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Heterokaryon incompatibility function of barrage-associated vegetative incompatibility genes (vic) in Cryphonectria parasitica

     Biology Department, Carleton University, Ottawa, Ontario, K1S 5B6 Canada

Michael G. Milgroom

     Department of Plant Pathology, Cornell University, Ithaca, New York 14853

Six vegetative incompatibility (vic) loci have been identified in Cryphonectria parasitica based on barrage formation during mycelial interactions. We used hygromycin B- and benomyl-resistance as forcing markers in C. parasitica strains to test whether heteroallelism at each vic locus prevents heterokaryon formation following mycelial interactions. Paired strains that had allelic differences at any of vic1, 2, 3, 6 or 7 but not vic4 displayed heterokaryon incompatibility function, as recognized by slow growth or aberrant morphology. While clearly forming barrages in mycelial interactions, paired strains with different alleles at vic4 formed stable heterokaryons. With examples from other fungi, this inconsistency at vic4 suggests that barrage formation and heterokaryon incompatibility are not different manifestations of the same process. Rather, the evidence indicates that heterokaryon incompatibility represents a component of a vegetative incompatibility system that may also use cell-surface or extracellular factors to trigger programmed cell death to modulate nonself recognition in fungi.

Key words: chestnut blight fungus, programmed cell death, virus transmission