HETEROCARYOSIS AND PROTOPLASMIC INCOMPATIBILITY IN NEUROSPORA CRASSA

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Abstract
The genes C and D and the alternate alleles c and d in N. crassa are concerned with heterocaryon formation, as previously reported. Heterocaryon tests with pairs of isolates of the same mating type proved that all pairs of the same heterocaryon genotype form compatible heterocaryons, whereas all other pairs are incompatible, i.e., fail to grow or grow erratically on minimal medium. These 2 classes of pairs were termed het+ and het-, respectively. The present experiments indicate that the reaction between members of all het- pairs involves protoplasmic incompatibility. Representa tive isolates of both het+ and het- pairs of the riboflavinless and the inositolless strains were observed microscopically during vegetative growth, with results as follows: In the het-pairs all interstrain hyphal fusions initiated an irreversible incompatibility reaction. Upon exchange of protoplasm through the new opening between the hyphae, the cells in the vicinity of the fusion gradually disintegrated. Since no exception was found in any het- pair and all interstrain hyphal fusions in all het+ pairs were normal, it is concluded that the incompatibility reaction is the mechanism whereby heterocaryotic growth in the het- pairs is inhibited or prevented.