Date Thesis Awarded
Honors Thesis -- Access Restricted On-Campus Only
Bachelors of Science (BS)
Joshua R. Puzey
Harmony J. Dalgleish
John C. Poutsma
The Monarch butterfly, Danaus plexippus, is projected to face substantial quasi-extinction risk over the next 20 years after decades of population decline (Semmens et al., 2016). The butterfly shares an exciting and oft-studied chemical relationship with its larval food source, milkweed. Monarchs have the ability not only to tolerate the plant’s toxic cardenolide defenses, but also to sequester them into their own tissue for defense against predators and parasites. Recent work demonstrating a strong correlation between the cardenolides of the milkweed host and the eventual parasite load of the metamorphosed adult motivated us to further investigate the passage of defenses from plant to herbivore (De Roode et al., 2016). Here we perform a feeding trial with seven unique genets of Common milkweed (Asclepias syriaca) and collect samples of Monarch larvae, pupae, adults and frass. We compare these samples’ cardenolide chemistry to that of their host plants using high-performance liquid chromatography. We find that cardenolide concentration varies significantly between D. plexippus and A. syriaca but not between the individual tissue types mentioned above. Our seven milkweed genets were obtained either from sites in North Carolina or Virginia and this geographic variation did not affect variation in cardenolide concentration. Our results suggest that cardenolide concentration varies over time (throughout development) in insect tissue. We show that some cardenolides identified are substantially more concentrated than others and that some appear only in milkweed or Monarch tissue. We find the relative concentrations of several most prominent cardenolides to vary significantly between plant and insect samples. We show that cardenolide profiles in Monarchs vary significantly from those in milkweed and vary significantly over time. We demonstrate that insects exercise substantial discrimination in their cardenolide sequestration and we show that this sequestered profile is stable throughout their development. The observation of many insect-unique cardenolides suggests some conversion of plant-derived defenses before sequestration. Replication and a larger dataset are needed to draw finer conclusions about differences in cardenolide content between milkweed, Monarchs, and their respective tissues.
Brammer, Jacob, "Are You What You Eat? Selective Sequestration of Toxic Milkweed Cardenolides in the Monarch Butterfly" (2017). Undergraduate Honors Theses. William & Mary. Paper 1033.
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