Figure 2: Stacked bar plot indicating the outcome for all inoculated wells over the three acquisition experiments in deep well boxes. The total height of the bar is proportional to the total number of wells inoculated of that strain type. The two darker bars together are proportional to the total number of wells that had growth in the acquisition experiments. The darkest coloured bar is proportional to the number of wells where resistance evolved based on the confirmation assays (excluding the populations verified to have contamination). No resistant diploid populations were found, while a large proportion of the haploid populations that grew in the acquisition experiments were reliably resistant to nystatin.
Abstract
The number of copies of each chromosome, or ploidy, of an organism is a major genomic factor affecting adaptation. We set out to determine how ploidy can impact the outcome of evolution, as well as the likelihood of evolutionary rescue, using short-term experiments with yeast (Saccharomyces cerevisiae) in a high concentration of the fungicide nystatin. In similar experiments using haploid yeast, the genetic changes underlying evolutionary rescue were highly repeatable, with all rescued lines containing a single mutation in the ergosterol biosynthetic pathway. All of these beneficial mutations were recessive, which led to the expectation that diploids would find alternative genetic routes to adaptation. To test this, we repeated the experiment using both haploid and diploid strains and found that diploid populations did not evolve resistance. Although diploids are able to adapt at the same rate as haploids to a lower, not fully inhibitory, concentration of nystatin, the present study suggests that diploids are limited in their ability to adapt to an inhibitory concentration of nystatin, while haploids may undergo evolutionary rescue. These results demonstrate that ploidy can tip the balance between adaptation and extinction when organisms face an extreme environmental change.