Transfer of aphid secondary symbionts within sympatric insectcommunities and the effect of ingesting symbionts on ladybirdbeetles Open Access

Abstract

Aphids cannot survive on their limited diet of plant phloem without the help of their primary

symbiotic bacterium, Buchnera. In addition to this obligate relationship, a number of

facultative symbionts can inhabit this sap-sucking insect. While efficient transmission from

mother to daughter is readily seen in the lab for both types, secondary symbionts are thought

to undergo horizontal transfer across species in the field. Genetic comparisons suggest these

jumps may even occur between very distantly related species, such as whiteflies or ladybirds.

Such horizontal transfer could play a critical role in both understanding the colonization of

new insect lineages with novel bacteria and in the general understanding of bacterial

migration. I explored the transfer among insects in the aphid community, screening field

samples of sympatric ants, ladybirds and aphids for three common aphid secondary

symbionts and conducting feeding experiments with live ladybirds. I found that ladybird

larvae fed aphids with symbionts were half as likely to die as larvae fed aphids without

symbionts, suggesting there is an advantage to consuming symbionts. There was decrease in

the persistence over time of the symbiont Serratia symbiotica in adult ladybird beetles fed

aphids with the bacteria, until no bacterial DNA was seen in the ladybirds after one week. No

ants or ladybirds were found in the field with any of the three main aphid symbionts,

suggesting that horizontal transfer is a rare event. Still, ingestion may need to be considered

as a route of symbiont acquisition.

Table of Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 1 Seasonal cycle of a pea aphid . . . . . . . . . . . . . . . . 2

Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

Figure 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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

Figure 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Table 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Future Directions and Concluding Remarks . . . . . . . . . 30

Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Appendix B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

About this Honors Thesis

Rights statement

• Permission granted by the author to include this thesis or dissertation in this repository. All rights reserved by the author. Please contact the author for information regarding the reproduction and use of this thesis or dissertation.

School	Emory College
Department	Biology
Degree	BS
Submission	Honors Thesis
Language	English
Research Field	Biology, Ecology Biology, General Biology, Microbiology
Keyword	insect community probiotics Aphid ecology ladybug Horizontal transfer Acyrthosiphon pisum Symbiont
Committee Chair / Thesis Advisor	Gerardo, Nicole Marie, Emory University
Committee Members	Gillespie, Thomas, Emory University De Roode, Jacobus, Emory University

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