Mutational processes in Dictyostelium discoideum: How mutations affect social behaviors and fitness

Abstract

Part I. Mutation is the most important biological force as it generates the variation that drives evolution and may play an important role in maintaining social structure in the social amoeba Dictyostelium discoideum. Using mutation accumulation lines of the social amoeba, I estimated the rate and degree of mutational effects on the social ability to form spores in chimeras by mixing equal proportions of cells of the ancestral clone with a mutated line and determining if the resultant spore proportion differs. Through the use of assays measuring growth, migration ability, and rates of spore germination, I assessed the fitness effects of mutation. In agreement with evidence that the majority of mutations are deleterious, I have found that the ability to get into the reproductive spores is diminished following mutation accumulation. Measuring growth rates on the selective medium revealed that approximately half of the lines showing a significant deviation from the ancestor have increased growth rates, possibly indicating the presence of beneficial mutations, while growth in a non-selective medium resulted in a loss of fitness. Additionally, spore germination decreased in lines with an abundance of mutations. Part II. Restriction Enzyme Mediated Integration (REMI) is a method of transformation that generates tagged mutations. We employed the REMI mutants to select for cheaters by competing pools of mutants over many generations, allowing the lines to fruit each time. We plated out high densities of spores in order to facilitate the lines bypassing the vegetative cycle but still allowing the social cycle. This process was repeated 20 times. At the end of this process, the frequency of each line was assessed and each line was sequenced to identify the genes that were affected by REMI mutagenesis. Once we had obligate cheaters, we assessed fitness in a variety of ways: axenic growth and growth on bacteria, rate of spore germination, and distance traveled by migrating slugs. We then looked for a correlation between cheating and fitness. We expect to see a tradeoff between the ability to preferentially produce spores in chimeric mixtures and other aspects of fitness.