| Summary: | In heterothallic basidiomycete fungi, sexual compatibility is restricted by mating types, typically controlled by two loci: <i>PR</i>, encoding pheromone precursors and pheromone receptors, and <i>HD</i>, encoding two types of homeodomain transcription factors. We analysed the single mating-type locus of the commercial button mushroom variety, <i>Agaricus bisporus</i> var. <i>bisporus</i>, and of the related variety <i>burnettii</i>. We identified the location of the mating-type locus using genetic map and genome information, corresponding to the <i>HD</i> locus, the <i>PR</i> locus having lost its mating-type role. We found the <i>mip1</i> and <i>β-fg</i> genes flanking the <i>HD</i> genes as in several Agaricomycetes, two copies of the <i>β-fg</i> gene, an additional <i>HD2</i> copy in the reference genome of <i>A. bisporus</i> var. <i>bisporus</i> and an additional <i>HD1</i> copy in the reference genome of <i>A. bisporus</i> var. <i>burnettii.</i> We detected a 140 kb-long inversion between mating types in an <i>A. bisporus</i> var. <i>burnettii</i> heterokaryon, trapping the <i>HD</i> genes, the <i>mip1</i> gene and fragments of additional genes. The two varieties had islands of transposable elements at the mating-type locus, spanning 35 kb in the <i>A. bisporus</i> var. <i>burnettii</i> reference genome. Linkage analyses showed a region with low recombination in the mating-type locus region in the <i>A. bisporus</i> var. <i>burnettii</i> variety. We found high differentiation between <i>β-fg</i> alleles in both varieties, indicating an ancient event of recombination suppression, followed more recently by a suppression of recombination at the <i>mip1</i> gene through the inversion in <i>A. bisporus</i> var. <i>burnettii</i> and a suppression of recombination across whole chromosomes in <i>A. bisporus</i> var. <i>bisporus</i>, constituting stepwise recombination suppression as in many other mating-type chromosomes and sex chromosomes.
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