Virus-like particles in the aquatic fungus, Rhizidiomyces.

Isometric virus-like particles (VLPs), 60 nm in diameter, have been observed by electron microscopy in eight isolates of thermal-shocked Rhizidiomyces, a zoosporic, unicellular member of the Hyphochytridiales. VLP pathogenesis was observed in all fungal developmental stages from the zoospores to the mature sporangia. Paracrystalline structures associated with mitochondria preceded VLP occurrence in multinucleate sporangia. Particles appeared first in the intact nucleus, and subsequently VLPs completely replaced the nucleus and cytoplasm of the fungal host. Cell wall breakdown followed with release of VLPs into the medium. Approximately 50% of each fungal isolate was destroyed by the VLPs under stress conditions of heat, low nutrition, or aging. However, sufficient numbers of zoospores were capable of maturing into sporangia which produced zoospores to maintain the isolates in culture. After cloning 10 generations of zoospores, VLPs could still be produced in the offspring, thus indicating vertical transmission of the VLPs from generation to generation.