The evolution of coopeation: insights from experimental populations of Pseudomonas fluorescens

Abstract

The field of experimental evolution is burgeoning under the power of microbial systems. Our ability to manipulate experimental design for use with microbes is only limited by our imagination. This thesis is a study that uses Pseudomonas fluorescens, a soil dwelling bacterium, as an experimental tool for understanding evolutionary processes. The evolution of cooperation has been a thorny issue for many years, because it initially seems to contradict the intrinsically selfish concepts established in Darwin’s theory of evolution by natural selection. Advances in microbiology and the ability to test important evolutionary theories using microbes, provides an exciting opportunity for those working in the field of experimental evolution. This thesis uses P. fluorescens to investigate four aspects of the evolution of cooperative behaviour organised into four results chapters (Chapters 2-5). The first describes the genotypic and phenotypic diversity of 26 independently derived ‘wrinkly spreader’ genotypes in order to analyse the genetic and phenotypic variation among morphotypes. Mutations were identified in 25 of the 26 wrinkly spreaders including a new locus mws and three new genes of known loci wspE, awsR and awsO. This new genetic information provided additional insight into the molecular causes of the wrinkly spreader phenotype. Multivariate analysis of the phenotypic traits revealed that wspF mutants were phenotypically distinct from other morphotypes at a level below the ecological niche. The second chapter extended existing studies on the evolution of wrinkly spreader genotypes within the well-known Haystack model for evolution in group-structured populations, by studying the population dynamics of cooperative genotypes with and without group structure, in a multi-level selection one framework. It was shown that the time spent in a haystack affects the fitness of cooperators, because the longer group-generation treatment conformed to the predictions of the Haystack model, while the shorter group-generation treatment did not. The third chapter was an investigation into how the fitness of the emergent group-level phenotype formed by cooperating wrinkly spreader cells was dependent on the density of wrinkly spreader cells. Contrary to prediction, no density dependence was observed when calculated in a multi-level selection one framework, but rather it was determined that the emergent fitness was dependent on time, implicating a role for a development-like process. The final results chapter of this thesis incorporated the hypothesised role for a development-like process into a novel theoretical model for the evolution of multicellularity in which fitness would be determined in a multi-level selection two framework. Novel apparatus and experimental design were developed to determine if it were possible to observe a response to a selective regime that selected simultaneously at the level of the individual cell and the level of the group of cells. A significant response was shown after only six group-generation cycles. In summary, this thesis exploits P. fluorescens as an experimental tool to gain insight into complex ecological and evolutionary phenomena such as cooperation, biofilm formation and the evolution of multicellularity, and provides insight into the molecular causes of the cooperation among wrinkly spreader genotypes.