"The relationship between legumes and rhizobia is an important model mutualism for several reasons. It is essential to agriculture, it provides a crucial ecosystem service by fixing atmospheric nitrogen, and it provides a tractable system for examining questions about mutualism, evolution, and ecology [1, 2]. This symbiosis remains generally mutualistic despite the fact that models predict that mutualistic rhizobia will eventually be out-competed and overwhelmed by parasitic rhizobial symbionts known as cheaters. Here I investigate one possible mechanism that could maintain the diversity of rhizobia strains: frequency-dependent selection. I also introduce a novel method of conducting rhizobia competition experiments that uses non-destructive macroscopic fluorescent imaging to identify rhizobial symbionts in root nodules."--Page ii.
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