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SUMMARY Stable auxotrophic mutants were induced in Humicola sp. strain 20-31 by irradiation with far ultraviolet light. Mixed inoculation of different auxotrophs on to minimal medium agar produced prototrophic mycelia in all seven combina- tions tested with a frequency of 0.5 to 2.5 %. These prototrophic mycelia were stable when subcultured by macerated hyphae, single hyphal tips, single aleurio- spores or single phialospores, and contained nuclei of approximately twice the volume of those in strain 20-3 I. They were more sensitive to growth inhibition by p-fluorophenylalanine (FPA) than 20-3 I and its auxotrophic derivatives and, at concentrations of 140 and 160 pg FPA/ml, gave rise to more vigorously growing sectors. The mycelia from these sectors segregated for morphological and auxo- trophic markers differentiating the parent auxotrophs and contained nuclei similar in size to strain 20-31. When reinoculated on to FPA media, these segregants showed the higher tolerance characteristic of strain 20-31 and failed to segregate further. It is concluded that in mixed culture, auxotrophic mutant derivatives of Humicola sp. strain 20-31 readily form somatic heterozygous diploids and that these are induced to haploidize by treatment with FPA. A difference in pigmenta- tion which segregates spontaneously in strain 20-31 is attributed to a single chro- mosomal gene and the six markers studied are assigned to at least four linkage groups. Parasexual genetic analysis may be of value in taxonomic studies of imperfect fungi.

Isolation and Haploidization of Heterozygous Diploid Strains in a Species of Humicola