Unencapsidated double-stranded RNA associated with membrane vesicles in isolates of Alternaria solani

Double-stranded RNA (dsRNA) elements of 8·3 and 5·5 kbp were detected in three of five geographically distinct isolates of Alternaria solani. Variation with respect to the number of dsRNA segments was detected among isolates of the fungus. No virus-like particles were purified from the mycelium, or observed in ultra-thin sections of hyphae. Utilizing differential centrifugation or density gradient centrifugation for subcellular fractionation, dsRNA was found associated specifically with fractions containing 80-100 nm spherical membrane vesicles. Similar vesicles were observed in cytoplasm of hyphae by electron microscopy. Treatment of dsRNA-containing fractions with ribonuclease A did not degrade the dsRNA, suggesting that it is located inside the vesicles. There was no correlation between the radial growth of the isolates in laboratory media and the presence or absence or dsRNA.