Unravelling the genetic diversity and population structure of Persian walnut in the Iranian Plateau

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 ( H O = 0.438, and H E = 0.437). Genetic differentiation among the populations was moderate ( F ST = 0.124), and genetic variation within the populations (79%) significantly surpassed among populations (21%). The gene flow ( N m = 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.