The Mating-Type Locus of Cryptococcus: Evolution of Gene Clusters Governing Sex Determination and Sexual Reproduction from the Phylogenomic Perspective

The Cryptococcus mating-type locus (MAT) locus has emerged as an exemplary model to elucidate general principles governing the assembly and function of gene clusters. Moreover, comparative genomic studies of the evolutionary trajectory of MAT in the pathogenic Cryptococcus species complex mirror evolutionary steps that have been hypothesized to have given rise to the more complex sex chromosomes in mammals, insects, fish, and plants. These then represent examples of convergent evolution to specialized genomic regions governing sexual reproduction throughout the eukaryotic tree of life. Although the molecular nature of the homothallic sexual cycle of Filobasidiella depauperata remains to be elucidated, possible models for the homothallic life cycle of F. depauperata are discussed. In summary, comparative genomic analysis of the structure of MAT loci from Cryptococcus-related species has important implications for the hypothesized evolutionary model. The MAT genes present in these species have implications for the gene content of the ancestral homeodomain and P/R loci in the model. The P/R locus boundaries of C. heveanensis have expanded to include not only the pheromone and pheromone receptor genes but also at least one of the pheromone response pathway genes, but not all of the C. neoformans and C. gattii MAT locus gene homologs, which illuminates the serial expansions that created evolutionary strata in the MAT locus genes of the pathogenic Cryptococcus species.