Contribution of bottom-up forces in the evolution of toxin resistance in milkweed butterflies
posterposted on 20.04.2020 by Derrick Yip, Marianthi Karageorgi, Elizabeth Ordeman, Naama Weksler, Anurag Agrawal, Noah Whiteman
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A central question in evolutionary ecology is whether herbivore host plant specificity is driven by chemical coevolutionary arms races between plants and herbivores or by predators of the herbivores (Ehrlich and Raven, 1964; Bernays and Graham, 1988). Milkweed butterflies breed on a range of toxic milkweed host plants; some of them, including the monarch butterfly, also sequester toxic cardenolides present in milkweeds to avoid predation. In a stepwise manner, milkweed butterflies have evolved resistance substitutions in their sodium pump, the target of the cardenolides (Petschenka et al., 2012). Previously, it was shown that cardenolide resistance in the monarch butterfly is mainly linked to sequestration, and not the need to cope with toxic milkweeds (Petschenka and Agrawal, 2015). In the present study, we aim to characterize whether milkweed butterflies evolved resistance substitutions in their sodium pump primarily to feed on milkweeds (bottom-up selective force) or to sequester the cardenolides and avoid predators (top-down selective force). To address this question, we use CRISPR-engineered Drosophila flies, previously generated in our lab, that carry mutations in their sodium pump that confer different levels of cardenolide resistance “mimicking” the milkweed specialists (Karageorgi et al., 2019) (Figure 1). To test the hypothesis that the resistance mutations evolved for feeding on milkweed, we plan to perform feeding experiments with the CRISPR- engineered Drosophila flies and compare their performance (survival/growth) in media containing milkweed tissue with different cardenolide levels. To test the hypothesis that the resistance mutations evolved for sequestration, we plan to perform cardenolide (ouabain) injections in the hemolymph of CRISPR-engineered Drosophila flies and compare their survival. Our study will illuminate the role of bottom-up and top-down forces in the evolution of host specificity in milkweed butterflies.
In the present poster, we present the series of experiments we will conduct to characterize the bottom-up hypothesis.