Data from: Population differentiation in common walnut (Juglans regia L.) across major parts of its native range - insights from molecular and morphometric data

Juglans regia is an economically highly important species for fruit and wood production in the warm temperate and sub-tropical zones of the Northern Hemisphere. Besides the natural influence of climatic and geomorphological barriers, its genetic structure has been strongly modified by humans and the population history is still unclear. For this reason, we investigated mainly natural walnut populations across the Eurasian continent on a molecular (44 populations, 581 trees) and morphometric level (23 populations, 1391 ripe nuts). Population genetic diversity and differentiation were examined by using seven microsatellite loci. Morphometric characteristics of the nuts (mainly roundness index and nut density) were used to estimate trait variation and population differentiation. Highest allelic richness R_s12 = 7.05 was observed in a Pakistani and the lowest value R_s12 = 3.04 in a Kyrgyz population. The genetic differentiation among populations was high (F_ST = 0.217; R_ST= 0.530) indicating a strong phylogeographic pattern. While variation of the roundness index within single populations was high, this trait neither differentiated geographical regions nor was it associated to genetic clusters. Approximated Q_ST based on this trait equalled F_ST, while approximated Q_ST based on nut density considerably exceeded F_ST, indicating selection. Nut density was moderately correlated with altitude, latitude and longitude, and differentiated populations according to their origin. Pakistani and Indian populations showed highest nut densities. These South Asian populations contain putatively ancestral nut forms, which probably have been lost in other populations as a consequence of human selection.