Evolutionary conflict in chimeras of the social amoeba, Dictyostelium discoideum
Strassmann, Joan E.; Queller, David C.
Doctor of Philosophy
Molecular biologists have extensively studied Dictyostelium discoideum and consider it a model organism for the study of cellular differentiation but paradoxically many aspects of the natural life and evolution of this organism are unknown. My study demonstrates there is high clonal diversity in nature and that different clones coexist in the same small soil sample. Thus, amoebae with different genotypes can join in the same multicellular organism making a genetic chimera in nature. I also show that a linear dominance hierarchy describes the interactions of genetically distinct clones in competition for survival in the multicellular stage. In fact, during this stage the amoebae differentiate into different types of cells allowing the pseudoplasmodium to form a fruiting body consisting of two principle cell types: the spore and the stalk cells. The formation of the fruiting body probably guarantees a better survival and dispersion of the spore but requires the death of 20% of the cells that initially comprise the pseudoplasmodium that differentiate into the stalk and the basal disc. Contributions of two clones in a chimera to spore and stalk are often unequal, with one clone taking advantage of the other's stalk contribution. To assess whether there was a hierarchy of exploitation among clones, I competed all possible pairs among seven clones. I found a clear linear hierarchy with one clone being most dominant, and the bottom clone losing in competition to all the others. The hierarchy at an earlier stage, evaluated with prespore and prestalk cells in the slug, was not as clear. These results suggest that there is a single principal mechanism for differential contribution to the spore and that it involves more than spore/stalk competition. My results legitimate the use of chimeric D. discoideum as a model organism for the investigation of issues relating to coexistence and conflict between cells during organismal development and thus as a simple model for the relationship between social organisms.
Molecular biology; Ecology; Microbiology