The production and characteristics of diploids inUstilago violacea

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Abstract
1. Forty-nine auxotrophic mutants were isolated after irradiation with ultraviolet light. The biochemical requirements were identified and the response to possible precursors tested in some of them.2. Diploid colonies were synthesized from compatible, auxotrophic haploid strains on an artificial medium by an adaptation of the balanced heterokaryon technique. Selection of diploid cells was especially convenient as the dikaryon cannot grow on such media, and is therefore only a transitory stage under these cultural conditions. Diploid cells were different in shape and size from haploids, and gave rise to colonies which could be distinguished by eye from haploids. The way in which diploid colonies arose from fusions between haploid cells was followed by microscopic observation. When known numbers of fused haploid cells were plated on minimal medium, diploid colonies occurred at a frequency of 3 × 10−4.3. Diploid cells, heterozygous for mating-type alleles, were incompatible with either haploid mating-type (i.e. neutral).4. Diploid cells could infect the host plant as a pure culture (i.e. they are solopathogenic).5. Meiotic and mitotic segregation of the large-celled strains was used to confirm diploidy. Spontaneous mitotic segregation was very rare. Mitotic crossing-over was induced at a high frequency by irradiation of diploid cells with ultraviolet light. A convenient technique for induced haploidization was devised usingp-fluoro-phenylalanine. Preliminary evidence using this technique indicates a haploid chromosome number of at least seven.6. It is considered thatU. violaceahas many advantages as an organism for genetical research, especially for analysis of the parasexual cycle.