In Saccharomyces cerevisiae, sexual reproduction has been studied for decades and remains an active field of genetic, biochemical, and structural studies in the actors of the signal transduction cascade that governs mating. Additional genes are in fact part of additional loci, similar to the silent HML and HMR loci in S. cerevisiae. The species closest to S. cerevisiae that have several sexual loci represent the majority of species examined and are discussed in this chapter. The chapter describes homologs of these genes encoded by putative MAT-like cassettes in the genomes of species containing multiple cassettes. The a1 gene from S. cerevisiae contains two introns that are conserved in Candida glabrata, the most closely related species among the yeasts with completely assembled genomes. Heterochromatin formation in hemiascomycetous yeasts is essential for the silencing of HMR- and HML-like loci to ensure mating-type determination. In the genomes of more-distant species with multiple cassettes, Kluyveromyces lactis and Kluyveromyces thermotolerans, only relics of HO are found, in which the LAGLIDADG motifs are conserved. HO gene sequences in genomes indicate that it was acquired at the same time as the appearance of the silent cassettes at the divergence of Candida and Kluyveromyces species. Further analyses of the natural habitat, lifestyle, and even ploidy of these species, and of the more recent adaptations of their genomes to the conditions under which we now study them, will help us to decipher the evolutionary strategies followed by each species.

Sexual cycle of an ho mutant of S. cerevisiae. Haploid α cells are shown in white, a cells in black, and diploids in gray. Mating-type switching in wild-type HO cells is not illustrated, for the sake of simplicity. See the text for details.

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Figure 15.1

Sexual cycle of an ho mutant of S. cerevisiae. Haploid α cells are shown in white, a cells in black, and diploids in gray. Mating-type switching in wild-type HO cells is not illustrated, for the sake of simplicity. See the text for details.

Regulation of mating type in S. cerevisiae. The drawing represents the promoters of asg, αsg, and hsg in different cell types, indicated at top (see the text for details), with the double helix representing the DNA, the arrow representing transcription, and transcription factors represented by various shapes with their names, shown as fixed to the DNA or not. In diploid cells, the a1/α2 complex is shown as inhibiting α1, while in reality it is repressed at the transcription level.

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Figure 15.2

Regulation of mating type in S. cerevisiae. The drawing represents the promoters of asg, αsg, and hsg in different cell types, indicated at top (see the text for details), with the double helix representing the DNA, the arrow representing transcription, and transcription factors represented by various shapes with their names, shown as fixed to the DNA or not. In diploid cells, the a1/α2 complex is shown as inhibiting α1, while in reality it is repressed at the transcription level.

Simplified representation of phylogenic relationship of all species analyzed. Data from B. Dujon (15a). Species names in black are from high-coverage genomes, and names in gray are from partial genomes. In the case of Ashbya gossypii, we have kept the name of the genus used by the sequencing group, instead of Eremothecium.

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Figure 15.3

Simplified representation of phylogenic relationship of all species analyzed. Data from B. Dujon (15a). Species names in black are from high-coverage genomes, and names in gray are from partial genomes. In the case of Ashbya gossypii, we have kept the name of the genus used by the sequencing group, instead of Eremothecium.

MAT-, HMR-, and HML-like loci in species with multiple cassettes. Species names are indicated on the left, close to chromosome or contig number. Attribution of type, HML-, HMR-, and MAT-like, to cassettes is shown above chromosomes. Chromosomes are drawn as double lines inside which cassette “boxes” are drawn (see the text for details); rounded ends represent telomeres.

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Figure 15.5

MAT-, HMR-, and HML-like loci in species with multiple cassettes. Species names are indicated on the left, close to chromosome or contig number. Attribution of type, HML-, HMR-, and MAT-like, to cassettes is shown above chromosomes. Chromosomes are drawn as double lines inside which cassette “boxes” are drawn (see the text for details); rounded ends represent telomeres.

Multiple alignments of Ho proteins. The HO translation from S. cerevisiae is the wild-type one (54), with the positions of residues that are different in the mutant underlined. Alignments were done using Clustal W, with sequences translated from genomic data. From C terminus to N terminus, boxed sequences correspond, in order, to the first LAGLIDADG motif, the first NLS motif, the second LAGLIDADG, and the second NLS.

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Figure 15.6

Multiple alignments of Ho proteins. The HO translation from S. cerevisiae is the wild-type one (54), with the positions of residues that are different in the mutant underlined. Alignments were done using Clustal W, with sequences translated from genomic data. From C terminus to N terminus, boxed sequences correspond, in order, to the first LAGLIDADG motif, the first NLS motif, the second LAGLIDADG, and the second NLS.