Cooperation, conflict, and experimental evolution in social amoebae
Kuzdzal, Jennie Jo Helene
Strassmann, Joan E.
Doctor of Philosophy
Cooperation and cheater control have helped shape life as we know it, but there is still much to learn. A eukaryote microbial model organism, like Dictyostelium discoideum , is an excellent system for advancing our understanding. When faced with starvation, multiple genetically distinct clones of D. discoideum aggregate together to form a chimeric fruiting body with a sterile stalk that holds aloft a sorus of hardy reproductive spores. One clone may be able to cheat and form disproportionately more spores, while forcing others to form more stalk. Here we discuss the impact of genetic relatedness on cooperation, and how social actions are temporally organized and can be affected by environmental conditions. First, we documented a potential strategy for facultative cheating within chimeras. We showed that the first cells to starve, and initiate the social stage, cheat cells that starved later. In another paper, we reviewed recent studies of social microbes, which demonstrate the importance of high relatedness in the evolution of cooperation and cheater resistance. In an experimental evolution study, we tested the hypothesis that de novo cheater mutants readily evolve under low relatedness conditions. We found that the majority of our lines evolved to cheat their ancestor. Further, we studied obligate cheaters, which pose a great threat to sociality. They gain a reproductive advantage in chimeras, but cannot cooperate clonally to form fruiting bodies. Wild obligate D. discoideum cheaters have never been documented, but we found that obligate cheaters readily evolved under low relatedness conditions in the laboratory. In another study, we looked at the effects of light level on spore production in D. discoideum and Dictyostelium citrinum . Overall, more spores were produced in the light than in the dark, probably because of reduced movement and cell loss during the motile multicellular slug stage. We found that these effects were species, clone, and environment dependent. Taken together, this work helps us understand how cooperation thrives in nature, despite the threat of cheaters.
Biological sciences; Cooperation; Social evolution; Microbiology; Evolution & development