4. Mating type loci in Coprinus cinereus

Formation of the dikaryon is controlled by two mating type loci, A and B. Fusing monokaryons have to exhibit different specialities at both loci for dikaryon formation to proceed. A and B control different steps in dikaryon formation as indicated in the following scheme.

The process of dikaryon formation and control by the mating type genes.
Both mating type loci have multiple specificities. The A mating type locus encodes two distinct types of homeodomain transcription factors in divergently transcribed gene pairs. To induce sexual development of C. cinereus, it needs one transcription factor from each class but they have to come from different nuclei. Alleles of the A genes have only low homology (40-70%). These gene dissimilarities are the base for protein recognition. An active transcription factor complex forms only between proteins from allelic gene pairs. The B mating type locus encodes sets of pheromones and pheromone receptors. Also the alleles of the B genes are dissimilar in sequence enabling distinctions between gene products. For sexual development, pheromones and pheromone receptors of different B specificities have to interact.

Transformation studies with cloned genes in monokaryons revealed that the A mating type genes repress asexual sporulation (oidia formation) and induce formation of hyphal knots, fruitbodies and resting structures (sclerotia, chlamydospores). Blue light counteracts all these A regulated functions. The B mating type genes were found to modify the action of A genes and the action of blue light on A mediated development. Furthermore, the B genes were shown to induce karyogamy and meiosis.

Due to the action of the heterologous mating type genes, oidia formation in the dikaryon is strongly repressed during dark unlike in monokaryons that constitutively produce oidia in high numbers. Blue light illumination partially overcomes oidia repression in the dikaryon. Oidia production on the dikaryon is however biased. One of the two possible nuclear types is preferrentially (in 90-98% of the cases) delivered into the spores. Mating type genes were shown to contribute to this bias of nuclear transfer. The results suggest two divergent gradients of mating type proteins to be present within a dikaryotic cell, each starting at the nucleus the respective proteins are produced from. Those nuclei giving rise to a weaker gradient will be repressed for migration into spores. Biased oidia production on the dikaryon is a special consequence of the fungal life cycle in that karyogamy does not directly follow plasmogamy and that the dikaryotic stage is maintained through long vegetative periods of the life cycle.

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