Date of Award

7-4-2023

Author's School

Graduate School of Arts and Sciences

Author's Department

Biology & Biomedical Sciences (Evolution, Ecology & Population Biology)

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

Social interactions are widespread in nature and can have important ecological and evolutionary consequences. Interactions are often categorized by who is benefited or harmed by the symbiosis. For example, if both sides benefit, the interaction is cooperative. If one side benefits at the expense of the other, it is an antagonism. The kind of interaction can have important evolutionary implications, but interactions rarely fit neatly into these interaction categories. Instead, interactions often involve elements of cooperation and conflict. My dissertation explores the consequences of interactions and how they can involve both cooperation and conflict using a range of experimental, genomic, and theoretical methods. Experimental work in this dissertation involves the symbiosis between Dictyostelium discoideum amoebae and Paraburkholderia bacteria. Previous work on this symbiosis showed that host D. discoideum can benefit or be harmed by symbiosis with Paraburkholderia depending on whether edible bacteria are present, but the effect of this context on the inedible Paraburkholderia symbionts is unknown. In my first chapter, I show that two species of Paraburkholderia symbionts are also affected by the presence of bacteria that are edible for hosts. Paraburkholderia grow to higher densities when food bacteria are scarce. Moreover, on the host side, I use simulations to show that symbiosis may be an adaptation for living in harsh soils with variation in the number of edible bacteria. I follow up on this idea in my second chapter by using host-symbiont co-occurrence data to test whether the prevalence of symbiosis is associated with variable soil conditions. I found that the prevalence of two Paraburkholderia species is associated with variable rainfall. In my third chapter, I investigate how food bacteria are carried or left behind during the symbiosis between D. discoideum and Paraburkholderia. This chapter shows that Paraburkholderia causes hosts to leave food bacteria uneaten, but that hosts gain the ability to carry food bacteria more often in food-poor environments. In my fourth chapter, I turn to interactions between members of the same species and test the hypothesis that pleiotropy can stabilize cooperation. Using genomic data from Pseudomonas aeruginosa, I found that quorum sensing genes involved in cooperation were more pleiotropic than genes involved in private functions. These results with P. aeruginosa support a role for pleiotropy in stabilizing cooperation. In my last chapter, I use theoretical models to understand how evolutionary conflicts affect long-term evolution. I show that conflict results in arms race dynamics that affect fitness and the kinds of mutations that become fixed in populations. The results from this dissertation advance our knowledge of how context affects symbiotic interactions, the stability of cooperation, and the long-term consequences of conflict.

Language

English (en)

Chair and Committee

David Queller

Committee Members

Joan Strassmann

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