A Single Mating-type Locus Composed of Homeodomain Genes Promotes Nuclear Migration and Heterokaryosis in the White-rot Fungus Phanerochaete Chrysosporium

The white-rot basidiomycete fungus Phanerochaete chrysosporium (Agaricomycetes) is a model species that produces potent wood-degrading enzymes. The mating system of the species has been difficult to characterize due to its cryptic fruiting habit and lack of clamp connections in the heterokaryotic phase. By exploiting the draft genome sequence, we reevaluated the mating system of P. chrysosporium by studying the inheritance and segregation of putative mating-type gene homologues, the homeodomain transcription factor genes (MAT-A) and the pheromone receptors (MAT-B). A pattern of mating incompatibility and fructification consistent with a bipolar system with a single MAT locus was observed, but the rejection response was much weaker than that seen in other agaricomycete species, leading to stable heterokaryons with identical MAT alleles. The homeodomain genes appear to comprise the single MAT locus because they are heterozygous in wild strains and hyperpolymorphic at the DNA sequence level and promote aspects of sexual reproduction, such as nuclear migration, heterokaryon stability, and basidiospore formation. The pheromone receptor loci that might constitute a MAT-B locus, as in many other Agaricomycetes, are not linked to the MAT-A locus and display low levels of polymorphism. This observation is inconsistent with a bipolar mating system that includes pheromones and pheromone receptors as mating-type determinants. The partial uncoupling of nuclear migration and mating incompatibility in this species may be predicted to lead to parasexual recombination and may have contributed to the homothallic behavior observed in previous studies.