The ultrastructure of hyphal anastomoses between vegetatively compatible and incompatible virulent and hypovirulent strains of Cryphonectria parasitica

Abstract

European hypovirulent (dsRNA-containing) Cryphonectria parasitica strain Ep-50 was paired individually with West Virginia virulent (dsRNA-free) strains Ep 15-7-7 (vegetatively compatible) and EP 7-5-1 (vegetatively incompatible) on cellophane membranes. Four to six hours after anastomoses formed, the strains were preserved using freeze-substitution and observed using transmission electron microscopy. Hyphal anastomoses between Ep-50 and Ep 15-7-7 showed complete cytoplasmic continuity, with microtubules and mitochondria extending through the fusion aperture. Spherical, membrane-bound virus-like particles, measuring 50–90 nm in diameter, were located in the Ep-50 hypha, the Ep 15-7-7 hypha, and the short anastomosis bridge between them. All anastomoses between the compatible strains involved a hyphal peg that grew toward a swelling that developed on the receiving hypha. Fusion took place between the swelling and the lateral wall of the peg. Anastomoses between the incompatible strains showed cellular collapse and cytoplasmic degeneration that extended away from the anastomosis area in hyphae of both strains. Because of this, vegetative incompatibility would seem to be a formidable barrier to hypovirulence conversion and biocontrol of C. parasitica. Key words: Endothia parasitica, hyphal fusion, virus-like particles, hypovirulence conversion.