Mating-type genes and hyphal fusions in filamentous basidiomycetes

In the filamentous basidiomycetes Coprinopsis cinerea and Schizophyllum commune, mating is regulated by the tetrapolar mating-type system consisting of two unlinked genetic complexes, named A and B. In the nineties, the molecular structure of A and B mating type loci and genes was revealed side by side in C. cinerea and S. commune, first the A complex and quite soon thereafter the B complex genes. The clear molecular structure of C. cinerea mating type genes has led to their use as models for genomic approaches to investigate several other filamentous basidiomycetes. In filamentous fungi, hyphal fusions are important for the distribution of available nutrients in a fungal colony. In ascomycetes and basidiomycetes they are also important for sexual reproduction. These aspects have been approached, especially in filamentous ascomycetes, but in filamentous basidiomycetes the role of fusions in the fungal life cycle has received less attention. Several proteins encoded by the genes required for hyphal fusion in filamentous ascomycetes show homology with proteins forming the striatin-interacting phosphatase and kinase (STRIPAK) complex in eukaryotic cells. Homologs to the genes encoding STRIPAK complex proteins can be identified in C. cinerea and S. commune genomes suggesting that a STRIPAK-like complex could also regulate hyphal fusions of filamentous basidiomycetes. The STRIPAK complex is a conserved signaling complex also homologous to the Saccharomyces cerevisiae Far complex involved in cell cycle arrest at yeast mating, while the fission yeast Schizosaccharomyces pombe SIP complex is involved in transition signaling from mitosis to cytokinesis. In filamentous basidiomycetes the signaling pathway regulated by the B mating type genes, the pheromone response pathway, is assumed to follow the same pattern as in the yeast S. cerevisiae. This poses an interesting question as to the relationship between the STRIPAK complex proteins and the proteins in cellular processes such as cell cycle, septal dissolution, nuclear migration, clamp cell development and fusion, known to take place after hyphal fusions. All these processes are also dependent on the activation of the B and A mating type pathways in C. cinerea and S. commune.