Abstract
Common walnut (Juglans regia L) is an economically important species cultivated worldwide for its high-quality wood and nuts. It is generally accepted that after the last glaciation J. regia survived and grew in almost completely isolated stands in Asia, and that ancient humans dispersed walnuts across Asia and into new habitats via trade and cultural expansion. The history of walnut in Europe is a matter of debate, however. In this study, we estimated the genetic diversity and structure of 91 Eurasian walnut populations using 14 neutral microsatellites. By integrating fossil pollen, cultural, and historical data with population genetics, and approximate Bayesian analysis, we reconstructed the demographic history of walnut and its routes of dispersal across Europe. The genetic data confirmed the presence of walnut in glacial refugia in the Balkans and western Europe. We conclude that human-mediated admixture between Anatolian and Balkan walnut germplasm started in the Early Bronze Age, and between western Europe and the Balkans in eastern Europe during the Roman Empire. A population size expansion and subsequent decline in northeastern and western Europe was detected in the last five centuries. The actual distribution of walnut in Europe resulted from the combined effects of expansion/contraction from multiple refugia after the Last Glacial Maximum and its human exploitation over the last 5,000 years.
Highlights
Common walnut (Juglans regia L.) is an economically important tree species, highly valued for its timber and edible nuts
Our objective was the integration of fossil pollen, cultural and historical data with population genetics [27], and the inference of demographic history using approximate Bayesian computation (ABC) [28] to evaluate if (1) ancient reservoirs of walnut diversity still exist in Europe, (2) clines of genetic diversity are present between Europe and Asian regions where walnut still grows naturally, (3) genetic clines in J. regia can be explained by the recolonization of western Europe from refugia of western Asia through the Balkans, or (4) whether western Asia and the southern European/ Balkans peninsulas represent two separate regions of walnut evolution
A significant decline in allelic richness was found with decreasing longitude (r = 0.522, P = 0.00015) and increasing latitude (r = 0.425, P = 0.00135)
Summary
Our objective was the integration of fossil pollen, cultural and historical data with population genetics [27], and the inference of demographic history using approximate Bayesian computation (ABC) [28] to evaluate if (1) ancient reservoirs of walnut diversity still exist in Europe, (2) clines of genetic diversity are present between Europe and Asian regions where walnut still grows naturally, (3) genetic clines in J. regia can be explained by the recolonization of western Europe from refugia of western Asia through the Balkans, or (4) whether western Asia and the southern European/ Balkans peninsulas represent two separate regions of walnut evolution
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