Evolution of energy storage traits in symbiotic rhizobia
2018-01
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Evolution of energy storage traits in symbiotic rhizobia
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2018-01
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The mutually-beneficial symbiosis between legume plants and nitrogen-fixing rhizobia involves an inherent conflict-of-interest over how rhizobia allocate the resources they receive from the host plant. In theory, rhizobia could enhance their future fitness by diverting resources from nitrogen-fixation into storage compounds like polyhydroxybutyrate (PHB). Although the conflict-of-interest between PHB accumulation and nitrogen-fixation has been discussed as a driving factor in the evolution of legume-rhizobia interactions, its role in natural populations is unclear. Therefore, this dissertation fills in key empirical gaps between what we know about the functional role of PHB and hypotheses about how natural selection might act on continuous variation in the amount of PHB that rhizobia acquire during symbiosis. The first chapter assesses the extent of heritable phenotypic variation within natural rhizobia populations interacting with soybean (Glycine max) and partridge pea (Chamaecrista fasciculata) and evaluates implications for fitness in the free-living stage after rhizobia are released from nodules. The results from my first chapter show that 1) natural populations of rhizobia contain heritable, quantitative variation in the amount of PHB they accumulate during symbiosis (a prerequisite for evolution by natural selection) and, 2) natural selection on PHB accumulation may be mediated by how rhizobia allocate PHB over time and among life functions, which could vary independently from traits underlying the amount of PHB acquired during symbiosis. The second chapter assesses phenotypic response to selection based on resident rhizobia populations from long-term agricultural plots varying in host (soybean) or non-host (maize) frequency over years. The mean PHB per cell (measured in nodules) was two times higher in rhizobia populations from plots with 5 or 30 years of continuous maize than from plots where soybean was grown in the previous year. An apparent decrease in mean PHB per cell after the first year of soybean following five years of maize supports the hypothesis that low-PHB rhizobia have higher reproduction in nodules, perhaps due to host sanctions against rhizobia that divert more resources to PHB. A model used to interpret the results suggests that, 1) PHB acquired during symbiosis may contribute to fitness variation for several years after the last host crop, and 2) host sanctions against less-beneficial rhizobia may be stronger in the first soybean crop due to a combination of lower initial rhizobia population size and negative frequency-dependent selection during symbiosis. Collectively, these findings provide empirical support for previously unsubstantiated hypotheses about how conflicts-of-interest over resource allocation contribute to the evolution of the legume-rhizobia mutualism and develop a more nuanced framework for future research.
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University of Minnesota Ph.D. dissertation. January 2019. Major: Plant and Microbial Biology. Advisor: Ford Denison. 1 computer file (PDF); vii, 115 pages.
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Muller, Katherine. (2018). Evolution of energy storage traits in symbiotic rhizobia. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/259777.
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