XVI International Botanical Congess

In ascomycetes hyphal fusions result in stable heterokaryons when strains have identical heterokaryon incompatibility (het) alleles. Pseudohomothallic N. tetrasperma a mating type heterokaryon has mechanisms for outcrossing that can produce incompatible het combinations. Wild isolates are homoallelic for het-c, but populations possess het-c variation. The presence of three trans-species alleles in related isolates prove that N. tetrasperma outcrosses in nature and creates a dilemma. Variation of het-c in populations seems desirable and is presumed to reflect balancing selection for self recognition. However matings that involve different het-c alleles lead to heterokaryon breakdown and interrupt pseudohomothallism. Thus outcrossed individuals have lower fitness with pseudohomothallism restored only by added outcrossings.