Abstract
The common walnut (Juglans regia L.) and iron walnut (J. sigillata Dode) are well-known economically important species cultivated for their edible nuts, high-quality wood, and medicinal properties and display a sympatric distribution in southwestern China. However, detailed research on the genetic diversity and introgression of these two closely related walnut species, especially in southwestern China, are lacking. In this study, we analyzed a total of 506 individuals from 28 populations of J. regia and J. sigillata using 25 EST-SSR markers to determine if their gene introgression was related to sympatric distribution. In addition, we compared the genetic diversity estimates between them. Our results indicated that all J. regia populations possess slightly higher genetic diversity than J. sigillata populations. The Geostatistical IDW technique (HO, PPL, NA and PrA) revealed that northern Yunnan and Guizhou provinces had high genetic diversity for J. regia while the northwestern Yunnan province had high genetic diversity for J. sigillata. AMOVA analysis revealed that significant genetic variation was mainly distributed within population as 73% in J. regia and 76% in J. sigillata. The genetic differentiation (FST) was 0.307 between the two walnut species (p < 0.0001), which was higher than FST values within populations (J. regia FST = 0.265 and J. sigillata FST = 0.236). However, the STRUCTURE analysis of the J. regia and J. sigillata populations revealed two genetic clusters in which gene introgression exists, therefore, the boundary of separation between these two walnut species is not clear. Moreover, these results were validated by NJ and UPGMA analysis with additional conformation from the PCoA. Based on the SSR data, our results indicate that J. sigillata is an ecotype of J. regia. Taken together, these results reveal novel information on population genetics and provide specific geographical regions containing high genetic diversity of the Juglans species sampled, which will assist in future conservation management.
Highlights
Walnuts trees have been valued since ancient times for their edible nuts and high-quality timber [1,2].Walnuts belong to the family Juglandaceae which includes more than 20 species, with the most well-known being J. regia L. [1,2,3,4,5]
The null allele test indicates a lower frequency of null alleles at 19 loci when compared to the threshold frequency (* = 0.15) across all the J. regia and J. sigillata populations studied while the probability of null alleles was slightly significant for the remaining six loci
The resulting phylogenies are congruent and similar to the Principal coordinate analysis (PCoA) and STRUCTURE results, demonstrating that the J. regia and J. sigillata populations do cluster into two groups according to their presumed walnut species; the results indicate gene introgression or hybridization between the different populations based on STRUCTURE
Summary
Walnuts trees have been valued since ancient times for their edible nuts and high-quality timber [1,2].Walnuts belong to the family Juglandaceae (genus Juglans) which includes more than 20 species, with the most well-known being J. regia L. (common walnut) [1,2,3,4,5]. J. sigillata is one of the most desirable and economically valuable hardwood timber tree species found in southwestern China. This species holds great religious and cultural importance in the region [11,12]. In southwestern China, J. regia and J. sigillata display sympatric distribution, most notably in the Yunnan and Guizhou provinces, and in Tibet [9,11]. Such distributions should enable the detection and extent of gene introgression between the two species and within different populations
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