It’s a very vulnerable point, and plants have targeted it

Wednesday, October 9th, 2019

Monarch butterflies eat only milkweed, a poisonous plant that should kill them, and even store the toxins in their own bodies as a defense against hungry birds:

Only three genetic mutations were necessary to turn the butterflies from vulnerable to resistant, the researchers reported in the journal Nature. They were able to introduce these mutations into fruit flies, and suddenly they were able to eat milkweed, too.

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Insects began dining on plants over 400 million years ago, spurring the evolution of many botanical defenses, including harsh chemicals. Certain plants, including milkweed, make particularly nasty toxins known as cardiac glycosides.

The right dose can stop a beating heart or disrupt the nervous system. For thousands of years, African hunters have put these poisons on the tips of arrows. Agatha Christie wrote a murder mystery featuring foxglove, which produces cardiac glycosides.

The toxins gum up so-called sodium pumps, an essential component of all animal cells. “It’s a very vulnerable point, and plants have targeted it,” said Susanne Dobler, a molecular biologist at the University of Hamburg in Germany.

These pumps move positively charged sodium atoms out of cells, giving their interiors a negative charge. Heart cells need sodium pumps to build enough electrical charge to deliver a heartbeat. Nerves use the pumps to produce signals to the brain. If the pumps fail, then those functions come to a halt.

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The researchers compared the genes that serve as blueprints for the sodium pump in poison-resistant species, like the milkweed beetle and the milkweed bug. Most of these species, it turned out, had gained the same three mutations.

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Monarchs share one of the mutations with a related butterfly that doesn’t eat milkweed, and a second mutation with a closer relative that eats milkweed but doesn’t store cardiac glycosides in its wings. The third mutation arose in an even more recent ancestor.

Gaining these mutations gradually altered the sodium pumps in the monarchs’ cells, Dr. Dobler suspected, so that the cardiac glycosides couldn’t disrupt them. As the butterflies became more resistant, they were able to enjoy a new supply of food untouched by most other insects.

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Noah Whiteman, an evolutionary biologist at the University of California, Berkeley, led the effort to test this hypothesis. “These three mutations may be the thing that unlocked the door” for the butterflies, he said.

He and his colleagues figured out how to use Crispr, the gene-editing technology, to introduce the mutations into fruit flies. The flies survive on rotting fruit, and even a small dose of cardiac glycosides can be deadly to them.

The researchers began by giving the flies the first mutation to arise in the ancestors of monarchs. The larvae that carried this mutation were able to survive on a diet of yeast laced with low levels of cardiac glycosides.

The second mutation let the flies withstand even more toxins, and the third made them entirely resistant. With all three mutations, the flies even ate dried milkweed powder.

The third mutation had another striking effect. When the flies with the gene developed into adults, their bodies carried low levels of cardiac glycoside, useful as a defense against predation.

O brave new world that has such insects in it!

Comments

  1. David Whitewolf says:

    I hope someone will start a project to make Monarch butterflies that thrive on plants other than milkweed, given that plant’s recent decline.

    Reading the prior post and comments right after reading this one on genetically modifying common flies to have the qualities of Monarchs was… interesting. Nicely juxtaposed!

  2. CVLR says:

    It’s worth taking a moment to reflect that we now live in a world in which the successful, reliable, reproducible transfer of high-import genes from one specie to another, while still newsworthy, is described in so casual a manner.

    https://youtube.com/watch?v=xSLlZh9yelk

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