Testing the selective sequestration hypothesis: Monarch butterflies preferentially sequester plant defences that are...

Herbivores that sequester toxins are thought to have cracked the code of plant defences. Nonetheless, coevolutionary theory predicts that plants should evolve toxic variants that also negatively impact specialists. We propose and test the selective sequestration hypothesis, that specialists preferentially sequester compounds that are less toxic to themselves while maintaining toxicity to enemies. Using chemically distinct plants, we show that monarch butterflies sequester only a subset of cardenolides from milkweed leaves that are less potent against their target enzyme (Na⁺ /K⁺ -ATPase) compared to several dominant cardenolides from leaves. However, sequestered compounds remain highly potent against sensitive Na⁺ /K⁺ -ATPases found in most predators. We confirmed this differential toxicity with mixtures of purified cardenolides from leaves and butterflies. The genetic basis of monarch adaptation to sequestered cardenolides was also confirmed with transgenic Drosophila that were CRISPR-edited with the monarch's Na⁺ /K⁺ -ATPase. Thus, the monarch's selective sequestration appears to reduce self-harm while maintaining protection from enemies.