We Owe Spicy Mustard to an 'Arms Race' Between Plants and Insects, Study Shows

Mustard is a summertime staple in the U.S., from the yellow spread on hot dogs to the piquant greens in salads. But while people have eaten it in various forms for several thousand years, its tang has a much longer — and less benign — history.

The origins of mustard, along with related foods like horseradish and wasabi, date back nearly 90 million years. As a new study explains, they're the result of an "arms race" between plants and insects that's been going on since the age of dinosaurs.

Despite humans' taste for mustard, it evolved as a pest deterrent. Mustard plants start by making compounds known as glucosinolates, which in turn produce pungent mustard oils when chewed or crushed. This was prompted by relentless nibbling from butterfly larvae, but as caterpillars evolved new ways to cut the mustard, plants had to up the ante — thus growing zestier and zestier over time.

The new study, published in the Proceedings of the National Academy of Sciences, sheds light on the genetics behind this co-evolution of butterflies and Brassicaceae, a plant family that includes more than 3,000 spicy species.

"We found the genetic evidence for an arms race between plants like mustards, cabbage, and broccoli and insects like cabbage butterflies," says co-author and University of Missouri biologist Chris Pires in a statement.

Mustard and catch-up

Plants began evolving glucosinolates sometime in the late Cretaceous Period, and eventually diversified to produce more than 120 varieties. These compounds are highly toxic to most insects, but certain species evolved ways to catch up with mustard by detoxifying the plants' chemical defenses.

This is an example of co-evolution, in which two species can mutually influence the way each other evolves. It was first revealed by scientists in a famous 1964 study, but the new research offers details on how it happened — and how humans might leverage this relationship for more than just a spicy condiment.

The researchers used genomes of nine Brassicaceae plants to make an evolutionary family tree, letting them see when new defenses emerged. They compared that with the family trees of nine butterfly species, revealing three big evolutionary waves over 80 million years in which plants debuted defenses and insects adapted.

"We found that the origin of brand-new chemicals in the plant arose through gene duplications that encode novel functions rather than single mutations," says Pat Edger, a former postdoctoral researcher at University of Missouri and lead author of the study. "Given sufficient amounts of time, the insects repeatedly developed counter defenses and adaptations to these new plant defenses."

The spice of life

The pressure of this rivalry led to more biodiversity, of both plants and insects, than in other groups without the same back-and-forth battles. It also led to the spicy flavors now enjoyed by modern humans, although we're starting to discover our debt to these caterpillars and plants may be even greater than we thought.

For one, learning the secrets of natural insect deterrents might help farmers protect crops without synthetic pesticides. "If we can harness the power of genetics and determine what causes these copies of genes," Pires says, "we could produce plants that are more pest-resistant to insects that are co-evolving with them."