Monarch butterflies’ predators copy their poison resistance at a genetic level

Monarch caterpillars live on poisonous milkweed, which makes the adult monarch butterflies noxious to most predators. Over the past decade, researchers have been able to spot a number of key genetic mutations which allow the monarchs to tolerate milkweed toxins.

Now, a study has shown that four of the monarch butterflies’ North American predators also have these mutations, in a fascinating display of convergent evolution.

A team of US researchers have found that a bird, a mouse, a wasp and a nematode have all evolved mutations in genes that code for a key protein, called the sodium-potassium pump.

These mutations are nearly identical, despite the vast morphological differences between the species.

These mutations also match those in the butterflies themselves, which the researchers spotted a couple of years ago.

A black-headed grosbeak on a branch
The black-headed grosbeak, one of the monarch butterflies’ milkweed-resistant predators. Credit: Mark Chappell, UC Riverside

The sodium-potassium pump is crucial to the function of heartbeats and nerve firing, so the milkweed toxins that interfere with its production are dangerous. Milkweed can cause heart attacks in creatures as large as humans and horses.

So it makes sense that mutations in the genes that code for this pump would make species more resistant to milkweed. Nonetheless, researchers were surprised by how similar these genes were.

“It’s remarkable that convergent evolution occurred at the molecular level in all these animals,” says co-author Simon Groen, assistant professor of evolutionary systems biology at the University of California, Riverside, in the US.

 “Plant toxins caused evolutionary changes across at least three levels of the food chain.”

Read more: Captive-bred migratory monarch butterflies don’t migrate

The researchers investigated the genomes of the black-headed grosbeak (Pheucticus melanocephalus), the eastern deer mouse (Peromyscus maniculatus), the small Trichogramma pretiosum wasp, and the Steinernema carpocapsae nematode.

Drawing of caterpillar, monarch butterfly, and black-headed grosbeak on a milkweed plant, with arrows & molecules denoting food chain
Monarch caterpillars eat milkweed plants, meaning that the adults are filled with toxins that dissuade most predators. But not the black-headed grosbeak, which has developed the same genetic mutations as the butterflies so that it can withstand milkweed toxins. Credit: UC Berkeley image by Julie Johnson

Both the bird and the mouse are known to eat monarchs, while the wasp parasitises monarch eggs and the nematode parasitises the caterpillars.

“It looks like, amazingly, [grosbeaks] are evolving resistance using the same kind of machinery in the same places in the genetic code as the monarch and the aphids, the bugs and the beetles, that feed on milkweeds, as well,” says Noah Whiteman, professor of integrative biology and of molecular and cell biology at the University of California, Berkeley, US.

The researchers aren’t sure whether there are other mutations that assist with milkweed tolerance, but they’re interested in finding out.

“My guess is, there are other parasitoids out there, and predators that have also evolved resistance mutations that are interacting with monarchs, and it’s just a matter of time before they’re discovered,” says Whitehead.

“We know that this isn’t the only way to evolve resistance to cardiac glycosides, but it seems to be the predominant way — targeting this particular pump.”

The researchers have published their findings in Current Biology.

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