Population genetics informs new insights into the phytogeographic history of Juglans regia L.

Juglans regia is an important perennial crop cultivated for its high-quality nuts and wood. It is generally believed that J. regia survived and expanded in almost completely isolated stands in Asia after the last glaciation. Humans subsequently dispersed J. regia through cultural expansion and trade. We evaluated the spatial genetic structure and genetic diversity of 2,929 J. regia samples from 150 populations using 14 Simple Sequence Repeats (SSRs) markers. Our study revealed that regions with the highest genetic diversity included Southern Asia, Western Asia, Western Europe, and China, as illustrated using a Geostatistical Inverse Distance Weighting (IDW) interpolation of observed heterozygosity (H-O), expected (HE) heterozygosity (H-E), percentage of polymorphic loci (PPL), the total number of alleles (N-A)(,) and Allelic richness (R-S) in Arc Geographic Information System (ArcGIS). The ecological Niche Model (ENM) showed J. regia had a high probability of association with Central Asian and Eastern Asian habitats. Population genetic structure, phylogeny, and Principal Coordinate Analysis (PCoA) identified three genetic groups corresponding to three geographic sources. Turkish and Georgian populations served as a bridge between Asian populations and Europe populations. We suggest that J. regia evolved in central Asian mountain ranges similar to 65 million years ago (Mya) and dispersed across Eurasia during climate shifts (similar to 65 to 3Mya). The population contracted into multiple refugia during the Last Glacial Maximum. The current distribution of J. regia across Eurasia was shaped by the cumulative effects of contraction or expansion of different refugia and human exploitation after LGM.

Automated summary

Juglans regia, an important perennial crop, has a high genetic diversity in Southern Asia, Western Asia, Western Europe, and China. The ecological niche model shows its association with Central Asian and Eastern Asian habitats. Three genetic groups were identified, with Turkish and Georgian populations serving as a connection between Asian and European populations. J. regia evolved in central Asian mountain ranges around 65 million years ago and its current distribution across Eurasia was shaped by the contraction or expansion of different refugia and human activities after the Last Glacial Maximum.