Monarch butterflies: beautiful poison

Monarch butterflies are something of an iconic butterfly species - think orange wings with large, regular black stripes and some white dots around the edges. They are famous for migrating en masse from the US and Canada to Southern California and Mexico for the winter. Those colourful wings signal more than just beauty - Monarchs are actually very, very poisonous, if even a human ate enough of them they would get very sick. But unlike most other toxic species, the monarch doesn’t make its own poison, as Phil Sansom found out earlier this year...

Anurag - The monarchs are the quintessential example of a butterfly that gets its poisons from its food. The monarch caterpillar feeds on the milkweed leaves and the milkweed leaves are producing poisons called cardiac glycosides

Phil - Cardiac glycosides, you said?

Anurag - Yeah - they bind to a universal animal enzyme and they stop it from functioning, which is what makes them poisonous to most animals.

Phil - What's the enzyme?

Anurag - The enzyme is the sodium-potassium pump and every animal cell, whether it's our human bodies or an insect’s body, uses this enzyme to shuttle salts across the cell membrane - it's a very critical cellular function. And without that, basically, the cell starts to have either too much salt inside or not enough, which basically causes the system to crash.

Phil - Sounds like a nasty poison!

Anurag - Nasty poison - and very general in the sense that all animals use these pumps so it's going to be very poisonous.

Phil - I just have to ask, you said they were called cardiac though. That means heart, right?

Anurag - Great question yeah. The name cardiac glycoside comes from the fact that these compounds have been used historically in traditional cultures to treat congestive heart failure. There've been several really interesting intersections of societal happenings with cardiac glycosides. And one of them is that Vincent van Gogh, as I assume all the listeners will know, in the last two years of his life and in his paintings, they took a turn. His paintings started having much more yellow, and halos around the lights that are so famous in starry night or in the sunflower paintings. Van Gogh was being treated for epilepsy at the time with extracts of the foxglove plant, which have cardiac glycosides. What we now know is that a side effect of too much of this medicine is yellow vision and seeing halos around bright objects.

Phil - That's amazing. Now I assume the monarch butterflies aren't having their cells unable to take salts in and out, and they're not getting their heart conditions treated. How do they not have all this stuff happen to them?

Anurag - The monarch butterfly has three specific mutations that we're aware of in the genes that code for its sodium potassium pump. Quite remarkably, those three single base pair changes alter the physical structure of that pump, making it about 200 times less likely that a cardiac glycoside will bind and stop that pump from functioning.

Phil - But then the butterflies and the caterpillars go beyond that don't they? Because you said that they actually not only get resistant to the poison, but start to use it themselves.

Phil - Absolutely. Yeah. I think one of the most fascinating things about monarch butterflies is they are themselves poisonous and they advertise it with that highly contrasting orange, black and white coloration. The monarch brings those compounds into its body, packs them away in its wings primarily, and that gets used then as the monarch's defense against predators like birds.

Phil - You know, it all sounds like a lot of effort to go through when you could just eat a plant that's not poisonous.

Anurag - No question - it's a lot of effort! But one of the consequences is you've got that resource largely to yourself. You know, one of the axioms in nature is that specialisation is beneficial. A jack of all trades is master of none. But we can't really think of it just as the Monarch butterfly deciding to specialise in eating the milkweed. What really more likely happened is that the monarch butterfly ancestor made initial steps towards feeding and specialising on milkweed. In response to that, the milkweed reciprocally evolved a host of defences to try to push the monarchs away. We call that coevolution when two species are going back and forth, evolving in response to each other.