How the monarch butterfly evolved its resistance to toxic milkweed

Science | AAAS | 10/2/2019 | Staff
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The beloved black and orange wings of the monarch butterfly are more than just beautiful. They warn potential predators that this insect is poisonous to eat—a trait acquired from the butterfly feeding on the toxic milkweed plant. Now, two research teams have independently discovered how this iconic insect became immune to its poisonous diet, and they have shown how three genetic mutations are key—by making those same changes in a fruit fly.

“They are impressive pieces of research,” says Marcus Kronforst, an evolutionary biologist at the University of Chicago in Illinois who was not involved with either group. He notes that these studies are rare examples where researchers verified the mutations responsible for a trait by making them in another species.

Milkweed - Plants - Compounds - Glycosides - Pumps

Milkweed plants produce compounds called cardiac glycosides, which disrupt molecular pumps that control the proper flow of ions in and out of cells. Monarch butterflies and other consumers of the plant, however, have evolved versions of these pumps that leave the animals unaffected. To find what changes these milkweed eaters had in common, Noah Whiteman, an evolutionary biologist at the University of California, Berkeley, and his colleagues recently matched up the gene for this molecular pump in 21 insects, including monarchs, that tolerate the plant to varying degrees.

They found three mutations that changed three amino acids in the protein pump. By looking at the distribution of these changes across the insect family tree, Whiteman and colleagues were able to predict which ones came first—and it turned out that the order in which they evolved mattered. They then tried to replay the tape of evolution by making combinations of those changes in fruit flies using the gene-editing tool CRISPR.

Mutation - Monarch - Ancestor

A single mutation predicted to have arisen in the monarch’s ancestor made...
(Excerpt) Read more at: Science | AAAS
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