The impacts of defensive symbionts and host plants on fitness and population dynamics of pea aphids, Acyrthosiphon pisum Öffentlichkeit

Abstract

The impacts of defensive symbionts and host plants on fitness and
population dynamics of pea aphids, Acyrthosiphon pisum

Populations are constantly changing in response to many factors including predation,
parasitism, disease, competition, and the environment. In many animals and plants, these
fluctuations may be impacted by interactions with beneficial microbial partners. Pea aphids,
Acyrthosiphon pisum, have evolved to host endosymbiotic bacteria that confer increased
resistance to various parasites and predators. Here, I explore how both the pea aphid genotype
and the bacterial endosymbiont, Regiella insecticola, influence the fitness of aphids under
alternative ecological conditions, namely in the presence competition, in the presence of
pathogens, and using alternative host plants. Previous studies have demonstrated that aphids
hosting Regiella have higher survival than aphids without Regiella when infected with Pandora
neoaphidis, an aphid-specific, entomopathogenic fungus. However, ecological surveys indicate
that only 16% of aphid populations harbor this beneficial symbiont. To determine if there are
competitive costs to harboring Regiella in the absence of a fungal infection, I conducted
population cage experiments on aphid strains with and without Regiella. Although there were
fecundity increases when hosting Regiella, I found that there was no significant cost or benefit
during competition. In contrast, I found that competition between aphids of different genotypes
with similar reproductive rates in the absence of competition led to one aphid strain consistently
outcompeting the other. Pea aphids are also able to utilize a range of host plant species, but some
aphid strains are adapted to using certain host plants but not others. Because the host plant
supplies the nutrients and comprises a major part of the external environment for the aphids, I
wanted to determine if the host plant affects aphid resistance to Pandora. I monitored the
survival of infected aphids on fava bean, red clover, and crimson clover. While host plant did not
affect resistance to infection, I did find that genotype influenced the aphids' performance on
crimson clover and fava bean. These findings show how host genotype, symbiotic partners, and
external biotic factors, including competition and host plants, can interact with one another to
shape aphid population dynamics.

Table of Contents

TABLE OF CONTENTS

INTRODUCTION ...………………………………………………………………………...……1

Figure 1……………………………………………………………………………....……5

Figure 2………………………………………………………………………………......8

METHODS ...…………………………………………………………………………..….......11

Figure 3…………………………………………………………………………….......12

RESULTS & DISCUSSION ...……………………………………………………….…….17

Figure 4……………………………………………………………………………….....17

Figure 5……………………………………………………………………………….....18

Figures 6 & 7...………………………………………………………………..……..20

Figure 8……………………………………………………………………………….....21

Figure 9………………………………………………………………………….....…..23

Figure 10..……………………………………………………………………….……..24

Figure 11..………………………………………………………………………….…..25

Figure 12..………………………………………………………………………….…..27

CONCLUSIONS ...…………………………………………………………………………....29

FUTURE DIRECTIONS………………………………………………………………..…....30

REFERENCES ...……………………………………………………………………………....31

APPENDIX A ...…………………………………………………………………………………38

APPENDIX B ...…………………………………………………………………………………39

APPENDIX C ...…………………………………………………………………………………40

About this Honors Thesis

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School	Emory College
Department	Biology
Degree	BS
Submission	Honors Thesis
Language	English
Research Field	Biology, Ecology Biology, General Biology, Entomology
Stichwort	fitness apparent competition competition adaptation pea aphid Population Biology Acyrthosiphon pisum host plant population dynamics
Committee Chair / Thesis Advisor	Gerardo, Nicole Marie, Emory University
Committee Members	Hammonds, Jason Edward, Emory University De Roode, Jacobus, Emory University

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