Kinship and the evolution of altruism in social amoebae and A model for the evolution of kin-limited interactions
Gilbert, Owen Michael
Strassmann, Joan E.
Doctor of Philosophy
For decades, social amoebae have served as a model supporting broader theories of social behavior. Owing to their peculiar aggregative life cycle, it has seemed reasonable altruism in social amoebae is possible because of adaptive mechanisms of kin discrimination, and kin discrimination evolves to maintain this altruistic behavior. Nonetheless, these hypotheses have not withstood critical tests in social amoebae or other organisms. As a result, general theories of social evolution have rested on a few abstract theoretical assumptions. I here use social amoebae as a model system to examine these assumptions through empirical study. First, I focus on the natural context of social evolution in the social amoeba Dictyostelium discoideum. I establish D. discoideum occurs frequently in a state of clonality during the social stage and that obligate cheaters (non-altruists) are not present in nature (Chapter 1). I then show that kin discrimination in D. discoideum has only a weak effect on genetic relatedness in the social stage (Chapter 2). Upon this finding, I propose a hypothesis that kin recognition evolves in response to facultative rather than obligate cheating (Chapter 2). I generalize this argument in Chapter 3, where I propose a new "selfish genome" model of kin recognition. This model is unique in that it accounts for the effects of genome-wide relatedness between individuals on one another's fitness. This model explains the adaptive basis of kin recognition-a trait thought to be crucial for major evolutionary transitions. I also describe two additional studies of social amoebae. In the first, I report on the finding of a large clonal patch of a social amoebae. This is the first example of such a phenomenon in a microorganisms (Chapter 4). In the second, compare two forms of migration and development in social amoebae (Chapter 5). This study shows social amoebae can be studied in a similar way to animals, with a focus on the multicellular phenotype. I argue the production of stalk during migration is an example of altruistic behavior.
Ecology; Evolution & development