Dissertation Research: Coevolution in the Legume-Rhizobiaceae Symbiosis: A Phylogenetic Test of Codivergence

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9623537 Riley Members of a bacterial family Rhizobiaceae are known for their nitrogen-fixing, symbiotic relationship with a variety of legume host plants. The soil bacteria forms nodules on legume roots, where it fixes atmospheric nitrogen into a form which the plant can use. The goal of this study is to test for coevolution between these two symbiotic partners. Similarity of branching patterns in phylogenies (or, phylogenetic congruence) is strong evidence of a shared evolutionary history between symbionts, and will be used to test for coevolution between the Rhizobeaceae and their legume hosts. Research will determine whether genetic information enabling the bacteria to become a symbiont of the plant, the sym region, has coevolved with genes of the host plant during the process of host plant diversification. Findings will also illustrate the contribution of geographic separation among spatially isolated strains to genetic divergence in rhizobia and show whether nodulation genes have been transferred between different rhizobial chromosomal lineages over the evolutionary history of rhizobia. Understanding the evolution of associations between Rhizobiaceae and legumes is important agriculturally. Legumes include the important crop plants, soybeans, peas, and chickpeas. The nitrogen-fixing symbiosis with rhizobia has also been exploited as a way to replenish soil with fixed nitrogen without using chemical fertilizers. Understanding how rhizobia/host relationships arose, the stability of the association throughout the diversification of legumes, and the types of evolutionary pressures which maintain the symbiosis in nature is critical if crop production and soil fertility are to be improved by manipulating the symbiosis.