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Diploid Derivatives of Ustilago violacea with Altered Mating-Type Activity. II. Polyploid Segregations and Mechanism of Origin

Diploid strains of Ustilago violacea, heterozygous for mating-type alleles a1 and a2 and neutral in phenotype, spontaneously produce frequent derivative types termed "opaques." Two of the four types of opaques that have been described mate as either a1 or a2 cells and are termed op-a1 and op-a2, respectively. These opaque derivatives are still diploid and unchanged in genotype from the original a1a2 diploid at all chromosomes except the one carrying the mating-type locus. Crosses of these opaque strains clearly show that op-a1 and op-a2 have become homozygous for one mating-type allele. Thus, op-a1 crossed with haploid a2 cells yielded teliospores (brandspores) which showed classical triploid segregation of all markers, including mating type. Similarly op-a1 crossed with op-a2 gave classical tetraploid segregation ratios of all markers. As ultraviolet light induces opaque formation to the same extent as it does mitotic crossing-over near other markers, opaques probably arise as a result of frequent mitotic crossing-over (or gene transpositions) near the mating-type locus to yield homozygous a1a1 or a2a2 types. Haploid-diploid dikaryons and diploid-diploid dikaryons of U. violacea were found to be strongly virulent, unlike diploid homokaryotic infections, and yield triploid and tetraploid teliospores, respectively. The volume of the teliospores increased more or less linearly with ploidy although there were interesting differences between spores derived from diploid monokaryons and those from haploid-haploid dikaryons.