-linguistic boundaries, used as a proxy of cultural similarities between human communities, on shaping the genetic structure of chestnut natural populations across Eurasia. Our data indicated that isolation by distance processes and landscape heterogeneity might have promoted simultaneously human language diversification and chestnut differentiation across the same geographic macro-regions. Hotspots of sweet chestnut genetic diversity were associated with areas of linguistic enrichment in the Trans-Caucasus and Pyrenees Mountains, where sweet chestnuts had sustained ties to human culture since the Early Bronze Age (EBA). Partial geographic coincidence between the distribution of five major cognitive sets of word-related terms and the inferred genetic clusters of sweet chestnut populations supported i) the indirect role of humans in shaping chestnut diversity across Eastern Mediterranean from the EBA, and ii) highlighted the importance of Romans on its cultivation history in the Eastern and Western Empire, where the role of Romans on the onset of chestnut domestication can be only postulated. The progressive linguistic and cultural homogenization of conquered territories in the Roman Empire as well as the exchanges of knowledge resulted in low biocultural diversity of chestnut and the predominant denomination of the chestnut with the Latinized form of Greek kástanon across Europe. Although selection and clonal propagation by grafting of chestnut cultivars for nut production is documented in the Western Empire since the 1st century BC, integration of data indicated that systematic nut production and long-distance dispersion of chestnut trees developed later. Indeed, specialized production of domesticated, large and tasty edible chestnuts, such as marron-type nut bearing cultivars, spread across Italy, France and Spain later, starting from the 14th century.