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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 Progeny Strains from the Parasexual Mating Cycle by Flow CytometryProgeny strains derived from the tetraploid RBY18 were grown in liquid YPD medium, as described in Materials and Methods. Strains P1 to P7 (C–I) were derived from growth of RBY18 on pre-spo medium, while strains S1 to S6 (J–O) were derived from growth of RBY18 on sorbose medium. In both cases, progeny strains were found to be diploid, or near diploid, by flow cytometric analysis. For comparison, a parental diploid strain (A) and tetraploid strain (B) were also analyzed by flow cytometry. The x-axis of each graph (Sytox) represents a linear scale of nuclear fluorescence, and the y-axis (Counts) represents a linear scale of cell number.
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pbio-0060110-g002: Analysis of Progeny Strains from the Parasexual Mating Cycle by Flow CytometryProgeny strains derived from the tetraploid RBY18 were grown in liquid YPD medium, as described in Materials and Methods. Strains P1 to P7 (C–I) were derived from growth of RBY18 on pre-spo medium, while strains S1 to S6 (J–O) were derived from growth of RBY18 on sorbose medium. In both cases, progeny strains were found to be diploid, or near diploid, by flow cytometric analysis. For comparison, a parental diploid strain (A) and tetraploid strain (B) were also analyzed by flow cytometry. The x-axis of each graph (Sytox) represents a linear scale of nuclear fluorescence, and the y-axis (Counts) represents a linear scale of cell number.

Mentions: To generate progeny strains that have undergone the parasexual mating cycle, the marked tetraploid strain RBY18 was induced to undergo chromosome loss on either pre-spo or sorbose medium and gal1− strains were selected by growth on 2-DOG medium. These 2-DOG resistant (DOGR) strains were subsequently analyzed by PCR to confirm that loss of MTL alleles on Chr 5 had accompanied loss of GAL1 alleles on Chr 1, an indication that cells had undergone a reduction in overall cell ploidy (unpublished data). PCR of the MTL loci was also used to detect possible jackpot effects, where several gal1− progeny might have been derived from a single cell having undergone a chromosome loss event. Where possible, progeny cells with different combinations of MTL alleles were used for subsequent analysis. Selected progeny strains were grown in YPD medium at 30 °C and analyzed by flow cytometry to determine the overall ploidy of each strain, as shown in Figure 2. Flow cytometric analyses confirmed that each strain was diploid, or close to diploid, in DNA content, as judged by staining of the DNA with sytox green [9]. Seven strains (P1 to P7) were derived from RBY18 by growth on pre-spo medium, and six strains (S1 to S6) were derived from RBY18 by growth on sorbose medium (Figure 2). Subtle differences were observed in the flow cytometry DNA profiles between isolates, where distinct peaks were evident representing non-replicated (G1 phase) and replicated (G2 phase) DNA. In some strains the majority of the cells contained replicated DNA (e.g., S5 and S6, Figure 2, panels N and O), while others had an almost equal distribution of cells with unreplicated and replicated DNA (e.g., P4, panel F). However, there was no obvious correlation between DNA profiles analyzed by flow cytometry and cell growth rates.


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 Progeny Strains from the Parasexual Mating Cycle by Flow CytometryProgeny strains derived from the tetraploid RBY18 were grown in liquid YPD medium, as described in Materials and Methods. Strains P1 to P7 (C–I) were derived from growth of RBY18 on pre-spo medium, while strains S1 to S6 (J–O) were derived from growth of RBY18 on sorbose medium. In both cases, progeny strains were found to be diploid, or near diploid, by flow cytometric analysis. For comparison, a parental diploid strain (A) and tetraploid strain (B) were also analyzed by flow cytometry. The x-axis of each graph (Sytox) represents a linear scale of nuclear fluorescence, and the y-axis (Counts) represents a linear scale of cell number.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2365976&req=5

pbio-0060110-g002: Analysis of Progeny Strains from the Parasexual Mating Cycle by Flow CytometryProgeny strains derived from the tetraploid RBY18 were grown in liquid YPD medium, as described in Materials and Methods. Strains P1 to P7 (C–I) were derived from growth of RBY18 on pre-spo medium, while strains S1 to S6 (J–O) were derived from growth of RBY18 on sorbose medium. In both cases, progeny strains were found to be diploid, or near diploid, by flow cytometric analysis. For comparison, a parental diploid strain (A) and tetraploid strain (B) were also analyzed by flow cytometry. The x-axis of each graph (Sytox) represents a linear scale of nuclear fluorescence, and the y-axis (Counts) represents a linear scale of cell number.
Mentions: To generate progeny strains that have undergone the parasexual mating cycle, the marked tetraploid strain RBY18 was induced to undergo chromosome loss on either pre-spo or sorbose medium and gal1− strains were selected by growth on 2-DOG medium. These 2-DOG resistant (DOGR) strains were subsequently analyzed by PCR to confirm that loss of MTL alleles on Chr 5 had accompanied loss of GAL1 alleles on Chr 1, an indication that cells had undergone a reduction in overall cell ploidy (unpublished data). PCR of the MTL loci was also used to detect possible jackpot effects, where several gal1− progeny might have been derived from a single cell having undergone a chromosome loss event. Where possible, progeny cells with different combinations of MTL alleles were used for subsequent analysis. Selected progeny strains were grown in YPD medium at 30 °C and analyzed by flow cytometry to determine the overall ploidy of each strain, as shown in Figure 2. Flow cytometric analyses confirmed that each strain was diploid, or close to diploid, in DNA content, as judged by staining of the DNA with sytox green [9]. Seven strains (P1 to P7) were derived from RBY18 by growth on pre-spo medium, and six strains (S1 to S6) were derived from RBY18 by growth on sorbose medium (Figure 2). Subtle differences were observed in the flow cytometry DNA profiles between isolates, where distinct peaks were evident representing non-replicated (G1 phase) and replicated (G2 phase) DNA. In some strains the majority of the cells contained replicated DNA (e.g., S5 and S6, Figure 2, panels N and O), while others had an almost equal distribution of cells with unreplicated and replicated DNA (e.g., P4, panel F). However, there was no obvious correlation between DNA profiles analyzed by flow cytometry and cell growth rates.

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