Sex Chromosomes Essay -- Biology, Heterothallic Fungi

The mechanisms responsible for the origin and maintenance of large non-recombining regions on sex chromosomes have been mostly studied in plants and animals, but the recent discovery of similar features on the fungal chromosomes carrying mating type genes in several species may shed new light on this phenomenon (Fraser et al. 2004). Sex chromosomes in plants and animals have evolved from an autosomal pair by the expansion of the non-recombining region around complementary genes determining sex-specific functions (Bergero and Charlesworth 2009). Such a multi-step expansion of the non-recombining regions in sex chromosomes, forming â€Å"evolutionary strata† (Lahn and Page 1999), is usually explained by the recruitment of genes determining sexually antagonistic traits (i.e. beneficial in males and deleterious in females, or conversely), via a selection for linkage to the sex-determining genes (Rice 1987, Charlesworth 2005). Selective forces driving the evolution of non-recomb ining regions are however likely to be different in fungi as cells of different mating types exhibit little phenotypic differences. In heterothallic fungi, syngamy can only occur between haploid cells carrying different alleles at the mating type genes, while in homothallic fungi, no such differences are required, allowing universal compatibility (Billiard et al. 2011). The two main fungal phyla have different mating type genes and organization: a single locus controls mating type in ascomycetes against two loci in basidiomycetes (i.e. haploid cells should carry different alleles at both 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... ...Smith et al. 2004) and Cryptococcus neoformans (Wang et al. 2002). In R. toluroides, the gene encoding ste20 is surrounded with genes encoding pheromones (Coelho et al. 2008). Other genes, such as the abc1, with more elusive role in mating and development of fungi have also been found in close proximity to the genes encoding the pheromone and its receptor in R. toluroides (Coelho et al. 2008). In this study, our goals were therefore to: 1) identify additional genes belonging to the mating type locus in Microbotryum, in particular the genes encoding the pheromones and the homeodomain proteins, all of which control mating types in most basidiomycetes, 2) assess whether the genealogies of genes in the mating type region as well as of loci of the previously proposed strata are consistent with the existence of evolutionary strata along the mating type chromosomes.