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Chapter 34 : Mating and Sexual Morphogenesis in Basidiomycete Fungi
Category: Microbial Genetics and Molecular Biology; Fungi and Fungal Pathogenesis
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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.
Life cycles of the filamentous hymenomycete C. cinerea (A) and the dimorphic heterobasidiomycete U. maydis (B). Details are given in the text.
Organization and function of genes at the S. cerevisiae MAT locus. (A) The MAT locus in MATa cells encodes a single protein, a1, that plays no role in regulating cell-specific a genes. The idiomorphic MAT locus in MATα cells encodes two proteins, α1, which activates cell-specific α genes, and α2, which represses cell-specific a genes. (B) In diploid cells with both alleles of MAT, α2 represses cell-specific a genes, and a new transcription factor generated by heterodimerization between α2 and a1 proteins represses all genes required for haploid mating functions.
Organization of the basidiomycete MAT loci encoding pheromones and receptor genes: U. maydis (A); S. reilianum (B); C. cinerea (C); and S. commune (D). The receptor genes are designated pra1 and pra2 in U. maydis; pra1, pra2, and pra3 in S. reilianum; rcb1, rcb2, and rcb3 in C. cinerea; and bar3 and bbr2 in S. commune. The prefix mfa is used to denote the pheromone genes of U. maydis and S. reilianum, phb in C. cinerea, and bap or bbp in S. commune. Different fill motifs represent different alleles of genes in U. maydis and S. reilianum and paralogous genes in different subloci in C. cinerea and S. commune. lga2 and rga2 in the U. maydis and S. reilianum a2 locus encode mitochondrial proteins, likely involved in uniparental inheritance. A compatible pheromone-receptor interaction that activates the pheromone response is shown for U. maydis.
Pheromone signaling network during mating in U. maydis. Components of a conserved MAPK module (hexagons) communicate with a conserved cAMP signaling pathway (left). Phosphorylation of Prf1 through PKA and MAPK signaling (circled P) is used to differentiate between a and b gene expression. The novel MAPK Crk1 as well as HMG box transcription factors Rop1 and Prf1 regulated prf1 expression transcriptionally (see the text for details) (figure modified from Feldbrügge et al., 2006 ).
Organization of the basidiomycete MAT loci encoding homeodomain proteins in U. maydis (A) and C. cinerea (B). Different fill motifs are used to represent different alleles of genes in both U. maydis and C. cinerea and paralogous genes in C. cinerea. bE, a1, b1, and d1 are HD1 genes, and bW, a2, b2, and d2 are HD2 genes. Crossed lines indicate compatible gene combinations. (C) Hypothetical heterodimer that results from heterodimerization between compatible HD1 and HD2 proteins.
Organization of mating-type loci of bipolar species U. hordei (A), C. neoformans (B), and C. disseminatus (C). Pheromones, receptors, and homeodomain proteins are indicated with the same symbols as in Fig. 3 and 5 . Large gaps between mating-type loci are indicated with the distance in kilobases. Pheromone and receptor genes shown for C. disseminatus are unlinked to MATA.
Comparative regulation of the sexual cycle in heterothallic basidiomycetes and ascomycetes. Evolution of heterothallism resulted in critical genes required to initiate the sexual cycle being localized to a MAT locus. In ascomycetes, the genes at MAT are translational activators that regulate differential expression of pheromone precursor and pheromone receptor genes and the many other genes required for processing the pheromones to active species. In basidiomycetes, the pheromone precursor and pheromone receptor genes are localized to the MAT locus.
Suggested steps in the evolution of MAT loci in ascomycetes and basidiomycetes. Refer to the text for details.
Receptor specificities of the predicted 11 pheromones encoded at the Ba3-B/32 locus of S. commune1