Department of Biological Sciences, 5722 Deering Hall, University of Maine, Orono 04469-5722, U.S.A.
A virulent field isolate of Rhizoctonia solani AG 3, Rhs 1AP, has given rise to three sectors in a period of several years. Subculturing of these sectors resulted in three hypovirulent isolates—Rhs 1A1, Rhs 1A2, and Rhs 1A3. We reported previously that five genetically different double-stranded (ds) RNAs occur in these four isolates, with each isolate containing a unique combination of two or more dsRNAs. We report here that all five dsRNA elements occur in the cytoplasm, and none in the nucleus. The mitochondria contains low to moderate concentrations of the four larger dsRNAs. The four isolates were paired in selected combinations in an attempt to transmit specific dsRNAs to cultures lacking these dsRNAs. This approach generated groups of near-isogenic lines possessing the same dsRNA elements. As many as six cultures possessing the same dsRNA genotype were found to have the same degree of pathogenicity. Consistently, acquisition of a 6.4-kb dsRNA brought about increased virulence, whereas the presence or acquisition of a 3.6-kb dsRNA resulted in diminished virulence in the recipient culture. All cultures had the same mitochondrial DNA haplotype as that of the original field isolate, Rhs 1AP. This, in conjunction with the fact that all cultures in this study had a single nuclear DNA origin (Rhs 1AP), indicates a strong correlation between certain phenotypic characters or changes thereof and dsRNA profiles or acquisition of particular dsRNAs, respectively.