Ultrastructure of a hypovirulent isolate of Sclerotinia sclerotiorum containing double-stranded RNA

Abstract

Sclerotia and hyphae of hypovirulent double-stranded RNA (dsRNA)-containing and virulent dsRNA-free isolates of Sclerotinia sclerotiorum were examined to describe and compare their subcellular structures using light and electron microscopy. Light microscopic analysis of virulent and hypovirulent tissues indicated that, in general, tissues from both isolates were similar. However, sclerotia of the hypovirulent isolate contained fewer protein bodies than those of the virulent isolate and both hyphae and sclerotia of the hypovirulent isolate displayed a more granular appearance throughout the cytoplasm. Electron microscopic analysis revealed that the granular appearance was associated with the presence of many small, double membrane bodies within all cells of the hypovirulent isolate. Comparable structures were not observed in any tissues examined from the virulent isolate. The average diameter of the double membrane bodies was 70 nm and they often were observed in continuum with the nuclear envelope. Particles representative of a mycovirus were not observed in any virulent or hypovirulent tissues. The results indicate that double-stranded RNA and hypovirulence in S. sclerotiorum is not associated with a typical mycovirus but may represent an unencapsidated mycovirus or viral-like agent packaged as double membrane bodies.