Spontaneous appearance of genetically distinct double-stranded RNA elements in Rhizoctonia solani

A tuberborne sclerotium of the plant-pathogenic basidiomycete Rhizoctonia solani gave rise to a culture (Rhs 1AP) that exhibited a sector (Rhs 1A1) with reduced pigmentation and growth rate. Rhs 1AP is virulent on potato, which is the only known host plant for Rhs 1AP. In contrast, Rhs 1A1 is hypovirulent or nonpathogenic on potato. The virulent isolate Rhs 1AP contains two double-stranded RNAs (dsRNAs) of 23 and 6.5 kb, whereas the hypovirulent Rhs 1A1 possesses three dsRNAs in addition to the two found in Rhs 1AP. The apparent sizes of the three novel dsRNAs are 25, 3.7, and 1.2 kb. We constructed complementary DNA libraries of these dsRNAs and conducted Northern blot hybridization analysis, which showed that the dsRNAs with corresponding sizes (23 and 6.5 kb) occurring in both cultures are genetically similar, if not identical. The five dsRNAs, however, are not related to one another. For over 12 yr, both the phenotype and the dsRNA content of Rhs 1AP and Rhs 1A1 remained stable until 2 yr ago, when Rhs 1AP gave two more sectors (Rhs 1A2 and Rhs 1A3) exhibiting a slow growth habit. Interestingly, both of the slow-growing subcultures lacked the original Rhs 1AP dsRNAs (23 and 6.5 kb). Rhs 1A2 and Rhs 1A3 have had stable phenotypes and dsRNA patterns since the time of their emergence. All of the cultures (Rhs 1A1, Rhs 1A2, and Rhs 1A3) derived from Rhs 1AP contained the 1.2-kb dsRNA. Polymerase chain reaction analysis of total DNA showed that sequence(s) genetically related to the 3.7-kb dsRNA element are found in genomic DNA from Rhs 1AP and Rhs 1A1