Effects of population size on fitness effects of mutations and long-term fitness trajectories in bacterial populations

Huansheng Cao, Fordham University

Abstract

My study examined the influence of effective population size (N e) on the fitness effects of mutations and long-term evolutionary trajectories in bacterial populations. I first studied the fitness of newly evolved intracellular bacteria in experimental evolution, which would either decline as predicted by Muller's Ratchet or increase as proposed in Wright's shifting balance theory. Using Escherichia coli evolved at high (10 9) and low (Ne) (2×102) in rich and minimal media for 4,000 generations, I found that fitness in experimental intracellular bacterial populations (rich medium/low Ne) remained unchanged, compared with substantial fitness gains in large populations. Thus intracellular bacteria do not appear to adapt in this new niche. My second chapter compared adaptive evolution between large and intermediate bacterial populations by testing two hypotheses: (1) intermediate populations (2×10 4) would reach higher fitness than large populations (109) over long-term (4,000 generations in my study) evolution, and, (2) orders of change in growth curve in intermediate populations would be different than those observed in large populations (an increase in growth rate followed by a reduction in lag phase duration). Using E. coli populations evolved at high and intermediate Ne in rich and minimal media for 4,000 generations, I found fitness gains in intermediate populations did not exceed those in large populations, but still were about 30% and 42% of them in minimal and rich media in 4,000 generations. The orders of change in growth curve in the first 500 generations between high and intermediate Nes were different in minimal but similar in rich medium, which were complicated by their genotypes. My third chapter examined the Lac- mutant that had an IS5 insertion in lacY, which founded half of the populations used in Chapters 1 and 2. Through mutagenesis and whole genome sequencing, I identified the mutation to be a single substitution in crp. Subsequent gene expression profiling showed 170 and 157 genes in Crp- were differentially expressed at high and intermediate Ne in minimal medium, which were involved in nutrient metabolism, stress response, motility, transcriptional regulation, and other functions. It indicates a suite of genes working together caused the fitness differences. ^

Subject Area

Biology, Genetics|Biology, Microbiology|Biology, Evolution and Development

Recommended Citation

Cao, Huansheng, "Effects of population size on fitness effects of mutations and long-term fitness trajectories in bacterial populations" (2012). ETD Collection for Fordham University. AAI3560061.
http://fordham.bepress.com/dissertations/AAI3560061

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