The relationship between milkweed and the monarch butterfly is a textbook case of coevolution: the plant makes heart-poisoning cardenolides, and the insect evolves enzymes that shrug them off. Research published in the Proceedings of the National Academy of Sciences shows the plant has a counter-move. Cornell scientists found that many milkweed species modify their cardenolides with a small nitrogen-sulphur ring that makes the toxins far better at binding the monarch's target enzymes, slipping past the butterfly's resistance.
The team, including Christophe Duplais of Cornell AgriTech, ecologist Anurag Agrawal and chemist Jeremy Baskin, analysed toxins from 52 milkweed species and modelled how the upgraded molecules bind to monarch enzymes. The modified structure appeared in nearly 70 per cent of the species studied, far more than earlier surveys suggested, and arose independently multiple times across lineages that diverged long ago, a striking example of convergent chemical innovation.
The findings complicate the classic model of chemical escalation, in which plants simply make more toxin and insects adapt in a linear arms race. Structural innovation, the authors argue, can create whole new classes of defence rather than just higher doses. The work, funded by the National Science Foundation, may also aid monarch conservation by identifying which milkweed species are safe or harmful for a butterfly whose populations are under pressure.