Development of Chloroplast Microsatellite Markers and Assessment of Genetic Diversity and Population Structure of Sophora tonkinensis Gagnep. in Southwestern China

Sophora tonkinensis Gagnep. is an important medicinal shrub native to the karst regions of southwestern China, where long-term overharvesting and habitat fragmentation have markedly reduced wild resources. Although recent phytochemical, transcriptomic, and chloroplast genomic studies have improved understanding of this species, its maternally inherited population structure has remained unclear. To address this gap, we developed nine novel chloroplast simple sequence repeat (cpSSR) markers and used them to genotype 274 individuals from eighteen wild populations. A total of 41 alleles were detected, with 2–10 alleles per locus, indicating moderate to high polymorphism at the species level. By combining the nine cpSSR loci, we further identified 25 chlorotypes, including 19 private chlorotypes. Within-population chloroplast diversity was generally low, and five populations were monomorphic, whereas HJSE and LYNG retained comparatively high chlorotype diversity. Genetic differentiation among populations was extremely strong (mean FST = 0.808), whereas historical gene flow was very limited (Nm = 0.112), and AMOVA showed that 85% of total chloroplast variation occurred among populations. Taken together, chlorotype network analysis, chlorotype geographic distribution, UPGMA, PCoA, and exploratory STRUCTURE analysis supported three geographically structured chloroplast groups, indicating long-term restriction of seed-mediated dispersal across the fragmented karst landscape. These newly developed cpSSR markers and the derived chlorotype framework provide a practical basis for tracing maternal lineages, prioritizing conservation units, guiding ex situ germplasm sampling, and informing future breeding of this nationally protected species. Overall, the present results describe chloroplast-based maternal structure rather than total genome-wide diversity in S. tonkinensis.