Dissertation Research: The Evolution of the Actinorhizal Symbiosis in Cercocarpus H.B.K. (Rosaceae)

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The mutualism (actinorhizal symbiosis) between the genus Cercocarpus, a shrub or tree in the rose family (Rosaceae) found in western North America, and Frankia, a nitrogen-fixing filamentous bacterium, is a system that may have experienced cospeciation. We propose to generate hypothesized evolutionary histories for Cercocarpus using DNA sequence and chloroplast restriction site data, and strains of Frankia infective on Cercocarpus using DNA sequence data. We will also test the host-specificity of these Frankia strains towards Cercocarpus species and other plants that have the ability to interact with Frankia. In addition, we will test whether the evolutionary histories of Cercocarpus and Frankia exhibit a pattern of cospeciation. The phylogeny produced for Cercocarpus will help clarify the confusing taxonomy, species boundaries, and relationships among species in this genus. Our study will be one of the first comprehensive examinations of possible cospeciation in groups involved in actinorhizal symbiosis. No comparably broad surveys of infective strains of Frankia exist for an actinorhizal genus, nor such broad scale identification and description of strains infective on members of the rose family. Information on the strains of Frankia infective on the members of the rose family can be used in comparisons with other known actinorhizal groups and our documentation of the diversity of Frankia may identify strains that can be used in agriculture.