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

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.T HE predicted advantages and disadvantages of hapotide site, and their level of dominance can be measured loidy and diploidy, like those of recombination (Kondirectly. drashov 1993), hinge on the ability of populations (a)Two factors are preeminent in determining the effect to cope with deleterious mutation (Kondrashov and of ploidy on the rate of adaptation (Orr and Otto 1994): Crow 1991) and (b) to adapt to new environments (Orr the waiting time for mutations to appear and the fixation and Otto 1994). Our goal in this study was to make a time required for mutations to spread to high frequency direct comparison of ploidy states by measuring the in a population in response to directional selection. Under relative abilities of isogenic haploids and diploids of the conditions of finite population size where the waiting yeast Saccharomyces cerevisiae to adapt to the presence of time for beneficial mutations is the rate-limiting step in the antifungal drug fluconazole (FLC). We followed the adaptation, diploids should, at first glance, have the faster initial, rather than the long-term, responses of haploid rate of adaptation. This is because diploids have twice the and diploid populations to pinpoint the relati...

Heterozygote Advantage Is a Common Outcome of Adaptation inSaccharomyces cerevisiae

“…In a study of fluconazole resistance in yeast, Anderson et al (2004) did not reveal any adaptive mutations that were overdominant in levels of resistance. However, this study focused on fungicide resistance (a component of fitness) rather than on the overall fitness, and thus it may have failed to detect fitness overdominance for some of the resistant mutations by overlooking increased pleiotropic costs of the homozygous resistant mutations.…”

“…There is substantial evidence that Haldane's sieve does operate in evolution, at least to some extent. For instance, fungicide resistance in haploid vs. diploid yeast is driven by distinct sets of mutations, and at least some of these differences can be attributed to the fact that many of the adaptive mutations in haploids are recessive in resistance and would be invisible in diploids (Anderson et al 2004). …”

“…If adaptation is limited by the waiting time for new adaptive mutations, this suggests that diploids might enjoy an adaptive advantage over haploids; indeed, it has been argued that the rate of adaptive evolution in diploids might be double that in haploids (Paquin and Adams 1983;Anderson et al 2004) [although see Zeyl et al (2003) and Gerstein et al (2011)].…”

Gene Expression and Evolution of Antifungal Drug Resistance

“…The central question was whether targeting of such genes whose altered expression is widely associated with resistance might impair the ability of fungal populations to evolve resistance in the first place. Our test of the ability to evolve resistance involved FLC but not AmB because resistance to FLC occurs with high frequency in wild-type populations and, unlike resistance to AmB, is easily assayed in a matter of days (4). We observed that the deletion of certain genes whose levels of expression permanently increased with the evolution of resistance to FLC and AmB impaired the ability of populations to evolve resistance to FLC.…”

A multispecies approach for comparing sequence evolution of X-linked and autosomal sites inDrosophila

“…Accordingly, we might expect to observe faster-X evolution, provided that new beneficial mutations are, on average, at least partially recessive. There is little relevant information on the levels of dominance of beneficial mutations, although indirect evidence from the genetics of species differences in highly selfing taxa of plants (Charlesworth, 1992), and from comparisons of rates of adaptive evolution of haploid and diploid laboratory populations of yeast (Zeyl et al, 2003 ;Anderson et al, 2004), is consistent with a predominance of at least partial recessivity of new, selectively favourable mutations.…”