Rethinking the history of common walnut (Juglans regia L.) in Europe: Its origins and human interactions

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

Ancient humans influenced the current spatial genetic structure of common walnut populations in Asia

Common walnut (Juglans regia L) is an economically important species cultivated worldwide for its wood and nuts. It is generally accepted that J. regia survived and grew spontaneously in almost completely isolated stands in its Asian native range after the Last Glacial Maximum. Despite its natural geographic isolation, J. regia evolved over many centuries under the influence of human management and exploitation. We evaluated the hypothesis that the current distribution of natural genetic resources of common walnut in Asia is, at least in part, the product of ancient anthropogenic dispersal, human cultural interactions, and afforestation. Genetic analysis combined with ethno-linguistic and historical data indicated that ancient trade routes such as the Persian Royal Road and Silk Road enabled long-distance dispersal of J. regia from Iran and Trans-Caucasus to Central Asia, and from Western to Eastern China. Ancient commerce also disrupted the local spatial genetic structure of autochthonous walnut populations between Tashkent and Samarkand (Central- Eastern Uzbekistan), where the northern and central routes of the Northern Silk Road converged. A significant association between ancient language phyla and the genetic structure of walnut populations is reported even after adjustment for geographic distances that could have affected both walnut gene flow and human commerce over the centuries. Beyond the economic importance of common walnut, our study delineates an alternative approach for understanding how the genetic resources of long-lived perennial tree species may be affected by the interaction of geography and human history

Rethinking the history of common walnut (<i>Juglans regia</i> L.) in Europe: Its origins and human interactions

<div><p>Common walnut (<i>Juglans regia</i> L) is an economically important species cultivated worldwide for its high-quality wood and nuts. It is generally accepted that after the last glaciation <i>J</i>. <i>regia</i> 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.</p></div

Correlation between genetic distances among walnut populations and human linguistic distances.

<p>^a Measures of genetic differentiation calculated among 39 common walnut populations using either F_ST [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135980#pone.0135980.ref027" target="_blank">27</a>] and D_<i>est</i> [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135980#pone.0135980.ref028" target="_blank">28</a>].</p><p>^b (A) Simple and Partial Mantel tests [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135980#pone.0135980.ref029" target="_blank">29</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135980#pone.0135980.ref030" target="_blank">30</a>] and (B) Multiple Regression Model analysis [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135980#pone.0135980.ref031" target="_blank">31</a>] of genetic (D_GEN) on geographic (D_GEO) and linguistic (D_LAN) matrices.</p><p>^c Partial correlation coefficient.</p><p>^d Significance of <i>r</i> values was tested using 5000 permutations as implemented in ZT software [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135980#pone.0135980.ref059" target="_blank">59</a>]: * P < 0.05, ** P < 0.01 and *** P < 0.001.</p><p>^e<i>P</i> values are based on 5000 permutations as implemented in R Ecodist package [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135980#pone.0135980.ref061" target="_blank">61</a>]: * P < 0.05, ** P < 0.01 and *** P < 0.001.</p><p>Correlation between genetic distances among walnut populations and human linguistic distances.</p

Delaunay connections associated with linguistic distance (D_LAN) and crossed by a statistically significant genetic barrier.

<p>^a Statistically significant genetic barriers were calculated using the Monmonier’s maximum difference algorithm as implemented in BARRIER software 2.2 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135980#pone.0135980.ref062" target="_blank">62</a>].</p><p>Delaunay connections associated with linguistic distance (D_LAN) and crossed by a statistically significant genetic barrier.</p

Common walnut population graph for 39 study sites in the Asian range.

<p>Nodes represent geographic sites with diameter proportional to within-site heterozygosity and length of edges connecting nodes equivalent to genetic differentiation among the sites calculated using 14 SSR markers. The color of each node represents the language phylum spoken by human communities living in the geographic sampling sites.</p

Spatial genetic structure of 91 walnut populations in Eurasia.

<p>Population structure inference for 91 walnut populations by Bayesian assignment using STRUCTURE for K = 4. Synthetic map of Inverse Distance Weighted (IDW) interpolations of the estimated mean population membership values (<i>Q</i>_<i>i</i>) (a) and bar plot showing assignment probabilities of individuals to K clusters (b). Abbreviations: CN = China, UZ = Uzbekistan, KG = Kyrgyzstan, TJ = Tajikistan, PK = Pakistan, IR = Iran, GE = Georgia, TR = Turkey, MD = Moldova, RO = Romania, HU = Hungary, SK = Slovakia, GR = Greece, IT = Italy, FR = France, ES = Spain.</p

Most likely demographic scenario for European walnut by the DIYABC approach.

<p>Posterior probability (<i>P</i>) and 95% confidence interval of <i>P</i> (in brackets) computed using a direct (<i>P</i>_<i>1</i>) and logistic regression (<i>P</i>_<i>2</i>) approach are provided for each scenario tested by the DIYABC approach. The most likely scenario for each stage is reported in grey. Confidence in scenarios was evaluated using type <i>I</i> error (False negative) and type <i>II</i> error (False positive) rates for logistic regression.</p