Academic journal article Genetics

Selective Advantages of a Parasexual Cycle for the Yeast Candida Albicans

Academic journal article Genetics

Selective Advantages of a Parasexual Cycle for the Yeast Candida Albicans

Article excerpt

SEX is costly and disrupts well-adapted allele combinations. It can also be advantageous by speeding up adaptation or by purging deleterious mutations. It has been difficult to establish how, precisely, these and other benefits outweigh the cost of sex. Indeed, abandoning sex in favor of clonal reproduction can be advantageous- asexual species arise frequently. Their life spans are short, however, indicating that sex may be essential for the long-term survival of species (Otto and Lenormand 2002; Rice 2002).

Sexual cycles have not been observed for ^20% of fungal species (Carlile et al. 2001). Whether these species are truly asexual or merely restrict the frequency of sex-a strategy believed to maximize its benefits (Heitman 2006)-is difficult to determine. Genetic marker distributions in such species often suggest limited recombination (Carlile et al. 2001). However, clonal reproduction will initially copy the genetic marker distributions that were generated by sex, and new mutations and genetic drift will only slowly erase evidence of past recombination (Schmid et al. 2004; Cox et al. 2013). With rare exceptions, fungi without observable sexual cycles are derived from recent sexual ancestors (Carlile et al. 2001; Schmid et al. 2004; Butler 2007). Thus genetic marker distributions may reflect limited ongoing recombination or be an "echo" of recombination events that occurred in sexual ancestors.

The yeast Candida albicans, a commensal colonizer and occasional opportunistic pathogen of humans (Odds 1988),canmate,butitisaconvolutedprocess.Mostisolates are diploid and carry both mating-type-like (MTL) locus alleles, preventing mating. Apparently incapable of meiosis, loss of an entire copy of chromosome 5 (chr 5), on which the MTL locusislocated,orlocalizedlossof heterozygosity (LOH), generate MTL-homozygotes, which can mate. To do so they must, in addition, undergo an epigenetic switch from the "white" phenotype to the matingcompetent "opaque" phenotype (Miller and Johnson 2002; Alby et al. 2009; Heitman 2010; Xie et al. 2013). A small fraction of mating encounters generates fusants with genetic markers from both parents (Lockhart et al. 2003; Bennett et al. 2005). These subsequently return to diploidy by chromosome loss (Bennett and Johnson 2003) and, in the process, intrachromosomal recombination can generate true recombinants (Forche et al. 2008). Alternatively, mating can be induced between haploids, which spontaneously arise from diploids by concerted chromosome loss, resulting immediately in diploid progeny (Hickman et al. 2013).

However, mating has never been observed in the human host, which is likely to be the natural environment of strains found on humans (Edelmann et al. 2005; Jacobsen et al. 2008; Wrobel et al. 2008) and the predominant environment of the species as a whole (Skinner and Fletcher 1960; Odds 1988). That mating is very rare is also indicated by the largely clonal population structure of the species (Gräser et al. 1996; Tibayrenc 1997; Tavanti et al. 2004; Bougnoux et al. 2008).

The existing data are consistent with two hypotheses: The first is that, while rare, mating in the natural environment produces progeny (fusant) lineages that surpass clonal lineages in fitness often enough to render the process biologically significant. If so, mutations that have turned mating into a convoluted process may have been selected because, for C. albicans, the benefits of mating are maximized by restricting its frequency (Heitman 2006). Alternatively, it is possible that mating so rarely generates fusants that are superior to clonal lineages that the process is no longer biologically significant. In that case, the mutations that make mating convoluted would indicate progressive mutational decay of a process that is no longer protected by selection-indeed mutations rendering strains entirely incapable of mating are not uncommon (Legrand et al. 2004). To distinguish between these two hypotheses we have investigated whether mating is under selection in C. …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.