The Parasexual Cycle in Candida albicans Provides an Alternative Pathway to Meiosis for the Formation of Recombinant Strains

Abstract

Candida albicans has an elaborate, yet efficient, mating system that promotes conjugation between diploid a and α strains. The product of mating is a tetraploid a/α 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.

Genomic evidence for ameiotic evolution in the bdelloid rotifer Adineta vaga

“…5). However, we cannot rule out that other forms of recombination occur in bdelloid populations in ways that do not require homologous pairing, such as parasexuality 26 . The high number of horizontally acquired genes, including some seemingly recent ones, suggests that HGTs may also be occurring from rotifer to rotifer.…”

Stress-Induced Phenotypic Switching inCandida albicans

“…To further analyze a potential relationship between growth rates and white-opaque switching, we tested progeny from the parasexual cycle of C. albicans for their doubling times and switching frequencies. Sexual reproduction in C. albicans occurs between opaque a and ␣ cells and is completed by an unusual mechanism of chromosome loss in tetraploid strains, generating recombinant diploid and aneuploid products (Bennett and Johnson, 2003;Forche et al, 2008). Previous studies demonstrated that many of the progeny derived from the parasexual cycle were slower growing than parental SC5314 strains, even in progeny with a euploid complement of chromosomes (Forche et al, 2008).…”

“…Sexual reproduction in C. albicans occurs between opaque a and ␣ cells and is completed by an unusual mechanism of chromosome loss in tetraploid strains, generating recombinant diploid and aneuploid products (Bennett and Johnson, 2003;Forche et al, 2008). Previous studies demonstrated that many of the progeny derived from the parasexual cycle were slower growing than parental SC5314 strains, even in progeny with a euploid complement of chromosomes (Forche et al, 2008). Analysis of euploid progeny identified several slow-growing diploid strains that also underwent switching at high frequency (e.g., isolates P6 and Ss10; Table 4).…”

Budding off: bringing functional genomics toCandida albicans

“…Although C. albicans is typically isolated as a diploid, it is also capable of propagating in the haploid or tetraploid states [22,40]. Furthermore, as noted above, aneuploid cells carrying an abnormal complement of chromosomes are frequently found among clinical C. albicans strains [11,[41][42][43], suggesting they can provide a selective advantage in some host contexts.…”

“…However, MLST karyotyping relied on genetic information at a limited number of loci, which may have obfuscated identification of hybrid strains or introgressed genomic regions resulting from sexual exchange [22].…”