Population Differentiation in Common Walnut ( Juglans regia L.) across Major Parts of Its Native Range—Insights from Molecular and Morphometric Data

<span class="paragraphSection"><div class="boxTitle">Abstract</div><span style="font-style:italic;">Juglans regia</span> is an economically highly important species for fruit and wood production in the warm temperate and subtropical 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 7 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 <span style="font-style:italic;">R</span>_s12 = 7.05 was observed in a Pakistani and the lowest value <span style="font-style:italic;">R</span>_s12 = 3.04 in a Kyrgyz population. The genetic differentiation among populations was high (<span style="font-style:italic;">F</span>_ST = 0.217; <span style="font-style:italic;">R</span>_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 <span style="font-style:italic;">Q</span>_ST based on this trait equalled <span style="font-style:italic;">F</span>_ST, while approximated <span style="font-style:italic;">Q</span>_ST based on nut density considerably exceeded <span style="font-style:italic;">F</span>_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.</span>

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