Chemical profiling of milkweed and monarch butterfly wing extracts via mass spectrometry

Herbivores that sequester toxins are thought to have cracked the code of
plant defenses. 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.