The evolution of virulence in heterogeneous host populations Restricted; Files Only

White, Patricia Signe (Fall 2019)

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Parasitism is a ubiquitous species interaction in which one partner gains a fitness benefit (i.e., increased survival and reproduction) to the detriment of the other by way of resource exploitation. Both partners impose reciprocal selection pressure on one another – hosts adapt to resist parasites, and parasites counteradapt to overcome host barriers – thus resulting in arms- race dynamics. However, host populations are complex, and the effects of factors such as genetic diversity and varying life stage on parasite evolution are largely unknown. Here, I empirically test theoretical predictions about the influence of host heterogeneity on the evolution of virulence (i.e., host mortality). I experimentally evolve populations of the roundworm Caenorhabditis elegans and its parasitic bacteria, Serratia marcescens. Host populations composed of multiple genotypes are expected to slow the rate of parasite adaptation, thereby reducing host mortality. I found that when evolving virulent S. marcescens parasites in populations with or without genetic diversity, host populations with genetic diversity were better able to reduce the effects of infection than in homogeneous host populations. In addition to genetic heterogeneity, the life stage at which a host is infected is predicted to influence infection dynamics. To determine if C. elegans life stages differed in ability to resist S. marcescens, I compared the dispersal life stage of C. elegans (dauer) with non-dauer life stages. I found that dauer is capable of avoiding S. marcescens, effects not found in non-dauer life stages. Further, host population size has been shown to be an essential factor in how quickly hosts can evolve resistance. I found that small populations of C. elegans are overcome by the random effects of genetic drift, and parasite resistance was either lost or unable to evolve at all. Overall, this dissertation provides evidence that many aspects of the host population can influence the outcome of host-parasite interactions. Host heterogeneity, life stage, and population size can all alter the evolutionary trajectories of host-parasite interactions. By understanding the factors that influence the evolution of virulence, we can better manage its effects in human populations, agriculture, and wildlife. 

Table of Contents

Chapter I: Introduction 1

1.1 Evolutionary forces 1

1.2 Virulence evolution: causes and effects 4

1.3 Heterogeneity of hosts 5

1.4 The experimental system: Caenorhabditis elegans and Serratia marcescens 7

Figure 1.1: The life cycle of Caenorhabditis elegans 10

1.5 Summaries of Chapters II, III, IV, V, and VI 11

Chapter II: Genetic drift limits host adaptation to a virulent parasite 12

2.1 Introduction 13

2.2 Methods 15

2.3 Results 18

Figure 2.1 Mortality rates of susceptible populations after 13 generations 20

Figure 2.2 Mortality rates of resistant populations after 13 generations 22

2.4 Discussion 23

2.5 Supplementary materials 25

Chapter III: Host heterogeneity mitigates virulence evolution 28

3.1 Introduction 28

3.2 Methods 31

3.3 Results 33

Figure 3.1 Change in host mortality when infected by evolved vs. ancestral parasite 36

Figure 3.2 Mortality rates of parasites infecting familiar vs. novel hosts 38

3.4 Discussion 39

3.5 Supplementary materials 42

Chapter IV: Dauer life stage of Caenorhabditis elegans induces elevated levels of defense against the parasite Serratia marcescens 49

4.1 Introduction 49

4.2 Methods 54

4.3 Results 60

Figure 4.1a-f Choice assay results in all strains and life stages 62

Figure 4.2 Choice assays between two benign food sources 64

Figure 4.3a-f Mortality rates of infected worms 66

4.4 Discussion 67

4.5 Supplementary Material 73

Chapter V: Phoresy: a quick guide 79

Figure 5.1: Examples of phoresy 84

Chapter VI: Discussion 85

6.1 Summary of results 85

6.2 Future work 87

References 95 

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