Mating and Sexual Morphogenesis in Basidiomycete Fungi

The mating pathways of filamentous ascomycete and basidiomycete fungi are clearly similar, relying on pheromones and cognate receptors for cell communication as well as specific transcription factors to regulate gene expression. Fungal mating-type (MAT) genes encode critical transcription factors, but in basidiomycetes, unlike ascomycetes, they also encode the pheromones and receptors. The reason for this difference between the two major divisions of the Dikarya will become evident when specific aspects of the basidiomycete lifestyle are considered. Understanding the role that these MAT genes play in cell-specific gene expression and mate attraction is particularly relevant to understanding mating in all other members of the Dikarya, even though the lifestyles of these fungi and the actual genes at the MAT loci may differ. The pheromones encoded at the MAT loci of all basidiomycete fungi studied belong to the Saccharomyces cerevisiae a lipopeptide pheromone family, and the receptors are correspondingly members of the S. cerevisiae Ste3p family. Signal transduction processes during mating are initiated by the binding of pheromones to their cognate receptors. Thereby, specific signaling cascades that trigger defined cellular events such as increased pheromone secretion or cell cycle arrest are elicited. Besides pheromones and cognate receptors, genes encoding the basidiomycete homologues of the a1 and α2 proteins of S. cerevisiae are found at the second MAT locus, called MATb in Ustilago maydis and MATA in hymenomycetes. Isolating mutants defective in morphogenesis has been simplified by the use of special strains with self-compatible mutations in both sets of MAT genes.