Polyploidy is observed across the tree of life, yet its influence on evolution remains incompletely understood1,2,3,4. Polyploidy, usually whole-genome duplication, is proposed to alter the rate of evolutionary adaptation. This could occur through complex effects on the frequency or fitness of beneficial mutations2,5,6,7. For example, in diverse cell types and organisms, immediately after a whole-genome duplication, newly formed polyploids missegregate chromosomes and undergo genetic instability8,9,10,11,12,13. The instability following whole-genome duplications is thought to provide adaptive mutations in microorganisms13,14 and can promote tumorigenesis in mammalian cells11,15. Polyploidy may also affect adaptation independently of beneficial mutations through ploidy-specific changes in cell physiology16. Here we perform in vitro evolution experiments to test directly whether polyploidy can accelerate evolutionary adaptation. Compared with haploids and diploids, tetraploids undergo significantly faster adaptation. Mathematical modelling suggests that rapid adaptation of tetraploids is driven by higher rates of beneficial mutations with stronger fitness effects, which is supported by whole-genome sequencing and phenotypic analyses of evolved clones. Chromosome aneuploidy, concerted chromosome loss, and point mutations all provide large fitness gains. We identify several mutations whose beneficial effects are manifest specifically in the tetraploid strains. Together, these results provide direct quantitative evidence that in some environments polyploidy can accelerate evolutionary adaptation.
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Gene Expression Omnibus
Sequence Read Archive
All aCGH data have been deposited in the National Center for Biotechnology Information Gene Expression Omnibus database under accession number GSE51017 and all WGS data have been deposited in the National Center for Biotechnology Information Sequence Read Archive database under accession number SRP047435.
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This work was supported by the Howard Hughes Medical Institute, the National Institutes of Health (R37 GM61345), the G. Harold & Leila Y. Mathers Charitable Foundation, the Dana-Farber Cancer Institute Physical Sciences-Oncology Center (U54CA143798), the Boettcher Foundation’s Webb-Waring Biomedical Research Program, the National Science Foundation (NSF 1350915), the National Institutes of Health (R01 GM081617), and an American Cancer Society Postdoctoral Fellowship.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Schematic representation of the construction of isogenic haploid, diploid, and tetraploid strains used in this study.
Relevant strain numbers are indicated for the CFP- and YFP-containing ancestors.
Extended Data Figure 2 Estimates from our mathematical modelling of the best-fit value of the beneficial mutation rate (μ) and the selection coefficient (s) of each ploidy evolution experiment.
a, Table of μ and s values that had the best-fit between the simulations and the experimental data; brackets, 95% confidence intervals. Values were determined on the basis of different assumptions about the underlying distribution of beneficial mutations, which included (b) uniform, (c) delta, and (d) exponential distributions. Estimates of μ and s were also obtained with (e) the equivalence principle model17, which assumes a delta distribution of beneficial mutations. Each two-dimensional plot includes the error range obtained by parametric bootstrap of 1,000 independent simulated data sets (Methods). The 95% confidence intervals of μ and s from those 1,000 data sets were defined as the error ranges. f, Schematic diagram of the three distributions of fitness effects that we used in our mathematical modelling: exponential (red), uniform (black), and delta (green) distributions. For illustration, we also provide a narrow Gaussian distribution (blue) that is close to a delta function. The real distribution of fitness effects probably has a more complex structure than any of the examples shown. The diagram illustrates the fact that the shape of the assumed distribution mandates differences in mutation rates. For example, if the mutations that mainly drive adaptation fall within the region of the double arrow, only a small proportion of the mutations from the exponential distribution will fall within this range, necessitating a much higher mutation rate to generate mutations in this region. By contrast, the delta distribution lies in the middle of the double arrow range; therefore, all of the mutations that arise from this distribution are strong enough to contribute to adaptation, resulting in a relatively lower mutation rate. The uniform distribution is intermediate between these two extremes. Only a small portion of the mutations of the uniform distribution is within the double arrow region, but the probability of these mutations is orders of magnitude larger than the exponential. Therefore, the mutation rate of the uniform is closer to the delta than to the exponential distribution. The values used to generate this figure are the best-fit values of μ and s of the haploid populations in the different three distributions. See Methods for more details.
Extended Data Figure 3 Experimental and computational analyses of the noise in our experimental measurements and of the methods used in our mathematical modelling (see Supplementary Information).
a, Three different methods were used to determine the percentage of YFP-expressing cells in mixtures of the 1N, 2N, and 4N CFP and YFP ancestor strains. Cells were analysed by flow cytometry (10,000 cells) and fluorescence microscopy (300 cells), and by single colony analysis (96 colonies) of the mixture before galactose induction. The percentage YFP determined by all three methods was highly correlated (Pearson correlation coefficient = 0.98). b, Table showing variation in flow cytometry replicate measurements: the standard deviation of the percentage YFP obtained from 48 replicate populations of six different CFP:YFP ratios, for each ploidy type. c, Table showing the average and standard deviation of the best-fit values for different ratios between beneficial (Ub) and deleterious (Ud) mutations, obtained from 100 independent data sets. d, Evaluation of different summary statistics by calculating the distribution of best-fit values from 1,000 replicate simulations. Four different summary statistics were used to analyse 1,000 replicates of a parameter pair, s and μ (see Methods). The summary statistic using ten bins has the highest mode and no outliers, and was used to generate our best-fit values. e, Criteria for exclusion of near-neutral mutations for implementation of the branching evolutionary model with an exponential distribution of mutations. Shown is the average deviation from equal percentages of YFP and CFP-expressing cells with different thresholds for neutral mutations. The threshold (Tr) represents the fraction of the average fitness effect (s), meaning every mutation whose absolute value was smaller than Tr × s was excluded. For this scenario (with parameters μ = 2 × 10−5 and s = 0.08), we can exclude every mutation with a fitness effect smaller than s (that is, Tr = 1, light blue) without changing the outcome relative to excluding no mutations (Tr = 0). However, when excluding all mutations with fitness effects smaller than 10× s (Tr = 10, dark green), the result changes substantially. Thus, for high mutation rates (Nμ > 1), we can exclude weak mutations20.
Aneuploidy was not detected in the parental 1N, 2N, or 4N strains. Genomic DNA from each strain was compared with that of an isogenic ancestor PY3295 (BY4741 MATa ura3 his3 trp1 leu2 LYS2) and log2 DNA copy number ratios were plotted using a custom Matlab script. To account for regions of complete deletion, the data were cropped at log2 ratios of ±2.0 and averaged across each chromosome using a sliding window of nine oligonucleotides. A log2 ratio of zero is indicated by the red line. Loci altered during strain construction are indicated (TRP1, pSTE5, URA3, STE4). Strain ploidy, determined by flow cytometry, is indicated on the right.
a, aCGH of eight haploid-evolved clones. Data are displayed as in Extended Data Fig. 4. No aneuploidy was detected. Clone 1N_131 acquired the HXT6/7 amplification (arrow). b, aCGH of eight diploid-evolved clones. No aneuploidy was detected, but all clones except 2N_233 acquired the HXT6/7 amplification. Log2 ratios were averaged across each chromosome using a sliding window of 29 oligonucleotides. The ploidy of the evolved clone, determined by flow cytometry, is indicated on the right.
aCGH data are displayed as in Extended Data Fig. 4. Note that whole chromosome or large segmental chromosome gain and loss events are observed in all clones except clone 4N_337. Ploidy of the evolved clone, determined by flow cytometry, is indicated on the right, with +/− indicating chromosome aneuploidy. Some highly aneuploid clones had widely different chromosome copy numbers for different chromosomes (for example, some chromosomes were disomic, others trisomic and tetrasomic).
Extended Data Figure 7 Analysis of recurrent and concerted chromosome loss events in the tetraploid-evolved clones.
a, Evolved tetraploids acquired large segmental aneuploidies (regions greater than the ∼7 kb HXT6/7 amplification); aCGH data for individual chromosomes with large segmental aneuploidies in 4N-evolved clones (plotted using Treeview52). All breakpoints occurred at or near Ty sequences (arrowheads). b, The pairwise patterns (Pearson correlation) of all chromosome copy number alterations in the 4N-evolved clones at generation 250 (n = 30, Supplementary Table 2). The copy numbers of some chromosomes were correlated (for example, chromosome XV and chromosome XVI), whereas others were anti-correlated (for example, chromosome VIII and chromosome IX), possibly reflecting the need for gene expression balance. c, Hierarchical clustering showing the copy number relationship among the chromosomes. d, Proportion of all chromosomes in the evolved tetraploid clones with the indicated copy number (black). The copy number of chromosome XIII (grey) in the 4N-evolved clones at generation 250 was significantly different from that of all other aneuploid chromosomes (Cochran–Armitage test, P < 1 × 10−7).
All 4N-evolved clones at (a) generations 35 and 55 and (b) generation 500 were aneuploid for multiple chromosomes or carried large segmental chromosome aneuploidies, except for clone 4N_gen35_503, which remained tetraploid. Data are displayed as in Extended Data Fig. 4. Ploidy of the evolved clone, determined by flow cytometry, is indicated on the right, with +/− indicating chromosome aneuploidy.
Extended Data Figure 9 aCGH from isogenic 2N and 4N strains with an extra copy of chromosome XIII or chromosome XII.
Data are displayed as in Extended Data Fig. 5b.
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Selmecki, A., Maruvka, Y., Richmond, P. et al. Polyploidy can drive rapid adaptation in yeast. Nature 519, 349–352 (2015). https://doi.org/10.1038/nature14187
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