Theses and Dissertations

Issuing Body

Mississippi State University


Ervin, Gary N.

Committee Member

Wallace, Lisa

Committee Member

Welch, Mark E.

Committee Member

Jordan, Heather

Date of Degree


Document Type

Dissertation - Open Access


Biological Sciences

Degree Name

Doctor of Philosophy


College of Arts and Sciences


Department of Biological Sciences


Local factors have the potential to generate genetic structure within species if populations respond differently to varying environmental conditions across their geographic range. In this project, spatial genetic structure was examined in the legume, Chamaecrista fasciculata, its symbiotic nitrogen fixing bacteria, and rhizosphere microbiomes. In the first chapter, the aim was to test for genetic structure among populations of C. fasciculata in the Southeast and to evaluate whether phenotypic variation in leaf pubescence is associated with genetic divergence among populations, which would be consistent with local adaptation. My results did not detect a significant association between genetic structure and phenotypic variation in leaf pubescence, but the role of environmental variables in generating the observed patterns of spatial genetic variation in C. fasciculata was demonstrated. In the second project, I analyzed genetic structure within a single population to test for the presence of fine-scale genetic structure of the host plant and to determine if genetic structure of symbiotic nitrogenixing rhizobia is influenced by host plants. Neighboring plants are expected to be more genetically similar than distant plants. If this expectation is supported and genotype x genotype interactions are important in this system, then I anticipated that spatially close host plants would show more genetically similar microbiota in their nodules and rhizospheric soil than distant host plants. The results indicated fine-scale genetic structure for both host plants and nodulating rhizobia, suggesting that the both organisms are influenced by similar mechanisms structuring genetic diversity or shared habitat preferences. In the third project, I characterized fine-scale structure and diversity of microbial communities of the rhizosphere of host plants within a single population. The results revealed significant differences in bacterial and fungal communities among host plants and a significant association between genetic distance of both microbial communities and spatial and plant genetic distances. These data confirm the importance of plant genotype and physical distance in shaping the genetic structure and diversity of bacterial and fungal communities of the rhizosphere. Overall, this study enhances our understanding of the degree to which intraspecific genetic variation in plants influences the diversity and structure of soil microbial communities.