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Abstract
Heterokaryotic colonies of Stagonospora nodorum consist of a fine mosaic of individual cells of the two component genotypes that are linked by frequent hyphal anastomosis. The intimate mixing does not extend to the colony edge where 1 mm3 blocks of hyphae all of the same genotype occur. Colonies developed from single pycnidiospores from heterokaryons were homokaryotic and segregated into the two component genotypes, demonstrating that all the nuclei in a spore are of the same genotype. Spore progenies from single pycnidia produced by heterokaryons, however, were frequently genetically heterogeneous, indicating that these asexual fruiting structures are not uninucleate in origin. The nuclear ratio in heterokaryons was characteristic of the component genotypes and could deviate markedly from 1:1 with no reduction in growth. Only 1.6% of pairs of physiologically-different nitrate non-utilizing mutants from different strains showed complementation, indicating that vegetative incompatibility is widespread in S. nodorum. The few compatible strains had common origins and may be clonally related. Tests on a limited number of combinations suggested that vegetative incompatibility could not be overcome by protoplast fusion. Attempts to isolate somatic heterozygous diploids and parasexual recombinants were largely unsuccessful. Heterokaryosis and parasexuality do not appear to be significant sources of natural variation in this fungus.