Long-term human impacts on genetic structure of Italian walnut inferred by SSR markers
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Life history traits, historic factors, and human activities can all shape the genetic diversity of a species. In Italy, walnut (Juglans regia L.) has a long history of cultivation both for wood and edible nuts. To better understand the genetic variability of current Italian walnut resources, we analyzed the relationships among the genetic structure of local walnut populations (inferred by SSR markers) and human migrations along ancient routes, using the territory of Royal Tratturo Candela-Pescasseroli (RT) as a case study. Sixteen J. regia provenances were collected along RT and compared with 13 Italian provenances and the landrace Sorrento. Although the level of SSR polymorphism we observed was moderately high, AMOVA revealed that most of the diversity was located within individuals (92.58%), and geographical differentiation was low (D est = 0.076). Evidence for human-mediated domestication bottleneck events was detected in about 95% of walnut provenances. A Bayesian approach divided 456 walnut samples into three clusters: (1) Sorrento genotypes, (2) trees from the island of Sicily, and (3) the remaining germplasm. The UPGMA tree based on Nei's distances distinguished northeastern provenances and weakly grouped 12 of 16 provenances of RT. The observed genetic differences derived mainly from gradations in allele frequencies. Separation of the Sicilian provenance from the mainland may be explained in terms of founder effects and prolonged geographic isolation. Two contrasting forces, selection, and frequent inter-regional transfer of propagules, appear to drive the patterns of genetic variability for J. regia.
KeywordsJ. regia SSR genetic structure Royal Tratturo
The study was developed in the framework of the Italian Project “FIMONT” (Metodi e sistemi per aumentare il valore aggiunto degli alimenti tradizionali ed a vocazione territoriale nelle zone montane) supported by Italian Ministry of Research, scientific coordinator Professor Giacomo Elias (University of Milan), financial coordinator Dr. Rosanna Farina (Italian Mountain Institution, Rome). The authors thank Giovanni De Simoni, Marcello Cherubini, Daniela Taurchini, and Daniele Canestrelli for their support in statistical and laboratory analysis and Dr. Claudia Mattioni (CNR-IBAF, Porano) and Dr. Isacco Beritognolo (CNR-ISAFOM, Perugia) for their critical review of the manuscript. A warm thanks to Francesco Pallotta for logistic support in Molise and Abruzzo. The use of trade names is for the information and convenience of the reader and does not imply official endorsement or approval by the United States Department of Agriculture or the Forest Service of any product to the exclusion of others that may be suitable.
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