Abstract:

Aims: Juglans regia, common walnut or Persia walnut, is an economically important tree crop, which is widely cultivated in northern China. Yet, the genetic diversity and population structure of J. regia in northern China is still under explored. In this study, we carried out population genetic analysis of J. regia in northern China to provide a scientific basis for germplasm conservation and utilization.
Methods: We sampled 491 individuals from 19 populations of J. regia from six provinces in northern China. A total of 31 polymorphic SSR loci were applied to genotype the samples. Different parameters (e.g., N_A, number of alleles; N_E, effective number of alleles; H_O, observed heterozygosity; H_E, expected heterozygosity; A_R, allelic richness; PIC, polymorphism information content) were calculated to characterize the genetic diversity, while genetic differentiation and population structure were assessed using analysis of molecular variance (AMOVA), STRUCTURE, principal coordinates analysis (PCoA) and neighbor-joining tree (NJ) analysis.
Results: The average genetic diversity parameters for the entire dataset were low (N_A = 2.620, H_O = 0.368, H_E = 0.368). AMOVA analysis showed that 16% of the genetic variation was partitioned among populations, and 84% within populations. The clustering results of STRUCTURE, PCoA and NJ revealed two groups in northern China, termed Group 1 and Group 2, respectively. Group 2 included one population from Qinghai Province, with the remaining 18 populations belonging to Group 1. The genetic variation was mainly from within groups, and with a high level of genetic differentiation between groups (F_ST = 0.32). However, the two groups shared a considerable proportion of alleles at contact region, and indeed, a genetic introgression signal was detected there.
Conclusion: We revealed two genetic groups with low genetic diversity of J. regia in northern China. The low genetic diversity of the J. regia populations may be ascribed to the cultivation history and long human-mediated selection and seed dispersal. Genetic differentiation between the two groups may be attributed to cultivation history and local adaptation in contrasting environments. Finally, based on the genetic diversity and genetic structure, we recommend that conservation priority should be given to populations in the Hainan Prefecture of Qinghai Province and Tianshui City of Gansu Province. Our study clarify the genetic diversity and population structure of J. regia from six provinces in Northern China, hence provides a basis for future conservation and utilization of walnut germplasm.

Key words: genetic diversity, population structure, Juglans regia, northern China, microsatellite marker