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Sex Chromosomes Essay

1746 words - 7 pages

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 “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-recombining 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 encodes two homeodomain proteins (HD1 and HD2) involved in regulating the dikaryotic stage. The cells carrying the different mating type alleles do not exhibit large phenotypic differences. In particular, even when anisogamy is present, it is not associated with mating types (Billiard et al., 2011). Each mating type can indeed produce small and large gametes, the size being not controlled by specific alleles. Nevertheless, chromosomes carrying the mating type genes can exhibit characteristics similar to those of sex chromosomes, with extended region over which they do not recombine and cytological differences between the two chromosomes in the pair (Hood 2002, Fraser et al. 2004, Menkis et al. 2008).
Recombination suppression on fungal mating type chromosomes has been suggested to proceed in successive stages (Menkis et al 2008; Votintseva & Filatov 2009, Fraser et al. 2004), leading to 'evolutionary strata' as in plants and animals (Lahn and Page 1999), i.e. discrete regions extending outward from the mating type genes that started diverging (i.e. ceased recombination) at different time points. However, evolutionary strata may not be expected in non-recombining region in fungal mating chromosomes, as there should...

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