| Social behaviors, including cooperation and altruism, are characteristic of numerous species. Kin selection theory provides a theoretical framework for studying the evolutionary origins and maintenance of social behaviors, but many aspects of its ecology and genetics remain unclear. The social amoeba Dictyostelium discoideum is a model system for the study of social evolution and provides insights into the nature and genetic basis of social cooperation. This species exhibits altruism during both asexual and sexual cycles of its life history, and recent studies have uncovered several possible genetic mechanisms associated with kin discrimination and cheating behavior during the asexual social cycle. By contrast, the molecular and evolutionary mechanisms that underlie sexual macrocyst formation remain largely enigmatic. In this dissertation, a study uses a molecular population genetics approach to understand the relationship between genetic dissimilarity and social cooperation in this species. Based on re-sequencing data from 137 gene fragments of 24 natural North American strains, surprisingly low levels of nucleotide variation and no strong population stratification is observed in this species. In addition, no clear relationship between nucleotide divergence and levels of social dominance and kin discrimination is observed. Kin discrimination experiments, however, show that sympatric strains show greater ability to distinguish self from non-self than do allopatric strains. Finally, a rapid decay of linkage disequilibrium is documented. The SNP data indicate that recombination is widespread within D. discoideum and that sex is likely to be an important aspect of the life cycle. To further characterize the genetics of kin discrimination, genome-wide gene expression during the development of D. discoideum is examined to identify differentially expressed genes during clonal vs. chimeric development. Four strains isolated from North Carolina, which have been shown to form a dominance hierarchy when co-developing in chimeras, are used. The transcriptomic profiles of D. discoideum cells in clone vs. different chimeras at five different developmental stages are analyzed using a customized microarray. Developmentally regulated genes identified using this approach are consistent with previous studies. Moreover, the effects of chimerism on global transcriptional variation, and developmental stage-dependent gene expression patterns associated with social interactions are also observed. |
