Analysis of genetic diversity and relationships among 86 Persian walnut (Juglans regia L.) genotypes in Tibet using morphological traits and SSR markers

Walnut (Juglans regia L.) trees have been seed-propagated in Tibet at an average altitude of 3,000 m asl under varied climates for many years (> 1,000). These factors may have resulted in the accumulation of an abundant and diverse genetic resource, however the genetic characteristics of walnut grown in Tibet have not been studied. We used 12 simple sequence repeat (SSR) loci and ten quantitative trait loci (QTLs) to reveal the extent of genetic diversity and relationships among 86 genotypes of Tibetan walnut. Nut weights ranged from 6.50 - 22.52 g, kernel weights from 2.52 - 10.89 g, and kernel percentages (kernel weight/nut weight) from 29.62 - 55.41% among the 86 genotypes studied. The 12 SSR primer pairs amplified 119 alleles, ranging from 4-18 per locus, with a mean value of 9.92 alleles per locus. The 86 walnut genotypes growing in Tibet showed a relatively moderate degree of genetic diversity based on SSR loci and morphological traits. Dendrograms based on either SSR markers or morphological traits indicated that the 86 genotypes did not cluster according to their geographic origin. A Mantel test showed that the correlation coefficient between morphological- and SSR-based genetic distances was not significant (r = 0.12; P = 0.23). Because the plant material sampled in this research was representative of walnut germplasm grown in high altitude regions, the results provide important new information for germplasm conservation and screening for superior germplasm.