Summary: Interspecific heterokaryons have been produced between auxotrophic mutants of and by polyethylene glycol induced fusion of somatic protoplasts. The heterokaryons grew very slowly producing colonies of irregular shape and, on complete medium, the parental strains were readily segregated. However, during long-term cultivation on minimal medium many of the heterokaryons gave rise to vigorously growing sectors characterized by the secretion of a brown pigment and by their stability when subcultured on complete medium. Protoplasts isolated from the heterokaryons gave rise to the same new colony type when regenerated on minimal medium, and to the new colony type plus the parental types on complete medium. The new colony type was assumed to be an interspecific ‘hybrid’ and showed normal vegetative morphology, regular colony shape and size but produced few conidia. Conidia formed early in the development of the ‘hybrid’ gave rise to ‘hybrid’ colonies. Various sizes of conidia were observed; the largest were uninucleate and the smallest were enucleate. Comparisons of conidial size, numbers of nuclei in cells and DNA content per nucleus for ‘hybrid’ and parental strains indicated that the ‘hybrid’ was diploid. Conidia from older cultures of the ‘hybrid’ were heterogeneous with ‘hybrid’, parental and recombinant colony types developing on germination. Cultivation of the ‘hybrid’ in the presence of benomyl gave segregation of parental and recombinant sectors.


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