Population Differences and Genetic Basis of Parasitic Resistance and Melanization in Monarch Butterflies Restricted; Files Only
Arega, Emmanuel (Spring 2024)
Abstract
Monarch butterfly (Danaus plexippus) larvae feed exclusively on the leaves of milkweed plants. They can consume secondary defense chemicals produced by milkweed called cardenolides. These chemicals are toxic to many other animals but have been linked to increased resistance to Ophryocystis elektroscirrha - an obligate, neogregarine protozoan parasite that infects monarch and queen butterflies with no other known hosts – in adult monarch butterflies. That being said, it is not yet known whether it is the cardenolides themselves that provide the resistance. Asclepias curassavica, a plant species in the milkweed family, is native to Puerto Rico and has a relatively higher cardenolide concentration than Asclepia incarnata, native to eastern North America. We crossed Eastern North American monarchs with Puerto Rican monarchs to generate a Hybrid population. We expected both monarch populations to be more effective at dealing with the protozoan parasites O. elektroscirrha when reared on A. curassavica as the consumption of milkweeds that produce more cardenolides has been linked to greater parasitic resistance in monarchs. We also expected differences in parasitic resistance between monarchs of different populations as previous work has shown that non-migratory monarchs are more resistant to infection when compared to migratory monarchs. As for the resistance of the hybrid monarchs, we projected that they would have intermediate levels of resistance between parental genotypes. We predicted this because it is likely that resistance to parasites in monarch butterflies is governed by multiple genes and, as a result, when hybridizing Eastern North American and Puerto Rican monarchs, which have different resistance profiles, their offspring are expected to inherit a blend of alleles from each parent resulting in an intermediate resistance phenotype. To test this, monarchs captured from Puerto Rico and North Florida were crossed in a typical F2 mating pattern to produce pure Eastern monarch families, pure Puerto Rican families and Hybrid monarch families. We then raised half of all Puerto Rican monarchs on A. incarnata and the other half on A. curassavica, half of all Eastern monarchs on A. incarnata and the other half on A. curassavica and half of all Hybrid monarchs on A. incarnata and the other half on A. curassavica. After accessing the severity of infection of each adult monarch, we found that Puerto Rican monarchs had a greater resistance to the protozoan parasite than Eastern monarchs and that Hybrid monarchs showed intermediate parasitic resistance, suggesting that parasitic resistance is a polygenic trait. Additionally, monarchs reared on A. curassavica presented greater resistance regardless of which population they belong to. Following these findings, we used pictures of fifth instar monarch larvae to assess larval melanization as the latter has been found to be a conserved immune response in other arthropods. Studies have found that Puerto Rican monarch larvae are darker than Eastern North American larvae, and these findings were reflected in our results. Interestingly, hybrid monarch larvae mean corporal blackness was significantly greater than that of Eastern monarchs, but we found no significant difference when compared to the Puerto Ricans. As these results do not mirror the latter parasitic resistance observations, melanization does not seem to be correlated to resistance.
Table of Contents
Table of Contents
Introduction 1
Methods 4
Matings 4
Inoculations 4
Handling larvae and butterflies 5
Assessing infection burden 6
Death checks 6
Color Analysis 7
Band Patterning Analysis 7
Statistical Analysis 8
Results 9
Parasitic Resistance 10
Larval melanization 14
Larval band patterning 19
Larval vs adult mean black coloration 23
Discussion 25
References 27
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File download under embargo until 24 May 2025 | 2024-04-30 23:15:55 -0400 | File download under embargo until 24 May 2025 |
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