Abstract: 【Objective】The objectives of this study are to investigate genetic structure of Rhizoctonia solani AG1-IA among different geographic populations from southern China, and to provide valuable information for the epidemiology of sheath blight disease on rice.【Method】R. solani AG1-IA population containing 188 isolates collected from 8 provinces (autonomous region) in southern China were genetically assessed using 8 fluorescence-labeled SSR markers. Genetic diversity parameters were calculated using POPGENE version 1.31, and inbreeding coefficient (F_IS) was generated with the computer program FSTAT 2.9.3. Deviation from Hardy-Weinberg equilibrium (HWE) was tested by the Markov chain model with chi-square test implemented in the GENEPOP 4.2. Analysis of molecular variance (AMOVA) was carried out in the software Arlequin 3.1, and gene flow (Nm) was estimated according to population differentiation index (F_ST). To illustrate genetic relationships based on their pairwise Nei’s genetic distances, UPGMA dendrogram was constructed with MEGA 5.0. Bayesian clustering analysis was undertaken to determine the population structure and the degrees of genetic admixture using the STRUCTURE program version 2.3.3. Correlation between genetic and geographic distances between populations was assessed with a Mantel test. 【Result】 For populations of R. solani AG1-IA, the mean observed and effective number of alleles were 4.025 and 2.071, respectively. Shannon’s information index ranged from 0.659 to 1.088 with an average of 0.859. Allele richness varied from 2.500 to 5.152 with a mean of 3.858. Observed heterozygosity ranged from 0.425 to 0.619 with an average of 0.506, and the expected heterozygosity ranged from 0.399 to 0.546 with an average of 0.472. The negative value of inbreeding coefficient (F_IS=-0.069) indicated excess of heterozygotes (or deficiency of homozygotes) in the total population. Six of the eight populations significantly deviated from Hardy-Weinberg equilibrium due to heterozygote excess or deficiency, supporting the evidence for a mixed reproductive system in populations, including both asexual and sexual reproduction, while the balance between asexual and sexual reproduction was population dependent. AMOVA attributed about 88.14% of the variance to individuals within populations, indicating that the main genetic variation existed within populations. A positive correlation (r=0.422, P=0.025) was detected between genetic and geographical distances of populations by Mantel test. UPGMA dendrogram indicated that all populations were separated into two genetically differentiated subgroups (F_ST =0.209-0.624), the first included two populations (GN and CT) located along the Pearl River was significantly distinct from the second made up of six populations located along the Yangtze River, which in agreement with STRUCTURE-based analysis. Extensive genetic admixture was observed in the Yangtze River populations, and high levels of gene exchange occurring within the subgroup (Nm = 2.525-8.447) were indicated by low population differentiation (F_ST=0.029-0.094). 【Conclusion】 R. solani AG1-IA with a wide distribution, a mixed reproductive mode, and long-distance dispersal via sclerotia or mycelium, these characteristics could account for the relatively high level of genetic diversity among populations in southern China. The migration of R. solani AG1-IA, to some extent, to prevent genetic divergence of pathogen populations to ensure that high levels of gene flow occured in populations located in the Yangtze River tributary region. Significant population subdivision was evident between the Yangtze River populations and the Pearl River populations, suggesting that restricted long-distance migration of R. solani AG1-IA was a plausible explanation for the spatial structures of genetic variation of pathogen populations.

Key words: Rhizoctonia solani AG1-IA, SSR marker, genetic diversity, genetic difference, gene flow