The intergeneric hybridization between Aspergillus niger and Trichoderma viride was studied. The fusion of protoplasts from auxotrophic mutant strains, using a polyethylene glycol solution, resulted in the formation of intergeneric fusants, the fusion frequencies being 10-5 to 10-4. The fusant strains were classifiable into two types according to their morphologies. The fusant strains of the first type formed prototrophic conidia and colonies showing an A. niger type morphology on both the minimal and supplemented media. These fusant strains were proved to be haploids by measurement of conidial sizes and DNA contents, and assumed to be recombinants. The fusant strains of the second type formed conidia showing the same nutritional requirements as those of the original auxotrophic mutant strains used for protoplast fusion. These fusant strains formed colonies showing mixed morphologies, between those of A. niger and T. viride, on the minimal medium and grew more slowly than the prototrophic parental strains of A. niger and T. viride. From these results and those of conidia analyses, the fusant strains of the second type were proved to be heterokaryons. Mycelial protoplasts of the fusant strains of the two types regenerated on the hypertonic minimal medium into colonies with morphological features identical to those of the original fusant strains.
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