Phenotypic and Mutational Consequences of Mitochondrial ETC Genetic Damage

摘要

Genetic mutation is the ultimate source of new phenotypic variation in populations. The importance of mutation cannot be understated, and constitutes a significant evolutionary force. Although single mutations may have little to no impact on organismal performance or fitness, when multiplied across the total number of potential sites within the genome, mutation can have a large impact. Accurate measurement of the rates, molecular mechanisms, and distributions of effects of mutations are critical for many applications of evolutionary theory. Despite the importance of both deleterious and beneficial mutations, their genome-wide patterns and phenotypic consequences are poorly understood when considering the mitochondrial genome. Mitochondria are organelles that are essential for eukaryotic life. They contain their own genome and generate bioenergy (ATP) necessary to sustain life via the electron transport chain (ETC). Because of their role in eukaryotic physiology, understanding how mitochondrial genetic and phenotypic variation can impact populations and evolutionary outcomes is essential. Past studies have implicated DNA-damaging oxidative stress as a source of mutations within somatic tissue, but there is a gap in knowledge regarding its role in heritable damage within the germ line. In this thesis, I aimed to test this possibility by characterizing the phenotypic and mutational consequences of high intracellular ROS levels caused by mitochondrial ETC genetic damage. I performed experiments using Caenorhabditis elegans ETC mutant, gas-1, and mutation-accumulation (MA) lines generated from this ancestral genotype. I quantified organismal fitness (fecundity and longevity), reactive oxygen species (ROS) levels, mitochondrial membrane potential (delta psi m), and ATP levels in these lines, and compared the results to those from a set of wildtype control lines. I begin with a general introduction to the hypothesis and the C. elegans system in Chapter I. In Chapter II, I report the findings from this work. In short, I found that while gas-1 MA lines began the experiment with low lifetime fecundity in comparison to the wildtype strain, their fecundity showed no further decline as expected, and even exhibited higher fecundity levels on days 3-5 of reproduction relative to the gas-1 progenitor. The gas-1 progenitor exhibited higher rates of ROS compared to wildtype, whereas the MA lines reverted back to wildtype levels; a similar pattern was observed for delta psi m, while ATP levels were low in the gas-1 progenitor and remained low in the MA lines. I interpret these findings in light of high-throughput sequencing results from these lines showing that, while nuclear and mitochondrial DNA mutation rates were equal to wildtype in these lines, the genomic pattern of mutation was highly nonrandom and indicative of selection for beneficial or compensatory sequence changes. Because ROS levels declined to wildtype in the evolved (MA), this study was unable to address whether ROS is a major contributor to heritable mutation in this system. I hypothesize that, in addition to their putatively compensatory genetic changes, gas-1 lineages experienced physiological compensation allowing them to survive, and that this was associated with a u22slow livingu22 phenotype. In Chapter III, I summarize general conclusions and implications of this study and end by providing suggestions for further study.