Acquisition and maintenance of specific bacterial symbionts in vertical and horizontal symbioses Open Access

Garcia, Justine Rebecca (2015)

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Symbionts, or microbial mutualists, can have profound effects on the ecology and evolution of multicellular organisms. These effects can differ in type or magnitude based on the genetic and phenotypic symbiont variation, so it is important to determine and understand the consequences of specificity in host-symbiont pairings. Using a framework wherein symbiont specificity can be determined at three points - in the external environment, within the host, and during transmission, I investigate how specificity can be imposed and the consequences of specificity in the host in vertical, or heritable, and horizontal, or environmentally acquired, symbioses. In the horizontal symbiosis between true bugs and Burkholderia bacteria, I show that specificity in Burkholderia acquisition is primarily driven by screening in the host, though external environmental conditions can determine the incidence of less prevalent Burkholderia in the host. I evaluated the location within the host where specificity is imposed by sequencing the bacterial communities in five distinct regions of the squash bug midgut using Illumina MiSeq. The vast majority of Burkholderia recovered throughout the midgut belonged to a single OTU, indicating that the host likely imposes specificity in a region anterior to the midgut. I used a vertical symbiosis, the pea aphid and its suite of facultative symbionts, to investigate how symbionts may promote their maintenance within the host by promoting aphid survival against parasitoid wasp attack. I found that one symbiont species, Regiella, enhanced the immune response used against parasitoids. The ability to enhance this immune response differed among Regiella strains, which may translate to differential maintenance of Regiella in host aphids. Finally, I evaluate symbiosis from the perspective of microbial symbionts, and suggest experiments and approaches that could incorporate symbiont fitness into investigations of symbiosis. The diverse approaches here, coupling phylogenetics, deep sequencing, and immunological assays highlight the challenges of developing a comprehensive understanding of the acquisition and maintenance of the world's diverse symbioses.

Table of Contents

Table of Contents

Chapter 1:Introduction

1.1 Introduction 1

1.2 Contact and acquisition 4

1.3 Maintenance 7

1.4 Transmission to the next generation 9

1.5 Evolutionary selection for the maintenance of symbioses 11

1.6 True bugs as a model for horizontal symbioses 13

1.7 The pea aphid, Acyrthosiphon pisum, as a model of 15

vertical symbioses

1.8 References 18

Chapter 2:Partner associations across sympatric broad-headed bug species and their environmentally acquired bacterial symbionts

2.1 Abstract 31

2.2 Introduction 33

2.2 Materials and methods 35

2.3 Results 49

2.4 Discussion 59

2.5 Supplemental Information 682.6 References 78

Chapter 3: Symbiont winnowing in the squash bug midgut

3.1 Abstract 85

3.2 Introduction 87

3.3 Materials and methods 92

3.4 Results 96

3.5 Discussion 100

3.6 References 106

Chapter 4: The symbiont side of symbiosis: do microbes really benefit?

4.1 Abstract 113

4.2 Introduction 114

4.3 An evaluation of assumed symbiont benefits 116

4.4 Recommendations for investigating symbiont fitness 120

4.5 References 128

Chapter 5: The encapsulation immune response in pea aphids is enhanced by a secondary symbiont in a genotype specific manner

5.1 Abstract 138

5.2 Introduction 140

5.3 Materials and methods 143

5.4 Results 148

5.5 Discussion 152

5.6 References 157

Chapter 6: Conclusions and Future Directions

6.1 Chapter 2: Considerations and future directions 164

6.2 Chapter 3: Considerations and future directions 166

6.3 Chapter 4: Considerations and future directions 168

6.4 Chapter 5: Considerations and future directions 169

6.5 Summary 171

6.6 References 172

List of Figures

Figure 1-1 3

Figure 2-1 48

Figure 2-2 51

Figure 2-3 52

Figure 2-4 57

Figure 2-5 59

Figure S2-1 68

Figure S2-2 69

Figure S2-3 70

Figure 3-1 91

Figure 3-2 98

Figure 3-3 99

Figure 3-4 100

Figure 4-1 120

Figure 5-1 149

Figure 5-2 150

Figure 5-3 151

Figure 5-4 151

List of Tables

Table 2-1 36

Table 2-2 55

Table 2-3 56

Table S2-1 71

Table S2-2 72

Table 3-1 93

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