Persian (Common) walnut (Juglans regia L.) is a famous fruit tree species valued for its nutritious nuts and high-quality wood. Although walnut is widely distributed and plays an important role in the economy and culture of Pakistan, the genetic diversity and structure of its populations in the country remains poorly understood. Therefore, using 31 nuclear microsatellites, we assessed the genetic diversity and population structure of 12 walnut populations sampled across Pakistan. We also implemented the geostatistical IDW technique in ArcGIS to reveal “hotspots” of genetic diversity. Generally, the studied populations registered relatively low indices of genetic diversity (NA = 3.839, HO = 0.558, UHE = 0.580), and eight populations had positive inbreeding coefficient (FIS) values. Low among-population differentiation was indicated by AMOVA, pairwise FST and DC. STRUCTURE, PCoA and neighbor joining (NJ) analysis revealed a general lack of clear clustering in the populations except that one population in Upper Dir was clearly genetically distinct from the rest. Furthermore, the Mantel test showed no correlation between the geographic and genetic distance (r = 0.14, p = 0.22), while barrier analysis suggested three statistically significant genetic barriers. Finally, the spatial interpolation results indicated that populations in Ziarat, Kashmir, Dir, Swat, Chitral, and upper Dir had high intrapopulation genetic diversity, suggesting the need to conserve populations in those areas. The results from this study will be important for future breeding improvement and conservation of walnuts in Pakistan.

Background Common walnut (Juglans regia L.) has a long cultivation history, given its highly valuable wood and rich nutritious nuts. The Iranian Plateau has been considered as one of the last glaciation refugia and a centre of origin and domestication for the common walnut. However, a prerequisite to conserve or utilize the genetic resources of J. regia in the plateau is a comprehensive evaluation of the genetic diversity that is conspicuously lacking. In this regard, we used 31 polymorphic simple sequence repeat (SSR) markers to delineate the genetic variation and population structure of 508 J. regia individuals among 27 populations from the Iranian Plateau. Results The SSR markers expressed a high level of genetic diversity (HO = 0.438, and HE = 0.437). Genetic differentiation among the populations was moderate (FST = 0.124), and genetic variation within the populations (79%) significantly surpassed among populations (21%). The gene flow (Nm = 1.840) may have remarkably influenced the population genetic structure of J. regia, which can be attributed to anthropological activities and wind dispersal of pollen. The STRUCTURE analysis divided the 27 populations into two main clusters. Comparing the neighbor-joining and principal coordinate analysis dendrograms and the Bayesian STRUCTURE analysis revealed the general agreement between the population subdivisions and the genetic relationships among the populations. However, a few geographically close populations dispersed into different clusters. Further, the low genetic diversity of the Sulaymaniyah (SMR) population of Iraq necessitates urgent conservation by propagation and seedling management or tissue culture methods; additionally, we recommend the indispensable preservation of the Gonabad (RGR) and Arak (AKR) populations in Iran. Conclusions These results reflected consistent high geographical affinity of the accession across the plateau. Our findings suggest that gene flow is a driving factor influencing the genetic structure of J. regia populations, whereas ecological and geological variables did not act as strong barriers. Moreover, the data reported herein provide new insights into the population structure of J. regia germplasm, which will help conserve genetic resources for the future, hence improving walnut breeding programs’ efficiency.

Background: Common walnut (Juglans regia L.) has a long cultivation history, given its highly valuable wood and rich nutritious nuts. The Iranian Plateau has been considered one of the last glaciation refugia and a center of origin and domestication for the common walnut. However, a prerequisite to conserve or utilize the genetic resources of J. regia in the Plateau is a comprehensive evaluation of the genetic diversity that is conspicuously lacking. In this regard, we used 31 polymorphic simple sequence repeat (SSR) markers to delineate the genetic variation and population structure of 508 J. regia individuals among 27 populations from the Iranian Plateau. Results: The SSR markers expressed a relatively high level of genetic diversity (HO= 0.438, and HE= 0.437). Genetic differentiation among the populations was moderate (FST= 0.124), and genetic variation within the populations (79%) significantly surpassed among populations (21%). The gene flow (Nm= 1.79) may have remarkably influenced the population genetic structure of J. regia, which can be attributed to anthropological activities and wind dispersal of pollen. The STRUCTURE analysis divided the 27 populations into two main clusters. A comparison of the neighbor-joining (NJ) and Principal coordinate analysis (PCoA) dendrograms and the Bayesian STRUCTURE analysis revealed the general agreement between the population subdivisions and the genetic relationships among the populations. However, a few geographically close populations dispersed into different clusters. Further, the low genetic diversity of the SMR and TKR populations necessitates urgent in situ conservation; additionally, we recommend the indispensable preservation of the RGR and AKR populations. Conclusion: These results reflected consistent high geographical affinity of the accession across the Plateau. Our findings suggest that gene flow is a driving factor influencing the genetic structure of J. regia populations, whereas ecological and geological variables did not act as a strong barrier. Moreover, the data reported herein provide new insights into the population structure of J. regia germplasm, which will help conserve genetic resources for the future improving walnut breeding programs’ efficiency.