Genetic mapping and nuclear interactions of an incompatibility response in Agaricus bisporus

During cultivation, mixing of different heterokaryotic individuals of the button mushroom, Agaricus bisporus, generally reduces yield. This phenomenon could be caused by direct antagonistic responses and/or reduced synchronization by not forming a chimeric hyphal network. In other fungi, highly divergent alleles for a set of genes affect successful network formation between individuals either by preventing fusion or, more commonly, triggering cell death post-fusion. To understand this process in A. bisporus, it is important to identify the allelic variants allowing these fungi to discriminate self from nonself. We leverage a recently described cell death staining method utilizing Evans Blue to visualize mycelial compatibility. Here, we provide results of a first genetic mapping of incompatibility alleles in A. bisporus. Crossing strains between A. bisporus var. bisporus and A. bisporus var. burnetti we find segregation ratios of compatible progeny generally consistent with three nuclear loci. To identify these regions, we first use a set of single Chromosome Substitution Lines (CSLs), produced by genotyping progeny with recombination skewed to the very chromosome ends. We localize the main effect to be between two and three chromosomes, depending on the common nucleus of interacting heterokaryons. Using genome-wide markers for 167 sexual progeny, we identify loci controlling mycelial compatibility on chromosomes 4, 6 and 7, the same chromosomes as indicated by chromosome substitution lines. Notably, while the choice of a common nucleus seemed to affect the compatibility of CSLs, it did not seem to affect the loci identified in the sexual progeny. The ability to mix different strains of this mushroom-forming fungus could allow additional cultivation approaches, combining strains with complementary characteristics. These results provide a starting point towards understanding the molecular mechanisms underlying this fundamental property of hyphal networks in basidiomycetes.