ORCID

https://orcid.org/0000-0002-4710-3993

Date of Award

12-4-2024

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

Organisms vary widely in their diet breadth, ranging from specialists that feed on few resources to generalists that feed on many. Specialists have honed their ability to catch and process specific prey, but generalists must deal with prey that differ in nutritional quality and defensive mechanisms. Have generalist predators overcome costs associated with consuming multiple prey species or do they experience costly trade-offs when handling different prey? Generalist predators also live in environments that vary in the type and abundance of prey species. How do generalists navigate this variation in prey abundance and profitability and seek out their preferred prey? My dissertation provides a new perspective on these questions by addressing the evolutionary challenges and adaptations associated with a generalist diet in a bacterivorous amoeba, Dictyostelium discoideum. In Chapter 2, I explored two understudied costs of diet generalism in these amoebas by varying the pattern of prey abundance of ecologically relevant prey bacteria. First, I found that amoebas experience a reduction in growth rate when they are switched to one species of prey bacteria from another, compared to controls that experience only the second prey. These switching costs typically disappear within a day, indicating adjustment to new prey bacteria. Switching costs also disappear if amoebas undergo spore formation prior to the prey switch. These results suggest that these switching costs are due to physiological trade-offs. Second, amoebas usually perform poorly in multi-prey communities compared to single-species prey communities. There were clear mixing costs in three of the six tested prey mixtures, and none showed significant mixing benefits. These results support the idea that, although amoebas can consume a variety of prey, they must use partially different methods and thus incur costs to handle multiple prey, either sequentially or simultaneously. Amoebas show variation in growth rate on their prey bacteria despite their generalist diet. In Chapter 3, I studied whether amoebas exhibit prey attraction behaviors that helps them distinguish suitable prey bacteria from unpalatable ones. I measured the innate attraction of these amoebas towards different species of prey bacteria and tested if this response was adaptive. I found that the levels of chemoattraction of naïve amoebas towards prey bacteria increased with increasing growth rate of the amoebas on those bacteria. This suggests that D. discoideum amoebas have an innate prey preference that is adaptive. Next, I tested whether experience with prey bacteria modified behavior in these amoebas. Given the number of bacterial species in soil, it might be impossible to possess an innate response that is adaptive on all encountered prey bacteria. Learning from experience should be advantageous. However, I found no evidence for learning. Instead, amoeba preference in learning experiments was partially explained by the innate attractiveness of the prey bacteria. These results suggest that generalist amoebas are innately attracted to the most profitable prey bacteria and this innate attraction cannot be overridden by recent experience. In Chapter 4, I tested the role of antagonistic pleiotropy in constraining diet breadth in these amoebas. I used D. discoideum lines that had been evolved on a monophagous diet of Klebsiella pneumonaie for 60 days, and tested their fitness on K. pneumonaie, and other species of prey bacteria. If antagonistic pleiotropy was important, then fitness improvement of the evolved lines on K. pneumonaie would be associated with a consistent fitness decline on other species of bacteria. I found modest support for fitness improvement on K. pneumonaie under some experimental conditions but saw no significant difference in the fitness of the evolved amoebas on other species of prey bacteria. These results suggest that antagonistic pleiotropy may not be important in constraining performance among the tested species of bacteria. Amoebas may indeed use modular techniques for hunting different prey bacteria, such that performance on each prey species can be independently maximized.

Language

English (en)

Chair and Committee

Joan Strassmann

Committee Members

David Queller; Jonathan Myers; R Fredrik Inglis; Rachel Penczykowski

Available for download on Tuesday, January 13, 2026

Included in

Biology Commons

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