CNR scientists investigate the origin of walnut
The first historical records of the use of the walnut fruit tree (Juglans regia L.) as a food date back to 9,000 years ago. Both Pliny the Elder, the Roman writer, admiral and naturalist, and Columella, the Roman writer on agriculture, report that the walnut was brought from Asia Minor to Europe by Greek settlers between the 7th and 5th centuries BC.
The cultivation of walnut, highly valued for the nutraceutical properties of the polyunsaturated fatty acid of its fruits, is spread in almost all temperate regions. The walnut is widely cultivated in France, Greece, Bulgaria, Serbia, Romania, China, California and Chile and, more recently, in New Zealand. Its original distribution in the wild was limited to the area between the southern Balkan Peninsula and central Asia.
Researchers at the Institute of Agro-environmental and Forest Biology (IBAF-CNR) and at the Institute for Archaeological and Monumental Heritage (IBAM-CNR) of the National Research Council (CNR) were able to identify the origin and expansion pattern of common walnut.
The research, published in Plos One, confirms the Asian origin of the plant and the existence, in the Caucasus and in the mountain valleys of Central Asia, of at least four areas (western Kyrgyzstan, western and south-central Asia, central and eastern Uzbekistan, the provinces of Xinjiang and Shandong in China) where walnut populations keep high levels of genetic diversity, probably survived in protected ecological refugia after the glaciations of the Pleistocene, identifying the major geographic barriers that blocked the natural gene flow and the trade routes and the cultural factors that overcame them.
“The trade in the area between Tashkent and Samarkand (central-eastern Uzbekistan), where the northern and central routes of the Northern Silk Road came together, might have caused the spatial separation of the genetic structure of autochthonous walnut populations and might have transferred to this area gene pools that are still detected today”, explained Angelo Massacci, IBAF-CNR Director, who added: “We integrated data obtained from population genetics of J. regia with the linguistic analysis of the word ‘walnut’ ‒ in relation to the distribution dynamics of the species ‒ and with archaeological, topographical and historical data that are key to detect the genetic structure of walnut populations, so far only generically related to human action”.
Over the last 30 years, the CNR researchers have assembled the largest collection existing today of wild walnut populations thanks to samples obtained from 16 countries of the Eurasian continent, between China and Spain, analysing the results of their selection.
“By integrating fossil pollen, cultural and historical data with population genetics, we demonstrated the presence in Europe, besides the Asian populations, of walnut in two glacial refugia in the Balkans and western Europe”, continued Massacci. “The spread, which took place between the Bronze Age and the Roman period, happened via the Roman trade routes in the trans-Danubian regions up to the Baltic and the borders of the Russian steppes. In addition, the subsequent expansion and contraction of the presence of the walnut over the last 4,000 years is due to variations in human exploitation”.
I would like to thank Maria Emilia Malvolti, Paola Pollegioni, Francesca Chiocchini, Marco Ciolfi, Irene Olimpieri and Virginia Tortolano from IBAF-CNR, Stefano Del Lungo from IBAM-CNR, and Sergio Mapelli from the Institute of Agricultural Biology and Biotechnology (IBBA-CNR) for their long and hard work. Their collaboration confirms the importance of transversal and interdepartmental cooperation between CNR institutes and shows that only a multidisciplinary approach can provide a holistic view of the factors that have determined the evolutionary history of the species”, concluded Massacci.