SPONTANEOUS APPEARANCE OF GENETICALLY DISTINCT DOUBLE-STRANDED-RNA ELEMENTS IN RHIZOCTONIA-SOLANI

A tuberborne sclerotium of the plant-pathogenic basidiomycete Rhizocto nia solani gave rise to a culture (Rhs 1AP) that exhibited a sector (R hs 1A1) with reduced pigmentation and growth rate, Rhs 1AP is virulent on potato, which is the only known host plant for Rhs 1AP. In contras t, Rhs 1A1 is hypovirulent or nonpathogenic on potato. The virulent is olate Rhs 1AP contains two double-stranded RNAs (dsRNAs) of 23 and 6.5 kb, whereas the hypovirulent Rhs 1A1 possesses three dsRNAs in additi on 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 libra ries of these dsRNAs and conducted Northern blot hybridization analysi s, which showed that the dsRNAs with corresponding sizes (23 and 6.5 k b) occurring in both cultures are genetically similar, if not identica l. 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 (R hs 1A2 and Rhs 1A3) exhibiting a slow growth habit. Interestingly, bot h 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 dsR NA patterns since the time of their emergence. All of the cultures (Rh s 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 se quence(s) genetically related to the 3.7-kb dsRNA element are found in genomic DNA from Rhs 1AP and Rhs 1A1.