The monarch butterfly is probably the most recognizable butterfly on the planet. Its iconic black and orange pattern is a prime example of aposematism: prey using bright colors to warn predators, “Don’t eat me, I taste bad!” But like poison dart frogs and other toxic critters, monarch butterflies aren’t simply born poisonous. How monarchs harness toxins, and the evolutionary rollercoaster it took to get there, is absolutely metal. 

Monarch butterflies lay their eggs exclusively on milkweed. A plant named for the sticky, milky latex inside its leaves. This latex contains cardiac glycosides, natural steroids, toxic to most species, that inhibit protein pumps at a cellular level and can stop your heart. Monarchs, however, have adapted.

Over millions of years they have evolved to thrive on the milkweed latex and defensively store the toxin in their bodies. These toxin reserves even stick around through metamorphosis, making the adult butterfly taste disgusting.

Though monarchs can safely ingest the toxic latex, they haven’t yet evolved a way to make it any less sticky. Whether their mouths are glued shut or their feet are cemented to the milkweed an estimated 60% of newborn monarch caterpillars die due to starvation. Perhaps that’s an evolution for another day!

Monarchs have decided the risk is worth it. Without milkweed they would become vulnerable, and tastier than ever. In fact monarchs lab raised on a healthy diet of decidedly nontoxic cabbage emerged from their chrysalises, according to some local blue jays, extremely delicious.

Scientists have identified three specific mutations that allow monarchs to withstand such toxins. These mutations each genetically alter the DNA responsible for creating amino acids inside the butterfly. The toxic cardiac glycosides from milkweed latex can’t bind with these mutated amino acids, which means they can’t inhibit protein pumps at a cellular level, which means they can’t stop the heart of a monarch butterfly.

The order of these three mutations is crucial. In fact scientists have determined the mutation that provides the largest toxic resistance will only evolve after the other two. Why wait for the good stuff? Well, everything has its trade offs. Flies genetically modified to have just the third and final mutation easily tolerated milkweed toxin, but were plagued with seizures and other neurological side effects. For monarchs, it's all or nothing.

But this adaptation may not have occurred by accident. While some mutations occur due to random errors in DNA replication, mutations can also be caused by mobile genetic elements. 

These little bits of genetic material are responsible for all sorts of genome variation, and play an important part in evolution. One group, Integrative Conjugative Elements, facilitate genetic material hopping from one species to another in a process known as horizontal gene transfer.

So did monarchs steal their ability to thrive on milkweed toxin from another species’s genome? We can’t definitively say, but we know they’ve done something similar before.

Enter the braconid wasp: a family of parasitoid wasps that lay their eggs inside butterfly larva. The eggs don’t come alone, the mother wasps cook up a virus to accompany their offspring inside the host. This bracovirus invades the caterpillar’s cells, disabling its immune system and making it the perfect snack for the newborn wasp larva.

There are thousands of species of braconid wasps and therefore thousands of unique bracoviruses, all finely tuned to attack and subdue a single species of butterfly or moth. But lay your eggs in the wrong caterpillar, and the next generation may not survive.

A bracovirus meant for another species wouldn’t be very effective. It’s possible elements of the virus’s genetic material may just jump across to the DNA of the still-very-much-alive caterpillar. And if they proved particularly advantageous, they could stick around for a while. Which is exactly what might have happened to the monarch butterfly.

Scientists have found DNA derived from wasps in monarchs from several million years ago. Genes carried by a virus created by one species found in another is truly incredible. The best part, horizontal gene transfer like this happens all the time and we’re just now beginning to realize it.

Just a nice little reminder that Mother Nature is way cooler than you think she is.