Genetic regulation of mating and dimorphism in Ustilago maydis

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Abstract
During its life cycle the phytopathogenic fungus Ustilago maydis adopts two different morphological forms. The haploid sporidia grow yeastlike and are nonpathogenic, the dikaryon grows filamentous and is able to infect corn plants. Cell fusion, pathogenicity, and dimorphism are under the control of two unlinked mating type loci, a and b. The a locus encodes a pheromone-based cell-recognition system and the b locus codes for two homeodomain proteins. Filamentous growth is dependent on the autocrine stimulation of the pheromone response pathway and on the presence of two nonidentical b alleles. We have identified genes whose expression is strictly limited to the filamentous phase and that are candidates for direct or indirect regulation by a and b. We also describe the development of a systematic screen for mutants that are affected in filamentous growth and (or) in pathogenicity. Key words: dimorphism, pheromones, homeodomain proteins, differential gene expression.