by Brian Thomas, M.S.

The leaves of the common roadside milkweed plant are poisonous to people and most other creatures. Alongside many other plants, its leaf tissues contain cardenolide poisons as a natural defense.

But many insects eat poisonous plants and survive. And milkweed is all that monarch butterfly caterpillars eat. What attributes let them do that, and how did they acquire those attributes in the first place?

To find some answers, scientists dug deep into the genetics and biochemistry of 18 different kinds of insects that all live on cardenolide-producing plants. Their results, published in the Proceedings of the National Academy of Sciences, were remarkable.

The authors wrote that cardenolides do their damage by inserting themselves into a specific binding pocket on a protein pump that animal cells—including insects—constantly use. Once bound, the tiny chemical disables the pump, and enough dysfunctional pumps disables the cell. The researchers discovered that, in creatures that can eat such plants, specific mutations change the shape of that binding pocket so that it excludes the cardenolide.

The scientists discovered through rigorous experimentation that all 18 insects had certain amino acids substituted at numbered positions 111 and 122 of the gene that codes for the cellular pump. Very few positions are altered in this gene. Since most alterations would diminish or halt its vital effectiveness, creatures tolerate very few such changes.

How is it that such different insects have exactly the same DNA base changes in this gene?…

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