Sex has no detectable net benefits for Candida albicans : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Molecular Microbiology at Massey University, Palmerston North, New Zealand

Abstract

Like many other important opportunistic human fungal pathogens, for more than a century Candida albicans was thought to be strictly asexual until a parasexual cycle was recently discovered in the laboratory. It is uncertain, however, whether sex is still a viable reproductive strategy for C. albicans. In this study I tested whether or not mating enhanced survival of parental genes in this yeast, by mating 10 clinical isolates and testing recombinants’ fitness. Clinical isolates of C.albicans usually are diploid, carrying both the MTLa and MTLα mating type alleles, each on a different copy of chromosome 5. These strains are apparently incapable of meiosis and cannot mate with each other, because the a1-α2 heterodimer suppresses mating. Through selection on sorbose-containing agar I induced loss of MTL-heterozygosity and generated 5 MTLa and 5 MTLα derivatives of clinical isolates. Existing mating techniques involve the use of auxotrophic markers, requiring time-consuming sequential disruption of two copies of biosynthetic genes if wild-type clinical isolates are to be mated. Furthermore, auxotrophy affects the virulence of a strain, and this can potentially interfere with comparing the fitness of recombinants with that of their parents. I therefore developed a method for mating clinical isolates marked with two drug resistance markers, the mycophenolic acid (MPA) resistance-conferring allele of IMH3 and the nourseothricin (NAT) resistance gene CaNAT1, allowing selection of recombinants on the basis of resistance to both agents. I marked all MTLa strains with the MPA resistance gene and all MTLα strains with the NAT resistance gene. This allowed 25 combinations for mating. Recombinants were obtained from 15 combinations of 9 strains. It was found that not all C. albicans clinical isolates could mate. Using growth rate as the criterion, I tested the fitness of clinical isolates, MTLhomozygous derivatives with and without resistance markers and recombinants during adaptation to a novel environment (YPD medium), maximizing the potential benefits of sex. After computationally correcting for the impact of experimental manipulations, I calculated the net benefit of sex as the difference in the number of offspring from two cells that become mating competent and engage in sex compared to the offspring they could have produced by continued clonal reproduction. My results indicated that, as a rule, engaging in sex reduces the chances of survival of C. albicans’ genes, in part because MTL homozygosis significantly reduced growth rates. Through fitness increase after recombination, sex may eventually confer a net benefit for some strain combinations in the laboratory, but this probably occurs too late to prevent elimination of recombinants by competition and genetic drift in nature. Sex in C. albicans therefore diminished parents’ chances to pass on their genes to future generations. These findings have a significant impact on the assessment of the role of sex in C. albicans and other “asexual” human fungal pathogens. Recent loss of the function of sex and incomplete decay of the sex machinery are the most likely explanation of C. albicans’s residual ability to mate, and one that also needs to be considered in other fungal pathogens.