Date of Award
Doctor of Philosophy (PhD)
Successfully investigating the evolution and maintenance of sex and mating systems can often have as much to do with choosing the right study system as it has to do with asking the right questions. Dictyostelium discoideum has long been the focus of researchers interested in understanding a number of biological processes, such as motility, chemotaxis and development. More recently, attentions have shifted to include questions about the evolution of social and sexual interactions both within and between species. The D. discoideum life cycles, both asexual and sexual, are uniquely social, each requiring a costly sacrificial act. This offers an ideal system for exploring questions about kin recognition, conflict, and the evolution of multicellularity, as well as the evolution of differential sexual investment and mating types. This dissertation focused on understanding the phylogenetic and geographical relationships between clones in D. discoideum and identifying the social and selective pressures that shape its mating system. I introduce this mating system in Chapter 1. In Chapter 2, I investigated genetic variation and population structure in D. discoideum to identify possible factors that could affect interactions between clones. I used DNA sequence data and phylogenetic techniques to show that though D. discoideum clones form a monophyletic group, there is evidence of genetic differentiation among locations (FST = 0.242, P = 0.011), suggesting geographic or other barriers limit gene flow between populations. In chapter 3, I again looked for population structure, this time concentrating on gamete size and sex ratio, to understand selective pressures maintaining multiple mating types in D. discoideum. Evidence suggests that both balancing selection and drift are likely acting on the D. discoideum mating system. I found no differences in gamete size across the three mating types and also no genetic differentiation across three wild populations at the mating type locus. However, I found that mating type frequency varied across these populations, likely due to drift. Chapter 4 focused on understanding the social dynamics of mating in D. discoideum. During macrocyst formation, two cells of complementary mating types fuse to form a zygote. This zygote then consumes hundreds of surrounding amoebae, likely clones of the original two cells, for use as protection and food. I varied the frequencies at which two clones of differing mating types interacted to investigate the possibility that one mating type cheats another by differentially contributing to the cannibalized cells. Contrary to previous claims that mating type I induces mating type II, coercing it to contribute disproportionately more of these cannibalized cells during macrocyst production, I found that these cells are likely contributed relative to their frequency in the population, regardless of mating type. However, I did find evidence for differential contribution to macrocyst production between some pairs of clones, suggesting that cheating can happen between partners during sex, but is rare and clone-specific. Overall, these studies looked for evidence of underlying population structure in D. discoideum that could impact our understanding of social and sexual interactions in this species. I also applied questions about the maintenance of sex usually only asked in two-sex systems to the unique social sexual interactions within D. discoideum in order to expand the understanding of how mating systems evolve and are maintained in nature. I developed and used new tools and techniques for observing the processes important to understanding this unique system and identified genetic and social factors that could impact how individuals interact during both the asexual and sexual life cycles.
Chair and Committee
Joan Strassmann, David Queller
Scott Mangan, Ken Olsen, James Umen
Douglas, Tracy Edwards, "Population Structure and Mating Dynamics in the Social Amoeba Dictyostelium discoideum" (2016). Arts & Sciences Electronic Theses and Dissertations. 843.