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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.

They can tolerate this food source because of a peculiarity in a crucial protein in their bodies, a sodium pump, that the cardenolide toxins usually interfere with.

In 2012, evolutionary biologist Noah Whiteman, now at the University of California, Berkeley, and a colleague proposed in a commentary that one could answer the question by engineering the monarch sodium pump mutations into fruit flies.

Some flies had one mutation of three seen in the sodium pump gene of monarchs, and some had combinations.

First, a mutation of small effect would have altered the structure of the sodium pump to provide some resistance, but also some neurological problems.