UC Merced

Nutritional symbionts are a vital component of many organisms’ survival strategies, supplying essential nutrients, metabolizing substances that are otherwise indigestible, and in some cases, conferring resistance to environmental stressors. Insects have a close and complex association with nutritional symbionts. This dissertation explores critical aspects of leafhopper symbiosis and evolution through a three-part investigation into the Hawaiian leafhopper radiation (Nesophrosyne) and the aster leafhopper (Macrosteles quadrilineatus). In Chapter 2, I focused on the symbiont genome evolution in two of the smallest known bacterial genomes, Sulcia and Nasuia, in a Hawaiian leafhopper radiation (<5 million years). The work found that multiple evolutionary processes are continually shaping this multi-partner symbiosis in a relatively short time frame. In Chapter 3, I sequenced the aster leafhopper genome, a polyphagous pest with a symbiotic association with Sulcia and Nasuia. Using the chromosomal-level sequencing of this leafhopper, along with three other leafhopper genomes with different ecologies and symbiotic associations, I explored how pest species biology and symbiotic associations shape leafhopper genomes. I found that genes related to insecticide resistance and adaptation to environmental pressures are conserved in leafhoppers, with some undergoing positive selection. I also found that despite the difference in symbiotic associations between all leafhoppers, genes involved in symbiont maintenance and support are evolutionary conserved. In Chapter 4, I investigated the potential role of DNA methylation in the differential gene expression in two symbiont tissues of the aster leafhopper. I leverage fluorescence-based cell sorting methods to isolate symbiont-containing cells and compare their methylation properties with non- symbiotic host cell types. I showed that DNA methylation is not the primary driver of gene regulation in bacteriocytes, but may play a role in some symbiosis- related genes. Collectively, this dissertation extends our understanding of leafhopper-symbiont interactions, pest adaptation mechanisms, and the role of epigenetics in symbiotic partnerships, thus contributing significant insights to the broader field of insect evolutionary biology and genomics.