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Small RNAs in Small Genomes: The Characterization of Expressed Small RNAs in Obligate Bacterial Symbionts of Hemipterans


Within hemipterans, the acquisition of bacterial symbionts has helped insects utilize the nutrient-limited resource of plant sap. Many of these bacterial symbionts have undergone a dramatic reduction in genome size, often losing key regulatory genes. It is therefore unclear how and if these symbionts regulate their gene expression. My dissertation explores the potential role that symbiont expressed small RNAs (sRNAs) have in gene expression. First, I use RNA-seq to characterize the sRNA expression profile of Buchnera, during two different life-stages (aphid ovarioles and maternal bacteriocytes) in which Buchnera has differential protein expression. The results from this experiement show that Buchnera sRNAs are differentially expressed between life-stages. My dissertation also provides in vitro evidence of the functionally of the Buchnera antisense sRNA carB. These results suggest that when Buchnera is in an extracellular state, free of the bacteriocytes, it can respond to changes in host nutritional demand. I then characterized Buchnera’s sRNA expression when its aphid host fed on two different host-plants which have been shown to have different nutritional and plant defense profiles. The results from this experiment show that Buchnera sRNA expression varies with aphid host-plant diet. These results suggest that Buchnera sRNAs can potentially impact the symbiosis in an adaptive nutritional manner, or stress response manner when aphids feed on a host-plant that is lower in nutrients. I also determined that sRNAs are expressed and conserved in one of the most reduced obligate insect symbionts, Candidatus Carsonella ruddii. Currently, many of the functional genomic tools that are optimized to work in a handful of model systems. As result, when working with non-model, unculturable systems, there is an additional challenge of optimizing and modifying current functional genomic tools. The last part of my dissertation tests the efficacy of novel RNAi delivery systems within three aphid species. This RNAi delivery system has resulted in successful gene knockdown in the soybean aphid. Overall, the findings of my dissertation strongly support the hypothesis that small bacterial genomes may utilize sRNAs to help regulate their own gene expression to help compensate for the loss of canonical regulatory proteins.

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