Birds Do It, Bees Do It, but Candida Albicans Does It Differently

Birds Do It, Bees Do It, but Candida albicans Does It Differently Richard Robinson | doi:10.1371/journal.pbio.0060121 The yeast Candida albicans lives an suggesting that many chromosomes do unnoticed and mostly harmless life as not harbor lethal recessive alleles, as a member of our gut flora. However, has been proposed for C. albicans. mainly in an immunocompromised Unexpectedly, the authors also host, it can proliferate and cause found evidence that, in some severe, life-threatening infections. strains, the remaining chromosomes Within this normally mild-mannered, had undergone homologous single-celled fungus beats the heart of recombination, one of the hallmarks a reproductive adventurer. For while of meiosis. C. albicans is known to it appears to be incapable of meiosis have genes that, in other organisms, and therefore true sex, it engages in an function specifically in meiosis, but no unusual and offbeat alternative—after role for them in C. albicans has been doi:10.1371/journal.pbio.0060121.g001 it mates, its progeny randomly cast off previously identified. The authors chromosomes to restore the diploid Configuration of the eight chromosomes showed that one such protein, Spo11p, number, or something close to it. In of Candida albicans in a recombinant strain which in other species cleaves double- a new study, Anja Forche, Richard derived from the parasexual mating cycle. stranded DNA as a prelude to meiotic Bennett, and colleagues show that this recombination, was expressed in process generates significant genetic been discovered, and its details and mitotically growing C. albicans. When diversity, which is further amplified by genetic consequences have not been the researchers deleted the gene, recombination between homologous well characterized. tetraploid cells could still reduce their chromosomes, using a protein that is The authors found that by chromosome number, but did not elsewhere used exclusively in meiosis. manipulating the medium used to undergo recombination. These results Meiosis is essential for sexual grow the yeast, they could induce stable indicate that in C. albicans, Spo11 reproduction because it prevents tetraploids to undergo a reduction cleaves double-stranded DNA to bring the number of chromosomes from in their chromosome number. They about recombination but, instead of doubling every generation. In humans, isolated and analyzed the chromosome- occurring during meiosis, this happens 23 pairs of chromosomes (the “diploid” reduced progeny, and showed that during chromosome loss in the number) split at the start of meiosis to each strain was diploid, or nearly so. parasexual cycle. give each gamete cell 23 singletons (the Colonies grown from these single cells The authors propose that “haploid” number). Union of sperm displayed a wide range of phenotypes, parasexuality may provide two and egg restores the diploid number. from smooth (few filamentous cells) to advantages over the conventional Genetic diversity is an important wrinkled (many filamentous cells). meiotic sexual cycle, both tied to the consequence of meiosis—members of To examine the genetic diversity of yeast’s ability to survive within the chromosomal pairs, called homologs, the offspring further, they turned to mammalian host. First, the randomness segregate randomly during meiosis, single nucleotide polymorphism (SNP) of chromosome reduction and the so that individual gametes contain profiling, combined with comparative high tolerance of the presence of a unique mix of paternally and genome hybridization (CGH). SNPs three copies rather than two (or maternally derived homologs. Diversity are short DNA sequences that differ four) increases genetic variety, a is increased when, before separating, between homologous chromosomes, potentially important feature for living homologs exchange segments, a thus allowing the identification of in a dynamic environment such as the process called recombination, or maternal versus paternal homologs intestine. Second, the parasexual cycle crossing over. in the offspring. CGH provides avoids the production of spores, often C. albicans, though, does none information on the copy number of the end result of fungal meioses. Spores of that. Its diploid number is 16, each homolog present. are believed to be highly conspicuous and a reductional division to an 8- Their analysis showed that of 13 to the immune system, which is exactly chromosome haploid has never been progeny strains, only three were true what such a low-profile resident of the observed. Instead of going down, the diploids, with exactly 16 chromosomes. gut wants to avoid. number goes up—during conjugation, The rest had three copies of at least two different mating strains unite one chromosome. Colonies from cells Forche A, Alby K, Schaefer D, Johnson AD, diploid cells to form tetraploid with extra chromosomes generally grew Berman J, et al. (2008) The parasexual cycle (32-chromosome) cells, which then more slowly than those of true diploids. in Candida albicans provides an alternative reduce back toward the diploid Viability did not require both maternal pathway to meiosis for the formation of number by chromosomal loss. This and paternal chromosomes in the same recombinant strains. doi:10.1371/journal. “parasexual” cycle has only recently cell—two copies of either would suffice, pbio.0060110 PLoS Biology | www.plosbiology.org 0919 May 2008 | Volume 6 | Issue 5 | e121.

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