Diploid Derivatives of Ustilago violacea with Altered Mating-Type Activity. I. Isolation and Properties

Abstract

Freshly isolated a1a2 diploid cells of Ustilago violacea grow in the vegetative phase (VP) by budding at temperatures above 20 C on complete medium but differentiate into sexual-precursor-phase (SPP) cells at temperatures below 20 C. At frequencies as high as 0.5%, strains arise spontaneously which lack the ability to differentiate into SPP cells under low temperatures. The strains are called opaque because their VP colonies are opaque as opposed to the translucent colonies of SPP cells. Three types of opaque strains are described: op-N (neutral in mating), op-a1 (a1 mating ability), and op-a2 (a2 mating ability). Analysis of all three types by mitotic haploidization indicates that they are still diploid and heterozygous for all marked chromosomes. Only the mating-type locus (and perhaps chromosome) appears to be altered during their formation. The op-N strains vary, some producing haploids of both mating types and others producing only one type of haploid (mainly a2). These op-N were still capable of differentiation into SPP but only under altered conditions of temperatures and nutrient. At least some op-N were solopathogenic, like the original diploid. The op-a1 and op-a2 strains yielded only the corresponding mating-type alleles on haploidization, could not develop into SPP cells under any conditions, and were not solopathogenic. Complementation tests indicated that the genes for SPP development were not impaired in either of these strains. Although the overall level of opaque formation was much higher, ultraviolet light induced opaque formation to the same extent as it induced mitotic crossing-over near a variety of genetic markers. It is, therefore, likely that op-a1 and op-a2 strains represent homozygous a1a1 and a2a2 strains formed by mitotic recombination. Possible mechanisms for the origin of opaques are discussed.