The mechanisms responsible for the origin and maintenance of large nonrecombinant regions on sex chromosomes have been primarily studied in plants and animals, but the recent discovery of similar features on fungal chromosomes carrying mating type genes in several species can shed new light on this phenomenon (Fraser et al. 2004). Sex chromosomes in plants and animals evolved from an autosomal pair through expansion of the nonrecombinant region around complementary genes determining sex-specific functions (Bergero and Charlesworth 2009). Such multi-step expansion of non-recombinant regions of sex chromosomes, forming "evolutionary strata" (Lahn and Page 1999), is generally explained by the recruitment of genes determining sexually antagonistic (i.e. beneficial) traits. in men and harmful in women). or vice versa), via selection for linkage to sex-determining genes (Rice 1987, Charlesworth 2005). The selective forces driving the evolution of nonrecombinant regions are, however, likely to be different in fungi, because cells of different mating types show few phenotypic differences. In heterothallic fungi, syngamy can only occur between haploid cells carrying different alleles at mating type genes, while in homothallic fungi no such differences are required, which allows universal compatibility (Billiard et al. 2011). The two major fungal phyla have different mating type genes and organization: a single locus controls mating type in ascomycetes versus two loci in basidiomycetes (i.e. haploid cells must carry different alleles at the two loci for successful mating). One of the two loci controlling mating types in basidiomycetes encodes pheromones and pheromone receptors involved in syngamy while the other locus...... middle of article...... Smith et al. 2004) and Cryptococcus neoformans (Wang et al. 2002). In R. toluroides, the gene encoding ste20 is surrounded by genes encoding pheromones (Coelho et al. 2008). Other genes, such as abc1, playing more elusive roles in fungal mating and development, have also been discovered near genes encoding the pheromone and its receptor in R. toluroides (Coelho et al. 2008) . In this study, our objectives were therefore to: 1) identify additional genes belonging to the mating type locus in Microbotryum, in particular genes encoding pheromones and homeodomain proteins, which control all mating types in most basidiomycetes, 2) evaluate whether the genealogies of the genes in the mating type region as well as the strata loci proposed previously are consistent with the existence of evolutionary strata along the mating type chromosomes.