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Interspecific and intraspecific heterokaryons of Verticillium albo-atrum and Verticillium dahliae were formed by protoplast fusion or microinjection. Diploid formation was increased 1000-fold following either treatment. Combinations of auxotrophic strains which normally never form heterokaryons were obtained with both techniques. The results suggest that a variety of incompatibility factors exist in Verticillium. Compatibility and incompatibility in their strict sense did not apply to the strains used in this work, and the terms ‘heterokaryon-former’ and ‘heterokaryon non-former’ have been introduced instead. The compatibility functions are assumed to be nuclear and somehow associated with cell wall formation or components affecting the cell wall. Temperature had a profound effect on both the morphology and the nutritional marker ratio of interspecific heterokaryons. Although the frequencies of diploid formation for intc specific and intraspecific diploids were very similar, the corresponding haploidization frequencies for these diploids were markedly different. An examination of random aneuploid conidia showed that spontaneous haploidization and mitotic crossing-over occurs in interspecific diploids of Verticillium. The genetic segregation of interspecific diploids grown in the presence of haploidizing agents suggested that the two fungi are closely related and that substantial chromosomal homology may exist between them.
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