SNHS Biology Blog 18: How Monarch Butterflies Evolved to Eat Milkweed

Sriram Yerneni

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The milkweed plant is something unpalatable for most animals. It contains bitter toxins called cardenolides that induce vomiting and even heart rhythm irregularities if enough is consumed. However, for the caterpillars of the monarch butterfly, milkweed is the only plant they can eat. Their tolerance to the milkweed is explained by a certain protein, the sodium pump, in their bodies. The sodium pump plays a critical function in cell physiology in animals; it helps physiological recovery after an electrical discharge occurs in cells, such as when heart muscle cells contract. Cardenolides inhibit the function of these pumps by binding to certain parts of them, which can make the heart function irregularly.

Monarchs have evolved a mutation in their genes for sodium pumps that made it more difficult for cardenolides to bind to them. Researchers wanted to find out more about why this trait emerged; it had to have taken multiple mutations for the milkweed to become edible, so speculation was raised around why exactly the first set of mutations had even occurred. Through testing done to fruit flies genetically engineered with sodium pump genes from monarch butterflies, researchers were able to shed light onto those questions. Flies would need to have gotten the genes in a certain order to be able to safely consume milkweed and not face other consequences; in one combination of genes, they could eat milkweed with no adverse effects, but they faced neurological issues. So, for the monarchs to be able to eat milkweed without consequence, they had to evolve resistance in stages. Through the first mutation, they developed some resistance to milkweed toxins and were faced some mild neurological problems as a result. The second mutation amended the neurological issues without affecting toxin resistance. The third mutation provided the strongest resistance to milkweed toxins, but by itself, it would’ve caused extremely severe neurological issues. This pattern of evolution is called an “adaptive walk,” in which mutations follow each other in a predictable succession to help organisms achieve a higher fitness. This evolutionary trait of the monarch butterflies has had an ecological ripple effect, opening up a new sources of food for them while also presenting the monarchs themselves as unpalatable for several predators. Often, birds learn the latter fact through experience. After enough attempts at making a meal out of monarchs and other insects that come in brilliant orange and yellow colors, birds and other predators learn that these brightly-colored insects would be a very unpleasant experience to eat.