Genetic diversity of Persian walnut (Juglans regia) in the cold-temperate zone of the United States and Europe

- There was strong evidence for a bottleneck in the U.S. population, but no evidence of inbreeding.
- Our results show that Midwestern USA J. regia germplasm is genetically distinct from existing Eastern European germplasm.
- Exchange of J. regia between North America and north Eastern Europe should be fruitful in both directions. Additional cold hardy J. regia germplasm will be useful for expanding the range of cultivation for the species as a nut crop and for production of hybrid J. × intermedia for timber production.

We compared the genetic diversity of Juglans regia L. growing in the cold temperate region of the eastern U.S. with J. regia growing in the cold-temperate and Mediterranean regions of Europe. Ten microsatellite (SSR) loci were used to assess the genetic relationships among 114 total trees originating from the Midwestern USA (n = 34), Hungary (n = 30), Slovakia (n = 28), and Italy (n = 22). All SSR loci were highly polymorphic with an average of 7.4 alleles per locus. All 114 trees were confirmed to be unique genotypes. Cluster analysis using Neighbor-joining (NJ) placed genotypes according to their geographic origin. STRUCTURE software confirmed the results of the NJ analysis and produced three main groups consistent with the geographic origins of the samples. According to Nei's genetic identity, samples from Slovakia and Hungary showed the highest similarity (0.94), while samples from the USA and Hungary had the lowest similarity (0.37). The genetic diversity of J. regia from the USA and Eastern Europe was relatively high compared to Italian samples. We found little genetic similarity between J. regia sampled from the eastern USA with J. regia currently growing near the Carpathian Mountains. There was strong evidence for a bottleneck in the U.S. population, but no evidence of inbreeding.