Genetic and epigenetic factors affecting adaptation in eukaryotes
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Adaptation is a complex process whose dynamics are poorly understood. This lack of understanding is largely due to our limited knowledge of the rate at which heritable variation is produced and the effects of that variation on fitness. Here, I investigate how genetic and heritable epigenetic variation influences the rate and outcomes of adaptation. In Chapter 1, I briefly review current theoretical predictions about the relationship between parameters of heritable variation and adaptation. In Chapter 2, I describe how epigenetic modification and/or selection on gametes can generate unusual patterns of adaptation. Some types of epigenetic modifications (X-inactivation and genomic imprinting) cause diploid loci to only express a single allele. Selection at these ‘effectively haploid’ loci is very similar to selection acting on haploid loci in sperm. In both cases, selection can favor the spread of traits that are only adaptive during one part of the life cycle, at one level of selection, or for one sex. In Chapters 3 and 4, I describe experimental evolution studies in the yeast, Saccharomyces cerevisiae, examining factors affecting adaptation rates in yeast. In Chapter 3, I present results from a mutation accumulation experiment estimating the rate of random mutations affecting fitness, the proportion of the mutations that are beneficial and the mean effect of mutations. In this study I found that a substantial proportion of random mutations improve adaptation in yeast. In chapter four, I present results of an experiment examining the relationship between the [PSI + ] prion and adaptation rates in the prion’s yeast host. [PSI + ] has previously been shown to improve yeast growth in some novel environments and has been proposed to increase yeast adaptation. The results of this study indicate that, in some environments, yeast carrying the [PSI + ] prion do adapt more rapidly than [psi - ] yeast.