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Abstract
SUMMARY: ‘Lucerne’ auxotrophs of Verticillium albo-atrum produced by ultraviolet irradiation and derived from the same parental strain formed heterokaryons on minimal agar 4-5 weeks after inoculation. A degree of heterokaryon incompatibility was shown between auxotrophs derived from different isolates of V. albo-atrum and one of V. dahliae. Conidial methods of heterokaryon synthesis failed but transference of agar inocula from complete medium agar cultures to minimal medium agar was successful. Conidia were uni-nucleate and nuclear migration subsequent to germ-tube fusion was observed. Heterokaryons grew slowly and were unstable, but by using di-auxotrophs a greater stability was achieved and a number were subcultured successfully for more than a year. Spore analysis yielded colonies of both parental types with various ratios. A heterokaryon test between a hyaline and normal dark auxotroph of V. albo-atrum indicated that an important system controlling morphogenesis might be linked with a cytoplasmic factor. A hypothesis is proposed to account for the production and degree of stability of the hyaéine variants.
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