Parasite Resistance and the Potential Longevity Costs of a High Cardenolide Diet in the Monarch Butterfly Open Access

Abstract

Monarch butterflies, Danaus plexippus, have adapted over time to utilize milkweed as a larval food source. Milkweed species contain varying amounts of toxic steroids known as cardenolides, which the larvae sequester as an anti-predation technique. Higher cardenolide concentrations have been associated with shorter adult lifespans in healthy monarchs, but the chemicals are beneficial under certain circumstances. Cardenolides confer resistance to the Ophryocystis elektroscirrha parasite, which infects monarch butterflies and reduces the host's lifespan. In most cases, high cardenolide diets increase both resistance and longevity of infected monarchs. However, recent evidence shows that after a certain concentration threshold is crossed, the benefits of increased resistance may be outweighed by the negative physiological cost of cardenolide consumption. While Asclepias physocarpa contains nearly three times the typical amount of cardenolides, the infected monarchs reared on it demonstrate lower longevities than a linear relationship would predict. The experiment presented here tests the potential costs of a prolonged diet on A. physocarpa in comparison to A. incarnata, a low cardenolide species. Monarchs were reared on varying diets of each species and analyzed for differences in pupal score, spore load and longevity. Treatment groups reared on A. physocarpa after infection had higher longevities than expected, and infected individuals reared on A. physocarpa during the early larval stages showed lower than predicted longevities. Therefore, the negative effects of a high cardenolide diet are most pronounced during the early stages of larval development. To see if smaller larvae are more susceptible to the costs of cardenolides, a new experiment is being developed where the larval diet will be varied for the first three days after hatching.

Table of Contents

BACKGROUND. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Monarch Butterfly. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Milkweed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Parasitism in the Monarch. . . . . . . . . . . . . .. . . . . . . . . . . . 2

Fig 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Parasite Resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..6

Cardenolides and Resistanc . . . . . . . . . . . . . . . . . . . . . . . . 7

Fig 2. . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . .8

Fig 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

Cost of Cardenolides. . . . . . . . . . .. . . . . . . . . . . . . . . . . .10

Fig 4. . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . 11

Asclepias physocarpa. . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Fig 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Fig 6. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . 13

MATERIALS AND METHODS. . . . . . .. . . . . . . . . . . . . . . . . . 15

Treatment Groups. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Fig 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Milkweed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

Pre-infection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Infection.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Post-infection.. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . 18

Pupal Score. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

Longevity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

Spore load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19

Statistical Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

RESULTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Pupal Score. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

Fig 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Longevity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

Fig 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Spore load.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

Fig 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .23

DISCUSSION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

Fig11 . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .26

FUTURE PROJECTS. . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . 30

REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32

About this Honors Thesis

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School	Emory College
Department	Biology
Degree	BS
Submission	Honors Thesis
Language	English
Research Field	Biology, Parasitology Biology, General
Keyword	Butterfly Biology Plant Milkweed Resistance Diet Parasitism Parasite Cardenolide Monarch
Committee Chair / Thesis Advisor	De Roode, Jacobus, Emory University
Committee Members	Beck, Christopher, Emory University Wakefield, Peter, Emory University

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