Dissertation: Burkholderia as bacterial symbionts of Lagriinae beetles Laura Victoria Flórez Friedrich-Schiller-Universität Jena, Nov. 2016 Symbiosis is ubiquitous in nature and can play a crucial role in shaping the biology of both eukaryotes and prokaryotes. Importantly, the interaction of microorganisms with eukaryotes can range from pathogenicity to mutualism, also shifting along this continuum. The ecological settings facilitating such lifestyle transitions are, however, poorly understood. This dissertation focuses on the symbiosis between Lagriinae beetles and Burkholderia gladioli, a bacterium mostly known for its plant pathogenic traits. In Lagria hirta and Lagria villosa beetles, I localized these bacteria on adults, larvae and eggs of both species confirming a vertical transmission route. The presence of B. gladioli in these and four other Lagriinae species suggested that the association is relatively ancient and evolved within this phytopathogenic bacterial group. Additionally, B. gladioli from L. villosa can successfully infect soybean plants, a food source for this beetle species, and negatively affect the plant’s reproductive output, implying that the insect symbiont conserves the ability to intimately interact with a plant. Presumably, the potential of plant pathogenic B. gladioli bacteria to produce potent bioactive substances was also essential for establishing a mutualism with the insect. In L. villosa beetles, I could show that B. gladioli on the surface of eggs inhibit the growth of antagonistic fungi. I thereby demonstrate a symbiont-mediated defense, which could be highly advantageous at the nutrient-rich and immobile egg stage. Furthermore, we elucidated four compounds (toxoflavin, caryoynencin, lagriene and lagriamide) that could be responsible for the protective effect by the symbionts. Finally, multiple symbiotic B. gladioli strains were found coexisting in individual beetles, bringing about interesting questions regarding the potential advantages of strain diversity in defensive symbiosis and the evolutionary dynamics supporting their long-term maintenance.