Genetic diversity and structure of natural Juglans regia L. populations in the southern Kyrgyz Republic revealed by nuclear SSR and EST-SSR markers

Persian walnut (Juglans regia L.), also known as common walnut, is an economically and ecologically important species in the Kyrgyz Republic. Natural forests of J. regia, walnut forests, grow between western and southwestern slopes of the Fergana and Chatkal mountain ridges in the southern Kyrgyz Republic. We investigated 11 natural populations of J. regia across its geographic range in the Kyrgyz Republic using eight nuclear microsatellite (nSSR) and eight expressed sequence tag (EST)-derived SSR (EST-SSR) markers. The SSR markers revealed comparatively high levels of genetic diversity (H-O=0.538, H-E=0.524). The overall genetic differentiation among populations was low, but statistically significant (F-ST=0.068). Significant correlation between genetic and geographic distances was observed, indicating an isolation-by-distance (IBD) model for J. regia. Spatial analysis of molecular variance (SAMOVA) and Bayesian analysis assigned individuals to two main genetic groups separating central region walnut populations (Jalal-Abad province) and southeastern region walnut populations (Osh province). Genetic diversity parameters such as gene diversity and allelic richness were significantly lower in the Osh province than in the Jalal-Abad province. Detection of significant genetic discontinuities suggested the existence of physical barriers to migration and gene flow among regions. We suggest that the Osh province and Jalal-Abad province populations should be considered as two evolutionary significant units (ESUs) for conservation efforts.