Mustard Is A Product Of Evolutionary Warfare Between Plants And Caterpillars

Plants produce mustard oils to fight off pests in a chemical conflict that’s been waged for millions of years

Butterfly on Mustard Plant
A small cabbage butterfly (Pieris rapae) hovers on a hedge mustard plant (Sisybrium officinale). While the butterfly might look harmless enough, its caterpillars engage in a chemical war with this mustard plant's cultivated relatives. Tom Blackwell/Flickr CC BY-NC 2.0

There’s an arms race going on in your garden, and it plays out on your kitchen table. Plants that want to stay alive, produce nasty compounds to deter insects. But the compounds that make the insects balk, make some humans drool.

Mustard oils are one example. They provide the basis of many common condiments, such as horseradish, mustard and wasabi. Compounds within the oils called glucosinolates provide sharp flavors that make tangy additions to a hotdog or sushi. But, to insects those oils can be toxic and deadly, as Roger Meissen explains in a post for the University of Missouri’s Decoding Science blog.

Brassicale plants — notably black and white mustards but also broccoli, kale and cabbage — produce glucosinolates to fight off insect pests. In response, some bugs like the cabbage butterfly have come up with chemicals aimed at detoxifying glucosinolates.

Both plants and insects have continually evolved new compounds and mechanisms to combat one another in an ongoing chemical war, researchers reported June 22 in Proceedings of the National Academy of Sciences. Today, brassicale plants churn out 120 different glucosinolates. A history of this arms race can even be seen in the organisms’ genes.

The researchers write that around 90 million years ago, the ancestors of brassicale plants started producing glucosinolates. About ten million years later, the caterpillars had developed their own defense arsenal. According to their research, the war has stages where the plants developed a new set of compounds and then butterflies developed a new set of toxin defenses to fight them. This happened three times over the past 90 million years. Both sides achieved this by making new copies of genes for glucosinolates, rather than just tweaking the originals.

“These plants duplicated their genome and those multiple copies of genes evolved new traits like these chemical defenses and then cabbage butterflies responded by evolving new ways to fight against them,” Chris Pires, a biologist at the University of Missouri and a co-author on the study, said. As the plants developed new compounds, sometimes they also diversified into new species.

Though the war wages on in most backyards, some caterpillars have given up and switched to mistletoe, the researchers found.