The edible fungus Pleurotus ostreatus (oyster mushroom) is an industrially produced heterothallic homobasidiomycete whose mating is controlled by a bifactorial tetrapolar genetic system. Two mating loci (matA and matB) control different steps of hyphal fusion, nuclear migration, and nuclear sorting during the onset and progress of the dikaryotic growth. Previous studies have shown that the segregation of the alleles present at the matB locus differs from that expected for a single locus because (i) new nonparental B alleles appeared in the progeny and (ii) there was a distortion in the segregation of the genomic regions close to this mating locus. In this study, we pursued these observations by using a genetic approach based on the identification of molecular markers linked to the matB locus that allowed us to dissect it into two genetically linked subunits (matB␣ and matB) and to correlate the presence of specific matB␣ and matA alleles with differences in monokaryotic growth rate. The availability of these molecular markers and the mating type dependence of growth rate in monokaryons can be helpful for marker-assisted selection of fast-growing monokaryons to be used in the construction of dikaryons able to colonize the substrate faster than the competitors responsible for reductions in the industrial yield of this fungus.Incompatibility systems are mechanisms for the creation of variability preventing selfing. The phytopathogenic fungus Ustilago maydis and the mushrooms Coprinus cinereus and Schizophyllum commune have been used as models to study mating incompatibility in basidiomycetes. In these species, mating is controlled by two unlinked multiallelic loci whose independent segregation generates four mating specificities in the progeny of a single individual (these fungi are then called tetrapolar) (for reviews, see references 2, 7, and 11). In tetrapolar basidiomycetes, a single basidiospore produces upon germination a hypha in which all nuclei are identical (homokaryon). Two hyphae with different mating alleles at the two incompatibility loci are able to fuse and give rise to a mycelium in which the two parental nuclei do not fuse throughout vegetative growth. This kind of mycelium is called dikaryotic, and the individual mycelium is called a dikaryon. Vegetative growth is maintained until a set of environmental conditions triggers fruit body formation. Karyogamy occurs within the basidia, and it is immediately followed by meiosis producing four uninucleate spores. The monokaryotic and dikaryotic conditions can be distinguished by the presence of clamp connections in dikaryons and their lack in monokaryons. Clamp connections are hookshaped structures involved in equal nuclei sorting to the daughter cells produced by mitosis.Genetic studies carried out in C. cinereus and S. commune have shown that the A incompatibility locus codes for homeodomain-containing transcription factors (2,13,14,18,21,24,27,28). The b mating-type locus of U. maydis is homologous to the A locus and also codes for homeodomain prote...
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