Genetics of Ustilago violacea .33. Genetic evidence for insertional mutations in the magenta locus

Three spontaneous mutants (m-1, m-2, and m-31) with a new sporidial colony color (magenta, m) were found in the stable pink 1.A1 a-1 and 2.A2 a-2 laboratory strains. The m-l and white (w) mutations were very closely linked (<0.1 cM); the m locus was assumed to be distal from the w locus and in the same chromosome arm. The color mutations formed a map of very closely linked (<1 cM), centromere-linked loci: orange (o)-pumpkin (p)-yellow (y)-centr-wA-wB-m. Crosses between the m-1 and m-2 mutants, between the m-2 mutant and laboratory strains with a different color, and between the m-2 mutant and strains from the herbarium/field collections gave nonsectored and sectored teliospore colonies with a nonparental color. All of the teliospores colonies from cross AV13 involving strains 1.C417* y and 2.C413 p had a nonparental color or different nonparental colors, including magenta. The W-irradiation of m-1, m-2, and m-31 sporidia gave mostly pink (+) colonies, ranging from 9% to 48%. Occasional nonsectored w and p colonies as well as one bisectored m/+ and one w/+ colony were found. Approximately 10(3) m-1 sporidia were UV-irradiated, and the following colonies with a nonparental color were found: w (one), p (three), o (one), +(f)(one), m(s) (supermagenta) (one), and m/+ (one). Crosses involving the m-2 mutant and the +(f), m(s), p, w, and y mutants indicated that the +(f) and m(s) mutants resulted from an insertional mutation and the others from a genic mutation. Sites of element insertion were (1) most likely in the pericentric regions to give the pink phenotype, (2) less likely in the y and p loci in the same chromosome arm and in the complex w locus, and (3) least likely in the o locus in one arm and m locus in the other arm. These observations were explained by proposing the transcentric transposition of a cryptic or a transactive element in the homologous chromosome with the color mutations during meiosis and to the UV-induced transposition of an element in sporidia.