ISOLATION AND CHARACTERIZATION OF CRYPHONECTRIA-PARASITICA MUTANTS THAT MIMIC A SPECIFIC EFFECT OF HYPOVIRULENCE-ASSOCIATED DSRNA ON LACCASE ACTIVITY

Hypovirulent, double-stranded (ds) RNA-containing strains of the chest nut blight fungus Cryphonectria parasitica were previously shown to pr oduce less laccase activity than virulent strains when grown on malt e xtract agar containing tannic acid (Bavendamm reaction). Three mutants that lacked this specific laccase activity were selected after UV mut agenesis of a dsRNA-free, wild-type strain. Complementation tests and sexual crosses showed that all mutations were recessive, two were alle lic (lacR1-1 and lacR1-2), and one (lacR2) was nonallelic. No linkage was detected between the two loci. None of the known C. parasitica lac cases (LAC1, LAC2, and LAC3) was substantially reduced in the lacR1 mu tants. The lacR2 mutant, in contrast, produced about 10-fold less extr acellular LAC1 and LAC3 activities than the wild-type strain. Intracel lular LAC2 was reduced to about 50% in this mutant. These results sugg est a role for both LAC1 and LAC3 in the Bavendamm reaction. The three mutations had no significant effect on fungal virulence, pigmentation , and sporulation, all phenotypes that were suppressed in an isogenic dsRNA-containing strain.