Spatial genetic structure of common walnut (Juglans regia L.) in central Asia

Landscape genetics is a powerful approach that combines ecology, geography, and population genetics to quantify the effects of landscape on evolutionary processes. Juglans regia is native to south-eastern Europe and west-central Asia, but its distribution was modified by human management during the last 3,000 years. Despite impressive morphological and phenological variation, an accurate overview of the genetic diversity and spatial genetic structure of Asian walnut is incomplete. Studies of the genetic structure of autochthonous populations, when combined with analyses of geographic data, can reveal how geographic barriers have shaped the distribution of walnut genetic resources. Sampled and analyzed walnut populations were geographically isolated by arid lowland, mountain slopes and highland steppes. Clustering approaches produced convergent results, dividing the samples into four main genetic clusters. Most genetic clusters were confined to particular mountain ranges, showing distinct phylogeographic structure. The spatial arrangement of genetic clusters of Eurasian walnut indicated that ancient trade routes facilitated walnut dispersal and gene flow over enormous distances. By combining genetic analysis with ethno-linguistic and historical data, we demonstrated a statistically significant association between ancient language phyla in Asia and the genetic structure of walnut. This result indicated that walnut's natural gene flow was substantially supplemented by human commerce over ancient trade routes such as the Persian Royal Road and Silk Road.