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dc.contributor.authorGallie, Jenna
dc.date.accessioned2010-03-29T21:45:33Z
dc.date.availableNO_RESTRICTIONen_US
dc.date.available2010-03-29T21:45:33Z
dc.date.issued2010
dc.identifier.urihttp://hdl.handle.net/10179/1215
dc.description.abstractSurvival in the face of unpredictable environments is a challenge faced by all organisms. One solution is the evolution of mechanisms that cause stochastic switching between phenotypic states. Despite the wide range of switching strategies found in nature, their evolutionary origins and adaptive significance remain poorly understood. Recently in the Rainey laboratory, a long-term evolution experiment performed with populations of the bacterium Pseudomonas fluorescens SBW25 saw the de novo evolution of a phenotypic switching strategy. This provided an unprecedented opportunity to gain insight into the evolution and maintenance of switching strategies. The derived ‘switcher’ genotype was detected through colony level phenotypic dimorphism. Further microscopic examination revealed the cellular basis of phenotypic switching as the bistable (ON/OFF) expression of a capsule. Transposon mutagenesis demonstrated that the structural basis of the capsule was a colanic acid-like polymer encoded by the Pflu3656-wzb locus. Subsequently, whole genome re-sequencing enabled elucidation of the series of mutational events underlying the evolution of capsule bistability: nine mutations were identified in the switcher. Present in both forms of the switcher, the final mutation – a point mutation in a central metabolic pathway – was shown to be the sole mechanistic cause of capsule switching; it ‘set the stage’ for a series of molecular events directly responsible for bistability. Two models were proposed to explain capsule switching at the molecular level: the genetic amplification-reduction model, and the epigenetic feedback model. Collective results of biochemical and genetic assays proved consistent with the epigenetic model, whereby a decrease in flux through the pyrimidine biosynthetic pathway activates an already-present feedback loop. Subsequent analysis of a second switcher (evolved independently of and in parallel with the first) revealed a radically different genetic route leading to phenotypically and mechanistically similar capsule switching. In addition to providing the first empirical insight into the evolutionary bases of switching strategies, the work presented in this thesis demonstrates the power of natural selection – operating on even the simplest of organisms – to forge adaptive solutions to evolutionary challenges; in a single evolutionary step, selection took advantage of inherent intracellular stochasticity to generate an extraordinarily flexible phenotype.en_US
dc.language.isoenen_US
dc.publisherMassey Universityen_US
dc.rightsThe Authoren_US
dc.subjectEvolutionary strategiesen_US
dc.subjectPhenotypesen_US
dc.subject.otherFields of Research::270000 Biological Sciences::270200 Genetics::270208 Molecular evolutionen_US
dc.titleEvolutionary and molecular origins of a phenotypic switch in Pseudomonas fluorescens SBW25 : a thesis submitted in partial fulfilment of the requirements for the degree of Ph.D. in Evolutionary Genetics at Massey University, Auckland, New Zealanden_US
dc.typeThesisen_US
thesis.degree.disciplineEvolutionary Geneticsen_US
thesis.degree.grantorMassey Universityen_US
thesis.degree.levelDoctoralen_US
thesis.degree.levelDoctoralen
thesis.degree.nameDoctor of Philosphy (Ph.D.)en_US


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