Biocultural diversity of common walnut (Juglans regia L.) and sweet chestnut (Castanea sativa Mill.) across Eurasia

A biocultural diversity approach integrates plant biology and germplasm dispersalprocesses with human cultural diversity. An increasing number of studies have identifiedcultural factors and ethnolinguistic barriers as the main drivers of the geneticdiversity in crop plants. Little is known about how anthropogenic processes haveaffected the evolution of tree crops over the entire time scale of their interactionwith humans. In Asia and the Mediterranean, common walnut (Juglans regia L.) andsweet chestnut (Castanea sativa Mill.) have been economically and culturally importantcrops for millennia; there, in ancient times, they were invested with symbolicand religious significance. In this study, we detected a partial geographic congruencebetween the ethno-linguistic repartition of human communities, the distributionof major cognitive sets of word-related terms, and the inferred genetic clusters ofcommon walnut and sweet chestnut populations across Eurasia. Our data indicatedthat isolation by distance processes, landscape heterogeneity and cultural boundariesmight have promoted simultaneously human language diversification and walnut/chestnut differentiation across the same geographic macro-regions. Hotspotsof common walnut and sweet chestnut genetic diversity were associated with areasof linguistic enrichment in the Himalayas, Trans-Caucasus, and Pyrenees Mountains,where common walnuts and sweet chestnuts had sustained ties to human culturesince the Early Bronze Age. Our multidisciplinary approach supported the indirectand direct role of humans in shaping walnut and chestnut diversity across Eurasiafrom the EBA (e.g., Persian Empire and Greek-Roman colonization) until the firstevidence of active selection and clonal propagation by grafting of both species. Ourfindings highlighted the benefit of an efficient integration of the relevant culturalfactors in the classical genome (G) × environmental (E) model and the urgency of asystematic application of the biocultural diversity concept in the reconstruction ofthe evolutionary history of tree species.