New research from Cornell University sheds light on the secret to the survival of monarch butterflies by revealing how the species developed immunity to fatal milkweed toxins.

Small genetic changes prevent a normally fatal poison from killing monarch butterflies. By editing these genes into laboratory fruit flies using CRISPR technology, scientists have reconstructed evolution and instantly conferred—in the flies—the same toxin resistance found in the butterflies.

“We experimentally went back in evolutionary time to reconstruct an event that happened naturally several times, several million years ago,” said Anurag Agrawal, professor of ecology and evolutionary biology at Cornell. Agrawal is a co-senior author on a new paper published in Nature.

“Monarch butterflies are a beloved insect,” Agrawal said. “They only eat milkweed, and survival of their annual migration depends on sequestering this plant’s toxins.”

Monarch caterpillars eat milkweed plants, which harbor a poison called cardiac glycoside. For most other caterpillars and animals that eat milkweed, the cardiac glycoside poisons them. The poison obstructs the body’s sodium pumps, which transport life-maintaining salt across cell membranes.

A specific variant of the monarch’s gene prevents its sodium pump from clogging with poison, effectively granting immunity to the butterflies. Most other species—particularly those that stay away from milkweed—don’t have this genetic change.

When researchers inserted the gene into the flies in the laboratory, they became immune to cardiac glycoside.

“Our study suggests that, although there is a universal ancestor to life on Earth, small modifications were critical in specific adaptations,” Agrawal said. “We now have the tools in evolutionary biology to reconstruct how organisms evolved over millions of years.”

The paper, “Genome Editing Retraces the Evolution of Toxin Resistance in the Monarch Butter,” published Oct. 2.

The research was funded by the National Science Foundation and the John Templeton Foundation.

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