The Impact of Maternally Transmitted Microbes on Animal Evolution
Funkhouser-Jones, Lisa Jean
Maternally transmitted microbes that infect host germ cells are perfectly poised to influence animal genome evolution, either directly through horizontal gene transfer or indirectly through selection for host genetic variants that control symbiont proliferation. In this dissertation, both of these scenarios are examined using the obligate, intracellular bacteria Wolbachia as a model for maternally-transmitted bacteria because of its widespread distribution across 40-50% of all arthropod species and its localization to the maternal germ line cells. In the first project, whole-genome sequencing of uninfected Chorthippus parallelus grasshoppers revealed that large regions of DNA from two different supergroups of Wolbachia had horizontally transferred into the grasshopper genome. While horizontal gene transfer of Wolbachia DNA to a eukaryotic host is common given Wolbachia?s proximity to the germ line genome, this is the first known example of two divergent Wolbachia strains contributing DNA to the same host genome. Furthermore, Wolbachia inserts were present in almost all of the grasshopper chromosomes and often differed between closely-related C. parallelus subspecies, indicating that horizontal gene transfer from Wolbachia is an unusually dynamic process in grasshoppers. In the second project, a forward genetic screen was conducted to find host genomic regions responsible for regulating an 80-fold difference in Wolbachia titers between two closely-related species of Nasonia parasitoid wasps (N. vitripennis and N. giraulti). Quantitative trait loci analyses and hybrid introgressions identified two genomic regions, one each on chromosomes 2 and 3, that act additively through a maternal effect to suppress Wolbachia titers in N. vitripennis. Thirty-three significantly differentially expressed genes are present in these regions, several of which function in pathways important for host control of intracellular bacteria including immunity, autophagy, and cell-to-cell trafficking. Additionally, staining of Nasonia ovaries with a nucleic acid dye revealed that N. vitripennis may keep Wolbachia out of developing oocytes by sequestering them in the neighboring nurse cells. Candidate genes that are overexpressed in N. vitripennis, such as trichohyalin, or those involved in Wolbachia trafficking, such as kinesin, are currently being evaluated using RNAi for their role in host regulation of Wolbachia titers and transmission.