Sexual reproduction is restricted to eukaryotic species and involves the fusion of haploid gametes to form a diploid cell that subsequently undergoes meiosis to generate recombinant haploid forms. This process has been extensively studied in the unicellular yeast Saccharomyces cerevisiae, which exhibits separate regulatory control over mating and meiosis. Here we address the mechanism of sexual reproduction in the related hemiascomycete species Candida lusitaniae. We demonstrate that, in contrast to S. cerevisiae, C. lusitaniae exhibits a highly integrated sexual program in which the programs regulating mating and meiosis have fused. Profiling of the C. lusitaniae sexual cycle revealed that gene expression patterns during mating and meiosis were overlapping, indicative of co-regulation. This was particularly evident for genes involved in pheromone MAPK signalling, which were highly induced throughout the sexual cycle of C. lusitaniae. Furthermore, genetic analysis showed that the orthologue of IME2, a ‘diploid-specific’ factor in S. cerevisiae1,2, and STE12, the master regulator of S. cerevisiae mating3,4, were each required for progression through both mating and meiosis in C. lusitaniae. Together, our results establish that sexual reproduction has undergone significant rewiring between S. cerevisiae and C. lusitaniae, and that a concerted sexual cycle operates in C. lusitaniae that is more reminiscent of the distantly related ascomycete, Schizosaccharomyces pombe. We discuss these results in light of the evolution of sexual reproduction in yeast, and propose that regulatory coupling of mating and meiosis has evolved multiple times as an adaptation to promote the haploid lifestyle.
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We thank J. Heitman, C. Lisset-Flores Mauriz, T. Noel, N. Hunter and J. Reedy for gifts of strains and plasmids, and S. Kabrawala and N. Balmuri for help with strain construction. We also thank T. Sorrells, L. Holt and members of the Johnson and Bennett laboratories for comments on the paper, and S. Jones for help with statistical analysis. This work was supported by National Science Foundation Grant MCB1021120 (to R.J.B.), by National Institutes of Health R01 Grant AI081704 (to R.J.B.), by T32GM007601 (to C.M.S.), by F31AI075607 (to R.K.S.), and by an Investigator in the Pathogenesis of Infectious Disease Award from the Burroughs Wellcome Fund (to R.J.B.).
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Schematic showing induction of pheromone-processing genes (a) and pheromone MAPK genes (b) during both mating and meiosis in C. lusitaniae.
Expression changes highlight genes induced during co-incubation of a and α cells on PDA medium (mating), as well as during growth of diploid a/α cells on PDA medium (induces meiosis and sporulation). In contrast, these genes are mating-specific in the related yeast S. cerevisiae. Scale indicates fold change for gene expression.
a, C. lusitaniae a/α cells divide as stable diploid cells on YPD medium. n = 3. b, On PDA medium, a/α cells are induced to enter meiosis and dyad spores (asterisks) begin to appear at 18 h. n = 3. c, Time course of meiosis in wild type, ste12Δ /ste12Δ and STE12-complemented C. lusitaniae strains. A morphology change (polarized growth) is evident in wild-type diploid strains grown on PDA medium starting at 12 h, whereas spore formation is apparent at 18–36 h. In the ste12Δ/ste12Δ mutant, both sporulation and morphology change are absent. d, Global transcriptional profile of meiosis in C. lusitaniae showing induced genes (yellow) and repressed genes (blue). C. lusitaniae diploid a/α cells (RSY432) were grown on PDA (sporulating) medium and expression changes compared to those on YPD (non-sporulating) medium. Genes changing more than four fold in expression are shown. All n values represent number of biological replicates. Scale indicates fold expression changes.
Extended Data Figure 3 Transcriptional profiling of C. lusitaniae wild type and ime2Δ mutants during mating and meiosis.
a, Full profile of wild-type and ime2Δ/ime2Δ strains during meiosis indicates that many transcriptional changes occur in ime2Δ/ime2Δ mutants as in wild-type strains. b, Analysis of cell cycle regulating genes during C. lusitaniae meiosis. Several cell cycle genes are induced in wild-type cells undergoing meiosis but not in ime2Δ/ime2Δ mutants (for example, APC4, CDC3, CDC10 and CDC14). c, Comparative expression of early (IME2, IME4, SPO11), middle (REC102, CDC3, SPS4, SPS1, NDT80, SWM1) and late (DIT1, DIT2) meiosis genes. Expression changes scaled as in Extended Data Fig. 1. d, The mating profiles of wild-type and ime2Δ strains were very similar. e, Comparison of meiosis genes induced in wild type and ime2Δ/ime2Δ mutants.
IME2 was reintegrated at the endogenous locus in an ime2Δ mutant (RSY437). Mating frequency was quantified by monitoring the formation of prototrophic products from auxotrophic parents. WT cross, RSY411 × RSY284, ime2Δ mutant cross, RSY406 × RSY437, IME2 complemented cross, RSY406 × CAY5022. Differences between the WT cross and the ime2Δ mutant cross, and between the IME2 complemented cross and the ime2Δ mutant cross were both significant. n = 9 (3 biological replicates in triplicate), P < 0.001, Kruskal–Wallis test. Data are representative of mean ± s.e.m.
Extended Data Figure 5 Schematic of genetically marked C. lusitaniae strains and control of meiosis by STE12.
a, C. lusitaniae mating experiments were performed between RSY284 (a strain) and RSY411 (α strain) and mating products selected based on auxotrophic makers. b, The a/α diploid strain RSY432 was induced to undergo meiosis on PDA medium and meiotic progeny identified by their red colour on YPD medium (ade− colonies) or by growth on medium containing cycloheximide (CHX-resistant colonies). c, Diploid a/α strains of C. lusitaniae were incubated on PDA medium for 3 days and analysed for the frequency of formation of meiotic progeny. Two independent ste12Δ/ste12Δ mutants were constructed in the a/α background and tested for meiotic progeny using both the CHXR and ADE2 markers. Mutants lacking STE12 were unable to undergo meiosis to produce haploid progeny, while reintegration of STE12 into the mutant background restored meiosis. Differences between both wild-type strains and ste12Δ/ste12Δ mutants, and between ste12Δ/ste12Δ mutants and STE12-complemented strains were significant (n = 3 biological replicates, P < 0.05, student’s t-test, two tailed). Data representative of mean ± s.e.m.
Extended Data Figure 6 Analysis of STE12 function in mating and meiosis in diverse hemiascomycete species.
The STE12 gene was deleted from haploid and diploid strains of K. lactis, P. pastoris and S. cerevisiae. The resultant strains were tested both for mating competency and the formation of meiotic progeny. Whereas deletion of STE12 blocked mating in haploid strains of all three species, loss of STE12 from diploid a/α strains did not have a significant effect on the formation of meiotic progeny. Thus, C. lusitaniae is unique among the hemiascomycete species tested in that STE12 is essential for meiosis only in this species. K. lactis mating and meiosis, n = 2, data combined for 3 independent mutants. P. pastoris mating, n = 3. P. pastoris meiosis n = 4. S. cerevisiae meiosis n = 3. All data presented as mean ± s.e.m. All n values represent number of biological replicates.
Extended Data Figure 7 Rewiring of the genetic programs that control sexual reproduction in hemiascomycetes.
In many hemiascomycete species, including the model yeast S. cerevisiae, mating and meiosis are controlled by two distinct transcriptional programs with STE12 as the master regulator of mating and IME2 as a key regulator of meiosis. However, in the opportunistic pathogen C. lusitaniae, STE12 has retained its role in regulating mating, but has also acquired control over meiosis. Similarly, C. lusitaniae IME2 has a conserved role in regulating mating, but also has a role in promoting mating. The programs controlling these two processes have therefore fused in C. lusitaniae, perhaps to facilitate a transient diploid state and more efficient return to the predominant haploid state.
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Sherwood, R., Scaduto, C., Torres, S. et al. Convergent evolution of a fused sexual cycle promotes the haploid lifestyle. Nature 506, 387–390 (2014). https://doi.org/10.1038/nature12891
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