Genetics of Ustilago violacea. XXXV. Transposition in Haploid and Diploid Sporidia and Germinating Teliospores.

Abstract

Ustilago violacea sporidia of the white (w) MAD strain (a-2 w lys-3 ino-1 thi) incubated on minimal medium containing 100 mM KClO3 (potassium chlorate) produced only colonies with the pink phenotype. Sporidia from these colonies retained their pink color on complex medium. Sporidia of the diploid D1 strain (a-1 y nic-1 thi/a-2 w met-1 arg-f Chl70 thi) and of the diploid D2 strain (a-1 y his-1 glu-1 thi/a-2 w lys-3 ino-1 thi) produced pink colonies on complex medium. Streaks of diploid D1 sporidia from the pink colonies were stable on complex medium. In contrast, streaks of diploid D2 sporidia, which are heterozygous for the MAD strain, were unstable, initially producing pink colonies on complex medium but then, with continued incubation, producing white termini. Sporidia from the white termini with diploid morphology continued to yield white colonies. Teliospore colonies from three crosses with the MAD strain as a common parent were uniformly pink or had a pink sector instead of the expected uniformly white colonies or colonies having a white sector. Four of 20 and 13 of 20 teliospore colonies, respectively, from two of the three crosses had both a-1 and a-2 sporidia, and the remaining colonies had only a-1 or only a-2 sporidia. All 40 teliospore colonies from the third cross had only a-1 or only a-2 sporidia. All of these observations indicated that the MAD strain may have two autonomous, transactive transposable elements in different chromosomes and that insertional mutations in at least two haplolethal loci were responsible for the teliospore colonies with only a-1 or only a-2 mating type. Crossing over between a haplolethal locus and the centromere would account for teliospore colonies with both a-1 and a-2 sporidia.