Analysis of genetic diversity in three Eastern European walnut germplasm collections

Walnut (Juglans regia L.) is a highly valuable, popular nut crop in many regions of the world. An effective management and utilization of plant genetic resources requires their comprehensive study. The current research was conducted to evaluate the genetic diversity and relationship of three Russian walnut collections, which are geographically located in Eastern Europe (Moscow, Crimea, and Kuban regions). Additionally, walnut varieties from USA, France, Moldova, and Ukraine were included in the plant materials under study. Totally, 141 walnut accessions were genotyped with 12 SSR markers. The number of alleles per locus varied from four (WGA72 and WGA4) to 21 (WGA276), with an average value of 10 alleles per locus. The level of polymorphism in the Russian subset of walnut genotypes was characterized as relatively high compared to the worldwide walnut germplasm. The analysis of genetic relationship within the experimental walnut accessions set involved using structure analysis, principal coordinate analysis (PCoA) and Fst - analysis. The structure analysis revealed seven main clusters of genotypes. Clustering generally correlated with the geographical origin of the walnut accessions. The NBG (Crimean) collection showed a relatively high level of influence from Moldovan and Ukrainian genetic backgrounds and also showed impacts from western European germplasms. Kuban local varieties and accessions were divided into two independent groups which are distanced from Kuban accessions that have Central Asian ancestral backgrounds. All sets of walnut accessions from the Kuban are distanced from the other studied groups of genotypes according to PCoA analysis. The data may reflect possible borders of contact between Asian and European walnut gene pools, along the line from Crimea to the north and between the basins of Dnieper and Don rivers. The results of this study can increase the effectiveness of management strategies, utilization of walnut genetic resources, and their combined applications in breeding.