Abstract
BackgroundThe red yeasts are an early diverged group of basidiomycetes comprising sexual and asexual species. Sexuality is based on two compatible mating types and sexual identity is determined by MAT loci that encode homeodomain transcription factors, peptide pheromones and their receptors. The objective of the present study was to investigate the presence and integrity of MAT genes throughout the phylogenetic diversity of red yeasts belonging to the order Sporidiobolales.ResultsWe surveyed 18 sexual heterothallic and self-fertile species and 16 asexual species. Functional pheromone receptor homologues (STE3.A1 and STE3.A2) were found in multiple isolates of most of the sexual and asexual species. For each of the two mating types, sequence comparisons with whole-genome data indicated that synteny tended to be conserved along the pheromone receptor region. For the homeodomain transcription factor, likelihood methods suggested that diversifying selection acting on the self/non-self recognition region promotes diversity in sexual species, while rapid evolution seems to be due to relaxed selection in asexual strains.ConclusionsThe majority of both sexual and asexual species of red yeasts have functional pheromone receptors and homeodomain homologues. This and the frequent existence of asexual strains within sexual species, makes the separation between sexual and asexual species imprecise. Events of loss of sexuality seem to be recent and frequent, but not uniformly distributed within the Sporidiobolales. Loss of sex could promote speciation by fostering the emergence of asexual lineages from an ancestral sexual stock, but does not seem to contribute to the generation of exclusively asexual lineages that persist for a long time.
Highlights
The red yeasts are an early diverged group of basidiomycetes comprising sexual and asexual species
In Basidiomycota, two functional classes of genes can be found within mating type (MAT) loci: in addition to transcriptional regulators, the genes encoding pheromones and pheromone receptors (P/PR) are located in the genomic regions that determine sexual identity
Updated phylogeny of the Sporidiobolales We investigated a group of 43 red yeast species representing the phylogenetic diversity in the order Sporidiobolales
Summary
The red yeasts are an early diverged group of basidiomycetes comprising sexual and asexual species. In Ascomycota, and possibly in Mucoromycotina and Chytridiomycota, the MAT loci occur in two alternate forms that determine two mating types or sexual compatibility groups [6,10,11,12]. In Basidiomycota, two functional classes of genes can be found within MAT loci: in addition to transcriptional regulators (homeodomain transcription factors - HD1/HD2), the genes encoding pheromones and pheromone receptors (P/PR) are located in the genomic regions that determine sexual identity. This formed the basis for the existence of a more complex mating system in some basidiomycetes. In this system two classes of MAT genes (HD1/HD2 and P/PR) are located in two genetically unlinked MAT loci ( designated as HD and PR loci, respectively)
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