Establishing a novel beneficial association using a nematode-bacterium model through experimental evolution Restricted; Files Only
Hoang, Kim (Fall 2019)
Published
Abstract
Microbes confer many benefits to their hosts, from nutrient provisioning to protection from enemies. These microbial symbioses are ubiquitous in nature, many of which evolved millions of years ago. However, it is unclear how these associations arise. My dissertation explores the roles of environmental and evolutionary contexts in the evolution of novel beneficial associations. I use the nematode Caenorhabditis elegans and the bacterium Bacillus subtilis as a model system to investigate these questions by experimentally evolving them under different conditions. Before experimental evolution, I find that B. subtilis protects C. elegans from heat shock by increasing host fecundity compared to the C. elegans standard diet, E. coli. To examine how B. subtilis influence C. elegans adaptation, I evolve nematodes in the presence or absence of ancestral B. subtilis with or without heat stress. Hosts evolving with ancestral B. subtilis under stress evolve the greatest fitness gain compared to in the absence of heat stress or in the absence of B. subtilis. These results show that association with novel beneficial microbes can help hosts adapt to stress under the right context. To determine how microbial evolutionary history affects host fitness, I passage B. subtilis through ancestral nematodes, in the absence of nematodes, or through co-passaged nematodes. I find that B. subtilis passaged through ancestral hosts evolve to benefit these hosts, but host adaptation is not necessary since bacteria passaged in the absence of hosts also increase host fitness. Finally, because co-passaged bacteria do not improve ancestral host fitness to the same level as bacteria passaged through ancestral hosts, I further investigate co-passaging by examining the fitness of co-passaged hosts. Co-passaged bacteria reduce fitness of their co-passaged hosts but not hosts that evolved with ancestral B. subtilis, suggesting that coevolution may impede the establishment of novel symbioses. By leveraging experimental evolution and a tractable model system, this work illuminates how environmental and evolutionary contexts shape the evolution of host-microbe associations.
Table of Contents
I. INTRODUCTION
I.1. THE ORIGIN OF BENEFICIAL SYMBIOSES
I.2. THE ROLE OF EVOLUTIONARY INTERACTIONS IN BENEFICIAL HOST-MICROBE INTERACTIONS
I.3. CAENORHABDITIS ELEGANS AND BACILLUS SUBTILIS AS A MODEL SYSTEM TO STUDY BENEFICIAL ASSOCIATIONS THROUGH EXPERIMENTAL EVOLUTION
I.4. SUMMARY OF DISSERTATION CHAPTERS
II. EXPERIMENTAL EVOLUTION AS AN UNDERUTILIZED TOOL FOR STUDYING BENEFICIAL HOST-MICROBE INTERACTIONS
II.1. INTRODUCTION
II.2. BENEFICIAL HOST-MICROBE INTERACTIONS: CURRENT QUESTIONS AND CHALLENGES
II.3. AN OVERVIEW OF EXPERIMENTAL EVOLUTION
II.4. UTILIZING EXPERIMENTAL EVOLUTION TO STUDY BENEFICIAL ANIMAL-MICROBE ASSOCIATIONS
III. THE EFFECTS OF BACILLUS SUBTILIS ON CAENORHABDITIS ELEGANS FITNESS AFTER HEAT STRESS
III.1. INTRODUCTION
III.2. METHODS
III.3. RESULTS
III.4. DISCUSSION
IV. ASSOCIATION WITH A NOVEL BENEFICIAL MICROBE FACILITATES HOST ADAPTATION TO A STRESSFUL ENVIRONMENT
IV.1. INTRODUCTION
IV.2. METHODS
IV.3. RESULTS
IV.4. DISCUSSION
IV.5. SUPPLEMENTAL INFORMATION
V. THE EFFECTS OF MICROBIAL EVOLUTION AND COEVOLUTION ON HOST FITNESS
V.1. INTRODUCTION
V.2. METHODS
V.3. RESULTS
V.4. DISCUSSION
VI. CONCLUSION
VI.1. SUMMARY AND DISCUSSION OF PREVIOUS CHAPTERS
VI.2. FUTURE DIRECTIONS
REFERENCES
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