Adaptive Evolution under Favorable and Unfavorable Population Genetic Conditions in Mitochondrial-deficient Caenorhabditis Elegans Nematodes

Abstract

Mutation is a fundamental process that drives evolutionary change; however, most new mutations are deleterious for organismal fitness and can readily propagate within populations under a broad range of conditions. Mutational processes able to counteract deleterious mutation accumulation include: 1) reversion mutation back to wildtype, 2) acquisition of generally beneficial mutations, and 3) compensatory mutations that specifically mitigate the effects of previously-acquired deleterious mutations through epistasis. The potential for any of these mutation types alters our expectations for the impact of deleterious mutation in populations, but since the fitness effects of individual mutations are rarely characterized, the relative importance of beneficial and compensatory epistatic mutations is unknown. In this thesis, I characterized the nuclear mutations that arose in a previous mutation accumulation (MA) experiment using Caenorhabditis elegans nematodes, in which mutations were allowed to accumulate under extreme drift conditions in replicate, independently evolving lines initiated from a low-fitness mitochondrial electron transport chain (ETC) mutant, gas-1. In contrast to the results of typical MA experiments, gas-1 MA lines improved fitness slightly compared to their ancestor. Here, I find that the gas-1 MA lines demonstrate little increase in amongline variance and that the gas-1 MA nuclear mutations are more narrowly functionally defined than wildtype MA nuclear mutations. When combined with evidence for zygotic or post zygotic selection these data suggest that selection-both purifying and positivecan be an extremely powerful force even in conditions of extreme genetic drift.Furthermore, functional characterization of a four-mutation set isolated from one of the ii gas-1 MA lines on gas-1 and wildtype backgrounds shows fitness improvements on both backgrounds. This beneficial four-mutation set is associated with a decrease in steady-