Virus-like particles in hyphae and conidia of European hypovirulent (dsRNA-containing) strains of Cryphonectria parasitica

Abstract

Hyphae and germinating conidia of European hypovirulent (dsRNA-containing) Cryphonectria parasitica strain Ep-50 and virulent (dsRNA-free) strain Ep-67 (isogenic derivative from Ep-50), and hyphae only of European hypovirulent (dsRNA-containing) strains Ep-4 and Euro-7 were freeze-substituted and examined for the presence of virus-like particles (VLPs) using transmission electron microscopy. Spherical, membrane-bounded VLPs, measuring 50–90 nm in diameter, were located in hyphae and conidia of hypovirulent strain Ep-50, but not virulent strain Ep-67. Hyphae of hypovirulent strains Ep-4 and Euro-7 contained VLPs similar to those found in Ep-50. The VLPs occurred in aggregates surrounded by rough endoplasmic reticulum or more rarely, scattered throughout the cytoplasm; most possessed an electron-dense core. Results of Bernhard's regressive staining technique and lipid extraction cytochemistry suggested that the particles consisted of RNA surrounded by a lipid membrane. A unique Golgi body was associated with the formation of VLPs in hypovirulent strain Ep-50. The VLPs do not resemble typical fungal viruses and may be the end product of a defense response on the part of the fungal host designed to wall off foreign nucleic acid. Key words: Endothia parasitica, mycovirus, defense response, hypovirulence, viroid, fungal Golgi.