Academic journal article Genetics

Haploidy, Diploidy and Evolution of Antifungal Drug Resistance in Saccharomyces Cerevisiae

Academic journal article Genetics

Haploidy, Diploidy and Evolution of Antifungal Drug Resistance in Saccharomyces Cerevisiae

Article excerpt

ABSTRACT

We tested the hypothesis that the time course of the evolution of antifungal drug resistance depends on the ploidy of the fungus. The experiments were designed to measure the initial response to the selection imposed by the antifungal drug fluconazole up to and including the fixation of the first resistance mutation in populations of Saccharomyces cerevisiae. Under conditions of low drug concentration, mutations in the genes PDR1 and PDR3, which regulate the ABC transporters implicated in resistance to fluconazole, are favored. In this environment, diploid populations of defined size consistently became fixed for a resistance mutation sooner than haploid populations. Experiments manipulating population sizes showed that this advantage of diploids was due to increased mutation availability relative to that of haploids; in effect, diploids have twice the number of mutational targets as haploids and hence have a reduced waiting time for mutations to occur. Under conditions of high drug concentration, recessive mutations in ERG3, which result in resistance through altered sterol synthesis, are favored. In this environment, haploids consistently achieved resistance much sooner than diploids. When 29 haploid and 29 diploid populations were evolved for 100 generations in low drug concentration, the mutations fixed in diploid populations were all dominant, while the mutations fixed in haploid populations were either recessive (16 populations) or dominant (13 populations). Further, the spectrum of the 53 nonsynonymous mutations identified at the sequence level was different between haploids and diploids. These results fit existing theory on the relative abilities of haploids and diploids to adapt and suggest that the ploidy of the fungal pathogen has a strong impact on the evolution of fluconazole resistance.

THE predicted advantages and disadvantages of haploidy and diploidy, like those of recombination (KONDRASHOV 1993), hinge on the ability of populations (a) to cope with deleterious mutation (KONDRASHOV and CROW 1991) and (b) to adapt to new environments (ORR and Otto 1994). Our goal in this study was to make a direct comparison of ploidy states by measuring the relative abilities of isogenic haploids and diploids of the yeast Saccharomyces cerevisiae to adapt to the presence of the antifungal drug fluconazole (FLC). We followed the initial, rather than the long-term, responses of haploid and diploid populations to pinpoint the relative advantages and disadvantages of each of the two ploidy states in adaptation and to minimize the potentially confounding effects of the genetic divergence that may accumulate with long-term evolution. Antifungal drug resistance is an excellent model for adaptation because the fitness increments with each mutation are large (ANDERSON et al. 2003), the underlying molecular mechanisms are well documented (SANGLARD et al. 1998; WHITE et al. 1998; LUPETTI et al. 2002; SANGLARD and ODDS 2002), the mutations can be readily mapped to the gene and nucleotide site, and their level of dominance can be measured directly.

Two factors are preeminent in determining the effect of ploidy on the rate of adaptation (ORR and OTTO 1994): the waiting time for mutations to appear and the fixation time required for mutations to spread to high frequency in a population in response to directional selection. Under conditions of finite population size where the waiting time for beneficial mutations is the rate-limiting step in adaptation, diploids should, at first glance, have the faster rate of adaptation. This is because diploids have twice the number of targets for mutations, resulting in a higher frequency of mutations conferring increased fitness. This advantage for diploids, however, accrues only when the mutations are sufficiently dominant in their effect on phenotype. Where the mutations are recessive, haploids should have the advantage because the fixation time, rather than the waiting time, is the rate-limiting step. …

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