Milkweed has found a new strategy in its epic evolutionary battle with monarch butterflies: upgrading its toxins to outmaneuver the monarch’s resistance. In a new study, published in the Proceedings of the National Academy of Sciences, researchers find that adding a small structural element containing nitrogen and sulfur to milkweed’s toxins circumvents monarchs’ ability to block them. The research sheds light on an underappreciated evolutionary tactic for plants: that not only can they increase their levels of toxicity, they can also structurally innovate to create new classes or subclasses of toxins.
“This structural innovation is a new axis for defining chemical toxins in the natural world,” said co-author Christophe Duplais, associate professor of entomology at Cornell AgriTech, in the College of Agriculture and Life Sciences (CALS). “This very simple modification makes a huge difference in terms of its ecological effect, because now this molecule is toxic to the monarch.”
Milkweed and monarchs have coevolved over millions of years, each building defenses and counter-defenses. One such defense is the monarchs’ ability to block milkweed’s toxins, called cardenolides, from binding to their target enzyme in the monarch’s cells. Monarchs have even evolved to sequester the toxins in their wings, to poison birds that peck at them.
