THE MOVEMENT OF VIRAL-LIKE RNA BETWEEN COLONIES OF CRYPHONECTRIA-PARASITICA

The viruslike dsRNA associated with hypovirulence of Cryphonectria (Endothia) parasitica moves between strains of the same vegetative compatibility types. To quantitate and determine the location of movement of the viral-like RNA between colonies in culture we have used colony in situ hybridization techniques. Virulent and hypovirulent (dsRNA-containing) strains of the fungus C. parasitica were inoculated in pairs on agar plates and covered with hybridization membranes at 24-hr intervals. After processing, the membranes were probed with a cDNA clone of part of the sequence of the plus-sense strand of the dsRNA. Presence of dsRNA in virulent colonies was detected between 1 and 2 days after the colonies touched. By 2 days after touching, dsRNA was detected throughout the periphery of the virulent colonies. DsRNA was always found in the highest concentration in the young peripheral mycelium. To confirm the in situ experiments, samples of hyphae were periodically removed along the length of paired colonies. The presence of dsRNA in these samples was assessed by dsRNA isolation and by colony morphology. The average rate of movement of the RNA over a 24-hr period was approximately 16 mm. This was three to four times faster than the colony growth rate during the same time period.