Hua Wang, Gang Pan, Qingguo Ma, Junpei Zhang, Dong Pei
Tree Genetics & Genomes
While walnuts are grown worldwide, two species are predominantly cultivated in China: Juglans regia and J. sigillata. J. regia is an introduced species, while J. sigillata is an indigenous species mainly distributed in southwestern China. In Tibet, unique plateau climatic conditions and relatively less intense agriculture are favorable for walnuts to preserve a greater variety of resources. Under these circumstances, understanding the genetic diversity and genetic structure of walnut populations in Tibet are essential for genetic variation conservation and superior germplasm resource screening. The objectives of this study were to reveal the genetic diversity and genetic structure of walnut populations in Tibet and infer the relationship between two species (J. regia and J. sigillata) using 12 molecular markers. Results based on 209 walnut trees from nine walnut populations suggested that the level of simple sequence repeat polymorphism was moderately high. However, the ratio of rare alleles (allele frequency﹤0.05) was higher than that reported in other studies. An analysis of molecular variance revealed that significant genetic variation not only existed among populations of J. regia (10.25%, P < 0.0001) and J. sigillata (11.07%, P < 0.0001), but also between the two species (7.91%, P < 0.0001). A Bayesian approach divided 209 walnut samples into two clusters and indicated the differentiation of these two species. The assignment analysis revealed the presence of J. intermedia hybrids among the sampled individuals. These results suggested that walnut populations in Tibet preserved some unique germplasm, and that the introgression between J. regia and J. sigillata may account for the convoluted boundary between the two species.