The effects of mutation accumulation on fitness: The role of environment, epistasis, population size, and time

The accumulation of deleterious mutations due to Muller's Ratchet is theorized as a major factor in the evolution of sex, recombination, and ploidy, and plays a major role in population genetics, speciation, conservation biology, and other biological phenomena. Though important, mutations are difficult to study due to multiple confounding factors that affect their expression. Deleterious mutations decrease the fitness of populations, and the effects of mutations and the type of mutations fixed in a population are not constant but are dependent on (1) population size; (2) the environment; and (3) their interaction(s) with the genetic background and with each other, called epistasis. Population sizes determine which mutations are fixed while environment and epistasis alter their impact.;Measurements of fitness suggest that small populations fix deleterious mutations which lowers overall fitness of all populations, while larger populations fix more beneficial mutations, thereby increasing their fitness. This change in mutational background seems to have no effect on epistasis. Measurement of the cost of T7 phage resistance showed that, on average, there was no directional epistasis.;However, environment had an enormous effect on mutation and epistasis. Mutational effects in general are magnified under the more stressful environment, DM. Furthermore, environment also changes the nature of epistasis. While, on average, no directional epistasis was detected in LB, a trend towards more antagonistic epistasis was observed in DM. In addition, the data showed that epistasis changes from synergistic to antagonistic through evolutionary time.;My results suggest that population size plays an important role in the type of mutation fixed. Small populations suffer from deleterious mutations while large populations could reverse these negative effects. Epistasis however, deviated from theoretical expectations of a predominant mode of interaction. Multiple types of epistasis were found suggesting that epistasis is situational.;To determine the interactions between these factors, a three stage experimental design was used. First, 15 populations of Escherichia coli were transferred daily through single cell bottlenecks for 200 days in a nutrient rich environment, LB. Bottlenecks facilitate the effects of Muller's Ratchet which increases the probability of deleterious mutation fixation. To determine the interaction between deleterious mutations or epistasis, the addition of a T7 phage resistance gene was used. In the second stage, these 15 evolved populations were moved to a minimal nutrient environment, DM. Here, environmental effects on mutation, epistasis, and the effect of epistasis through time can be evaluated. Last, a population flush was performed to assess the mutations fixed by large versus small populations. The effects of epistasis were analyzed to determine the consequence of having an alternate mutational background.