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Elucidating the mating system of Phaffia rhodozyma, an astaxanthin-producing yeast with biotechnological potential
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In fungi belonging to the phylum Basidiomycota, sexual identity is usually determined by two genetically unlinked MAT loci, one named PR locus, which encodes one or more pheromone receptors (Ste3) and pheromone precursors (Mfa), and the other, named HD locus, that comprehends at least one pair of divergently transcribed genes encoding homeodomain transcription factors (HD1 and HD2). The two MAT loci work as two distinct mating compatibility check points. Most basidiomycete species are heterothallic, meaning that sexual reproduction requires mating between two sexually compatible individuals harboring different alleles at both MAT loci. However, some species are known to be homothallic, one individual can complete the sexual cycle without mating with a genetically distinct partner. While the molecular underpinnings of the heterothallic life cycles of several basidiomycete model species have been dissected in detail, much less is known concerning the molecular basis for homothallism. The general aim of this research was to study the molecular mechanisms of sexual reproduction in fungi, specifically those governing the homothallic life cycle of P. rhodozyma.
Six MAT genes were found in P. rhodozyma, organized in two MAT loci, most likely located on different chromosomes. The PR locus was shown to be composed of two clusters, at approximately 5 kb from one another, each encoding one STE3 gene and one MFA gene, while the HD locus encompassed two divergently transcribed homeodomain transcription factors genes, HD1 and HD2. Functional genetic analysis was performed by targeted gene deletion of the MAT elements found in P. rhodozyma and the results allowed the proposal of a molecular model controlling the homothallic sexual behavior of P. rhodozyma. In this model (i) each pheromone interacts with the pheromone receptor of the other cluster (Mfa1 activates Ste3-2 while Mfa2 activates Ste3-1); since neither pheromone receptor is required per se for sporulation they seem to be functionally redundant; and (ii) both homeodomain proteins appear to work together to regulate genes required for sexual development. Comparison of the MAT regions of additional Phaffia species and of other representatives within the order Cystofilobasidiales, indicate that transitions to homothallism probably occurred several times independently. Furthermore, it revealed a particularly dynamic pattern of MAT gene evolution, with the generation of new receptors within each genus and exceptionally large numbers of mature pheromones encoded in the genomes of some of the species. In conclusion, this work allowed for the first time the elucidation of the basic molecular mechanisms governing the homothallic life cycle of a basidiomycete. At the same time the genetic manipulation of the MAT genes of P. rhodozyma also allowed the generation of preferably outcrossing strains, which may be potentially useful for further improvement of this yeast as an industrial organism by way of selective breeding.
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Basidiomycetes Homothallism Phaffia rhodozyma MAT genes