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.

Keywords: anthropogenic processes; common walnut; human linguistic diversity; population genetics; sweet chestnut.

Figure 1

Graph network of 91 common walnut and 73 sweet chestnut populations in Eurasia. Nodes represent geographic sites and length of edges connecting nodes equivalent to genetic differentiation among the sites calculated using SSR markers for 91 common walnut (a) and 73 sweet chestnut (b) populations in Eurasia. The color of each node represents the language phylum spoken by human communities living in the geographic sampling sites

Figure 2

Spatial coincidences between genetic richness and stratification of walnut (a) and chestnut (b) linguistic terms. Hot spots of common walnut and sweet chestnut genetic diversity calculated using SSR markers and stratification of walnut and chestnut linguistic‐related forms across Eurasia. Inverse Distance Weighted (IDW) interpolations of the estimated allelic richness (Rs) values were modified from Pollegioni et al. (2017) and Mattioni et al. (2017)

Figure 3

Linguistic evolution of the six major cognate sets (*KVrV, *ŋuńV‐, *a‐/an‐gōza, *HwV́rƛ̣V, *ḳV̆rḳV, *ṭVɫV) referred to walnut across Eurasia. The spatial and time distribution of walnut terms was derived using the consensus language trees with approximate estimation of the divergence times computed between languages of ^a Dravidian family (Kolipakam et al., 2018), ^b Altaic phylum‐Turkic family (Mikic et al., 2011; Savelyev & Robbeets, 2020), ^c Uralic phylum‐Finno‐Ugric family (Honkola et al., 2013), ^d Indo‐European phylum (Chang, Cathcart, Hall, & Garrett, 2015, in accordance with steppe theory), and ^e Kartvelian phylum (Koryakov, 2002) included into the putative Eurasiatic macro‐phylum as proposed by Pagel, Atkinson, Calude, and Meade (2013). ^f Afro‐asiatic phylum‐Semitic family (Kitchen et al., 2009), and ^g the putative Dene‐Sino‐Caucasian macro‐phylum (Van Driem, 2008) including Basque (Valdiosera et al., 2018), Burushaski, North‐Caucasian phylum (Koryakov, 2002), and Sino‐Tibetan phylum (Sagart et al., 2019) were also included in the reconstruction

Figure 4

Linguistic evolution of the four major cognate sets (*kastAno‐, *derw‐, *blwt, *kVl) referred to chestnut across Eurasia. The spatial and time distribution of chestnut terms was derived using the consensus language trees with approximate estimation of the divergence times computed between languages of ^aDravidian family (Kolipakam et al., 2018), ^bAltaic phylum‐Turkic family (Mikic et al., 2011; Savelyev & Robbeets, 2020), ^cUralic phylum‐Finno‐Ugric family (Honkola et al., 2013), ^dIndo‐European phylum (Chang et al., 2015, in accordance with steppe theory), and ^eKartvelian phylum (Koryakov, 2002) included into the putative Eurasiatic macro‐phylum as proposed by Pagel et al. (2013). ^fAfro‐asiatic phylum‐Semitic family (Kitchen et al., 2009), and ^gthe putative Dene‐Sino‐Caucasian macro‐phylum (Van Driem, 2008) including Basque (Valdiosera et al., 2018), North‐Caucasian phylum (Koryakov, 2002), and Sino‐Tibetan phylum (Sagart et al., 2019) were also included in the reconstruction

Figure 5

Geographic coincident between (a) the population genetic structure of walnut inferred across Eurasia and (b) the distribution of the six major cognate sets (*KVrV, *ŋuńV‐, *a‐/an‐gōza, *HwV́rƛ̣V, *ḳV̆rḳV, *ṭVɫV)) referred to walnut. Inverse Distance Weighted (IDW) interpolations of the estimated mean population membership values (cluster analysis: Qi) were modified from Pollegioni et al. (2017). Gray arrows indicated migration rates per generation. Word forms connected to proto‐words but changing meaning over the time are reported as empty dots. The earliest date of appearance of the attested walnut linguistic terms was provided for each cognitive set

Figure 6

Geographic coincident between (a) the population genetic structure of chestnut inferred across Eurasia and (b) the distribution of the four major cognate sets (*derw‐, *kastAno‐, *blwt, *kVl) referred to chestnut. Inverse Distance Weighted (IDW) interpolations of the estimated mean population membership values (cluster analysis: Qi) were modified from Mattioni et al. (2017). Gray arrows indicated migration rates per generation. Word forms connected to proto‐words but changing meaning over the time are reported as empty dots. The earliest date of appearance of the attested chestnut linguistic terms was provided for each cognitive set