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

Abstract

A biocultural diversity approach integrates plant biology and germplasm dispersal processes with human cultural diversity. An increasing number of studies have identified cultural factors and ethnolinguistic barriers as the main drivers of the genetic diversity in crop plants. Little is known about how anthropogenic processes have affected the evolution of tree crops over the entire time scale of their interaction with humans. In Asia and the Mediterranean, common walnut ( Juglans regia L.) and sweet chestnut ( Castanea sativa Mill.) have been economically and culturally important crops for millennia; there, in ancient times, they were invested with symbolic and religious significance. In this study, we detected a partial geographic congruence between the ethno‐linguistic repartition of human communities, the distribution of major cognitive sets of word‐related terms, and the inferred genetic clusters of common walnut and sweet chestnut populations across Eurasia. Our data indicated that isolation by distance processes, landscape heterogeneity and cultural boundaries might have promoted simultaneously human language diversification and walnut/chestnut differentiation across the same geographic macro‐regions. Hotspots of common walnut and sweet chestnut genetic diversity were associated with areas of linguistic enrichment in the Himalayas, Trans‐Caucasus, and Pyrenees Mountains, where common walnuts and sweet chestnuts had sustained ties to human culture since the Early Bronze Age. Our multidisciplinary approach supported the indirect and direct role of humans in shaping walnut and chestnut diversity across Eurasia from the EBA (e.g., Persian Empire and Greek–Roman colonization) until the first evidence of active selection and clonal propagation by grafting of both species. Our findings highlighted the benefit of an efficient integration of the relevant cultural factors in the classical genome (G) × environmental (E) model and the urgency of a systematic application of the biocultural diversity concept in the reconstruction of the evolutionary history of tree species.

Genetic Diversity among Some Walnut (Juglans regia L.) Genotypes by SSR Markers

“…On the other hand, the introduction of walnut to its new habitats may be dispersed easier by similarities in human language over large geographic areas. This may lead to the genetic homogenization of different populations known as the human role in walnut evolution by merging the plant biology and germplasm dispersal processes data with human cultural and linguistic diversity [54].…”

The late Holocene introduction of Juglans regia (walnut) to Cyprus

“…A combined study of ethnolinguistics and genetics suggested a spread of Juglans from the Early Bronze Age, driven by Greek, Roman and Persian expansion/trading (Pollegioni et al 2020). The persistent presence of Juglans pollen from around 2,000 cal yr bp in AM18-5 would be consistent with a Roman expansion of walnut on Cyprus (Fig.…”

“…A Bronze Age to Roman period spread of Juglans has been proposed from palynological, ethnolinguistic and genetic studies (Bottema 1980(Bottema , 2000Eastwood et al 1998;Pollegioni et al 2017Pollegioni et al , 2020. Genetics show refugia in southern Europe and Anatolia, which provides a starting point to consider how this important economic species spread (Pollegioni et al 2017(Pollegioni et al , 2020. In the Levant, pollen…”

The Roman Legacy on European Chestnut and Walnut Arboriculture

“…It is thus generally agreed that the expansion of the Roman Empire played a decisive role in the spread of these two tree species in Europe and their cultivation, both as fruit trees and for timber production (Conedera et al 2004a;Pollegioni et al 2020). As a result of such a rapid, human-assisted and spatially defined introduction, originating from the territories already conquered by the Romans, both species highlight a relatively low biocultural (i.e.…”

“…As a result of such a rapid, human-assisted and spatially defined introduction, originating from the territories already conquered by the Romans, both species highlight a relatively low biocultural (i.e. genetic and ethno-linguistic) diversity in Western Europe with respect to the Transcaucasian (both species) and Asian (walnut) areas of Last Glacial Maximum (LGM) refugia (see Mattioni et al 2013Mattioni et al , 2017Fernandez-Lopez et al 2021 for Castanea sativa and Pollegioni et al 2020 for Juglans regia).…”