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The parasexual cycle in Candida albicans provides an alternative pathway to meiosis for the formation of recombinant strains.

Forche A, Alby K, Schaefer D, Johnson AD, Berman J, Bennett RJ - PLoS Biol. (2008)

Bottom Line: We show that deletion of SPO11 prevented genetic recombination between homologous chromosomes during the C. albicans parasexual cycle.These findings suggest that at least one meiosis-specific gene has been re-programmed to mediate genetic recombination during the alternative parasexual life cycle of C. albicans.We discuss, in light of the long association of C. albicans with warm-blooded animals, the potential advantages of a parasexual cycle over a conventional sexual cycle.

View Article: PubMed Central - PubMed

Affiliation: Department of Genetics, Cell Biology and Development, University of Minnesota, Minneapolis, Minnesota, United States of America.

ABSTRACT
Candida albicans has an elaborate, yet efficient, mating system that promotes conjugation between diploid a and alpha strains. The product of mating is a tetraploid a/alpha cell that must undergo a reductional division to return to the diploid state. Despite the presence of several "meiosis-specific" genes in the C. albicans genome, a meiotic program has not been observed. Instead, tetraploid products of mating can be induced to undergo efficient, random chromosome loss, often producing strains that are diploid, or close to diploid, in ploidy. Using SNP and comparative genome hybridization arrays we have now analyzed the genotypes of products from the C. albicans parasexual cycle. We show that the parasexual cycle generates progeny strains with shuffled combinations of the eight C. albicans chromosomes. In addition, several isolates had undergone extensive genetic recombination between homologous chromosomes, including multiple gene conversion events. Progeny strains exhibited altered colony morphologies on laboratory media, demonstrating that the parasexual cycle generates phenotypic variants of C. albicans. In several fungi, including Saccharomyces cerevisiae and Schizosaccharomyces pombe, the conserved Spo11 protein is integral to meiotic recombination, where it is required for the formation of DNA double-strand breaks. We show that deletion of SPO11 prevented genetic recombination between homologous chromosomes during the C. albicans parasexual cycle. These findings suggest that at least one meiosis-specific gene has been re-programmed to mediate genetic recombination during the alternative parasexual life cycle of C. albicans. We discuss, in light of the long association of C. albicans with warm-blooded animals, the potential advantages of a parasexual cycle over a conventional sexual cycle.

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Analysis of the Parasexual Mating Cycle in C. albicans(A) Overview of the mating cycle in C. albicans. White MTLa and MTLα cells must switch to the opaque state to undergo mating and formation of a mononuclear tetraploid a/α cell. A reduction in ploidy back to the diploid (or near diploid) can occur by random chromosome loss.(B) A scheme for selection of diploid progeny strains from tetraploids. A tetraploid strain, RBY18, heterozygous for the GAL1 gene on Chromosome (Chr) 1 and for all four MTL alleles on Chr 5 was constructed by mating MTLa and MTLα diploid strains, as shown. After induction of chromosome instability, strains that had undergone a reduction in ploidy were selected for by growth on 2-deoxygalactose (2-DOG) medium, as only strains that have lost the GAL1 gene are able to grow on medium containing 2-DOG. Progeny strains were subsequently analyzed by PCR of the MTL locus and by flow cytometric analysis to confirm they were diploid strains. Strains were then analyzed by SNP and CGH microarrays to determine their genetic content.
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pbio-0060110-g001: Analysis of the Parasexual Mating Cycle in C. albicans(A) Overview of the mating cycle in C. albicans. White MTLa and MTLα cells must switch to the opaque state to undergo mating and formation of a mononuclear tetraploid a/α cell. A reduction in ploidy back to the diploid (or near diploid) can occur by random chromosome loss.(B) A scheme for selection of diploid progeny strains from tetraploids. A tetraploid strain, RBY18, heterozygous for the GAL1 gene on Chromosome (Chr) 1 and for all four MTL alleles on Chr 5 was constructed by mating MTLa and MTLα diploid strains, as shown. After induction of chromosome instability, strains that had undergone a reduction in ploidy were selected for by growth on 2-deoxygalactose (2-DOG) medium, as only strains that have lost the GAL1 gene are able to grow on medium containing 2-DOG. Progeny strains were subsequently analyzed by PCR of the MTL locus and by flow cytometric analysis to confirm they were diploid strains. Strains were then analyzed by SNP and CGH microarrays to determine their genetic content.

Mentions: The parasexual cycle of C. albicans, as currently envisaged, is shown in Figure 1A. Note that no meiotic program has been observed in C. albicans, despite the presence of many genes in the genome whose homologues function specifically in meiosis in other fungi [18]. However, C. albicans strains have been found to undergo a parasexual cycle; tetraploid strains become genetically unstable when incubated on certain laboratory media, losing chromosomes and generating diploid (and aneuploid) progeny strains that are themselves mating competent. The chromosome loss process is concerted, with loss of one or more chromosomes predisposing the cell to lose additional chromosomes, and the diploid state being the final product [16]. While tetraploids are stable when grown on YPD medium at different temperatures, two culture conditions were identified that induced genetic instability in C. albicans: (i) growth of tetraploid strains on S. cerevisiae “pre-sporulation” (pre-spo) medium at 37 °C, and (ii) growth of tetraploid strains on medium containing L-sorbose at 30 °C. The latter condition was previously shown to also induce chromosome loss in diploid C. albicans strains [19]. More specifically, diploid strains were unable to grow on L-sorbose medium unless they first underwent loss of one copy of Chromosome (Chr) 5, becoming monosomic for this chromosome. In contrast, diploid strains were relatively stable when grown on pre-spo medium, indicating that diploid and tetraploid strains exhibit very different selective pressures when cultured on this medium.


The parasexual cycle in Candida albicans provides an alternative pathway to meiosis for the formation of recombinant strains.

Forche A, Alby K, Schaefer D, Johnson AD, Berman J, Bennett RJ - PLoS Biol. (2008)

Analysis of the Parasexual Mating Cycle in C. albicans(A) Overview of the mating cycle in C. albicans. White MTLa and MTLα cells must switch to the opaque state to undergo mating and formation of a mononuclear tetraploid a/α cell. A reduction in ploidy back to the diploid (or near diploid) can occur by random chromosome loss.(B) A scheme for selection of diploid progeny strains from tetraploids. A tetraploid strain, RBY18, heterozygous for the GAL1 gene on Chromosome (Chr) 1 and for all four MTL alleles on Chr 5 was constructed by mating MTLa and MTLα diploid strains, as shown. After induction of chromosome instability, strains that had undergone a reduction in ploidy were selected for by growth on 2-deoxygalactose (2-DOG) medium, as only strains that have lost the GAL1 gene are able to grow on medium containing 2-DOG. Progeny strains were subsequently analyzed by PCR of the MTL locus and by flow cytometric analysis to confirm they were diploid strains. Strains were then analyzed by SNP and CGH microarrays to determine their genetic content.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2365976&req=5

pbio-0060110-g001: Analysis of the Parasexual Mating Cycle in C. albicans(A) Overview of the mating cycle in C. albicans. White MTLa and MTLα cells must switch to the opaque state to undergo mating and formation of a mononuclear tetraploid a/α cell. A reduction in ploidy back to the diploid (or near diploid) can occur by random chromosome loss.(B) A scheme for selection of diploid progeny strains from tetraploids. A tetraploid strain, RBY18, heterozygous for the GAL1 gene on Chromosome (Chr) 1 and for all four MTL alleles on Chr 5 was constructed by mating MTLa and MTLα diploid strains, as shown. After induction of chromosome instability, strains that had undergone a reduction in ploidy were selected for by growth on 2-deoxygalactose (2-DOG) medium, as only strains that have lost the GAL1 gene are able to grow on medium containing 2-DOG. Progeny strains were subsequently analyzed by PCR of the MTL locus and by flow cytometric analysis to confirm they were diploid strains. Strains were then analyzed by SNP and CGH microarrays to determine their genetic content.
Mentions: The parasexual cycle of C. albicans, as currently envisaged, is shown in Figure 1A. Note that no meiotic program has been observed in C. albicans, despite the presence of many genes in the genome whose homologues function specifically in meiosis in other fungi [18]. However, C. albicans strains have been found to undergo a parasexual cycle; tetraploid strains become genetically unstable when incubated on certain laboratory media, losing chromosomes and generating diploid (and aneuploid) progeny strains that are themselves mating competent. The chromosome loss process is concerted, with loss of one or more chromosomes predisposing the cell to lose additional chromosomes, and the diploid state being the final product [16]. While tetraploids are stable when grown on YPD medium at different temperatures, two culture conditions were identified that induced genetic instability in C. albicans: (i) growth of tetraploid strains on S. cerevisiae “pre-sporulation” (pre-spo) medium at 37 °C, and (ii) growth of tetraploid strains on medium containing L-sorbose at 30 °C. The latter condition was previously shown to also induce chromosome loss in diploid C. albicans strains [19]. More specifically, diploid strains were unable to grow on L-sorbose medium unless they first underwent loss of one copy of Chromosome (Chr) 5, becoming monosomic for this chromosome. In contrast, diploid strains were relatively stable when grown on pre-spo medium, indicating that diploid and tetraploid strains exhibit very different selective pressures when cultured on this medium.

Bottom Line: We show that deletion of SPO11 prevented genetic recombination between homologous chromosomes during the C. albicans parasexual cycle.These findings suggest that at least one meiosis-specific gene has been re-programmed to mediate genetic recombination during the alternative parasexual life cycle of C. albicans.We discuss, in light of the long association of C. albicans with warm-blooded animals, the potential advantages of a parasexual cycle over a conventional sexual cycle.

View Article: PubMed Central - PubMed

Affiliation: Department of Genetics, Cell Biology and Development, University of Minnesota, Minneapolis, Minnesota, United States of America.

ABSTRACT
Candida albicans has an elaborate, yet efficient, mating system that promotes conjugation between diploid a and alpha strains. The product of mating is a tetraploid a/alpha cell that must undergo a reductional division to return to the diploid state. Despite the presence of several "meiosis-specific" genes in the C. albicans genome, a meiotic program has not been observed. Instead, tetraploid products of mating can be induced to undergo efficient, random chromosome loss, often producing strains that are diploid, or close to diploid, in ploidy. Using SNP and comparative genome hybridization arrays we have now analyzed the genotypes of products from the C. albicans parasexual cycle. We show that the parasexual cycle generates progeny strains with shuffled combinations of the eight C. albicans chromosomes. In addition, several isolates had undergone extensive genetic recombination between homologous chromosomes, including multiple gene conversion events. Progeny strains exhibited altered colony morphologies on laboratory media, demonstrating that the parasexual cycle generates phenotypic variants of C. albicans. In several fungi, including Saccharomyces cerevisiae and Schizosaccharomyces pombe, the conserved Spo11 protein is integral to meiotic recombination, where it is required for the formation of DNA double-strand breaks. We show that deletion of SPO11 prevented genetic recombination between homologous chromosomes during the C. albicans parasexual cycle. These findings suggest that at least one meiosis-specific gene has been re-programmed to mediate genetic recombination during the alternative parasexual life cycle of C. albicans. We discuss, in light of the long association of C. albicans with warm-blooded animals, the potential advantages of a parasexual cycle over a conventional sexual cycle.

Show MeSH
Related in: MedlinePlus