Mating-Type Locus Organization and Mating-Type Chromosome Differentiation in the Bipolar Edible Button Mushroom Agaricus bisporus

In heterothallic basidiomycete fungi, sexual compatibility is restricted by mating types, typically controlled by two loci: , encoding pheromone precursors and pheromone receptors, and , encoding two types of homeodomain transcription factors. We analysed the single mating-type locus of the commercial button mushroom variety, var. , and of the related variety . We identified the location of the mating-type locus using genetic map and genome information, corresponding to the locus, the locus having lost its mating-type role. We found the and genes flanking the genes as in several Agaricomycetes, two copies of the gene, an additional copy in the reference genome of var. and an additional copy in the reference genome of var. We detected a 140 kb-long inversion between mating types in an var. heterokaryon, trapping the genes, the gene and fragments of additional genes. The two varieties had islands of transposable elements at the mating-type locus, spanning 35 kb in the var. reference genome. Linkage analyses showed a region with low recombination in the mating-type locus region in the var. variety. We found high differentiation between alleles in both varieties, indicating an ancient event of recombination suppression, followed more recently by a suppression of recombination at the gene through the inversion in var. and a suppression of recombination across whole chromosomes in var. , constituting stepwise recombination suppression as in many other mating-type chromosomes and sex chromosomes.