Investigating the effects of genetic variation and environmental fluctuations on the evolution of laboratory populations of Escherichia coli

Abstract

Variation is central to evolution by natural selection. It is both a prerequisite for evolution and a major factor driving the course of evolution. Temporal heterogeneity or fluctuating environments, play crucial roles in the maintenance of variation and diversity. For my doctoral work, I have systematically investigated the interaction of variation and fluctuating environments, on the evolutionary dynamics of laboratory populations of Escherichia coli.

First, I investigated how increased supply of variation due to elevated mutation rates, and the physiological state of the organism influence genomic and phenotypic evolution. Ultraviolet radiation (UV) was used to increase the mutation rates during two different phases of the growth cycle, namely the lag and the exponential phase. Genome evolution showed signatures of the growth phases, likely due to differences in mutational distributions and/or selection. However, UV resistance phenotype was convergent across growth phases. In the second part of this study, populations were subjected to fluctuations in UV exposures, in the presence of an antibiotic. In addition to UV and antibiotic resistance, these populations had accumulated greater variation in neutral traits showing increased pre-adaptation and cross-resistance to novel antibiotics.In a second study, I explored the role of migration as a source of variation, in populations adapting to fluctuating environments. In contrast to a number of studies in lethal environment, when populations are faced with suboptimal and unpredictably fluctuating environments, migration without variation had negative effect on fitness. This negative effect was countered, and migration was beneficial, only when migrants carried variation. Finally, I studied how predictability and frequency of fluctuation affects the extent of adaptation. Predictability of fluctuations has no effect while frequency of fluctuations has only a minor effect on adaptation.Taken together, these results show that increased supply of variation is largely beneficial, despite most mutations having deleterious fitness effects. In addition, it demonstrates the role of complex interactions with internal or external environments of organisms in determining the tempo and extent of adaptation. Fluctuating environments, on the other hand, favor as well as promote the maintenance of variation while factors such as predictability and frequency of fluctuations had little to no effect on adaptation.