Date of Award
Doctor of Philosophy (PhD)
Microbes exist against a backdrop of other organisms, and the interactions between microbes have major consequences on their traits, their evolution, and their impact on the world. Microbial interactions and the adaptations that enable them are extremely diverse – they can unlock abilities beyond the reach of individual cells or lead to a population’s destruction, they can be temporary or permanent, they can be between genetically identical cells or different species entirely. The first chapter of this dissertation reviews microbial interactions and the related concept of the evolution of conflict and cooperation.To be certain a trait is an adaptation at all, a researcher must demonstrate both that the trait benefits an organism’s fitness but also how it benefits fitness – that is, what the selective pressures are, if any, that resulted in the trait’s evolution and maintenance. Microbes’ small size and the resulting difficulty of observing microbial interactions directly makes this an especial challenge for microbes and behooves the use of creative approaches. This dissertation focuses upon the social amoeba Dictyostelium discoideum, which engages in interesting interactions both within its own species and with other microbial species. To infer the existence of adaptations in D. discoideum and its partners to these interactions, I employed an experimental evolution approach. Unlike many experimental evolution experiments, which involve applying an artificial selective pressure to a population and observing how it adapts, my studies instead focus on removing existing selective pressures as a way to infer what they were in the first place. I am looking for adaptations by evolving them away. In my second chapter, I apply this concept to look for evidence of D. discoideum adaptations to cheating. D. discoideum undergoes a social life cycle wherein potentially unrelated cells aggregate to produce chimeric multicellular structures that should be especially vulnerable to exploitation by uncooperative cheater genotypes. While this phenomenon is well studied in the laboratory, some disagreement remains about its adaptive role in nature. To what extent is cheating (or resisting being cheated by others) an important selective pressure on D. discoideum in nature? In my study I evolved D. discoideum under conditions in which cheating was impossible and thus any selective pressure to cheat should be relaxed. I found evidence that under these conditions, D. discoideum evolves a reduced ability to cheat (or an increased susceptibility to being cheated). These results are consistent with other studies indicating an adaptive role for cheating in nature, and validate my approach as a way to study other microbial traits with difficult-to-assess adaptive values. In my final chapter, I apply a similar approach to a more complicated interaction that occurs between D. discoideum and three endosymbionts in the genus Paraburkholderia. In nature D. discoideum-Paraburkholderia interactions appear to be common and past studies suggest there are positive and negative fitness consequences for both partners. In an attempt to characterize whether these interactions have a history consistent with an overall cooperative or antagonistic relationship, I experimentally evolved multiple strains of D. discoideum and several bacterial species without access to one another. Without partners, any selective pressures normally exerted by D. discoideum and its endosymbionts upon one another should be relaxed. My results provided evidence of antagonistic adaptations on the part of some symbionts, but few overall or species-level trends. Instead, different strains appeared to respond to experimental evolution differently. These results suggest that D. discoideum and Paraburkholderia’s relationship may differ considerably in character depending on the strains involved, and reinforce the need to design experiments with strain variation in mind when studying microbial interactions.
Chair and Committee
Joan E. Strassmann David Queller
Jonathan B. Losos, Carlos A. Botero, R F. Inglis,
Larsen, Tyler John, "Inferring adaptation in social microbes from experimental evolution under relaxed selection" (2022). Arts & Sciences Electronic Theses and Dissertations. 2801.