Mustard, wasabi and horseradish are just some of the delectable, sharp-tasting meal accompaniments that we enjoy as a result of a 90-million-year-old arms race between insects and the plant order Brassicales. Mustard oils began as a chemical defense that vegetable ancestors developed against pesky insects – who, in turn, evolved counter-tactics so that they could keep feeding on them with getting sick. Plants fought back and so on, for millions of years. 

Compounds called glucosinolates are what create the extra zing that we like with our hotdogs, sushi, and Bloody Marys. For insects that haven’t evolved detoxification mechanisms, glucosinolates turn their guts inside out. According to new findings published in Proceedings of the National Academy of Sciences, gene and genome duplications helped to escalate the complexity of the chemical defenses of plants, spurring on more caterpillar counter-adaptions within 10 million years.

Half a century ago, scientists revealed that cabbage butterflies and plants – ranging from cabbage and mustard to broccoli and kale – influence each other’s development and evolution. To explore the underlying genetics of this coevolutionary interaction, a team led by Chris Pires from the University of Missouri, Columbia, and Stockholm University’s Christopher Wheat looked at the evolutionary histories of the plants and the bugs side by side. 

The team examined nine existing Brassicales genomes and also sequenced several transcriptomes, a cell’s set of RNA. By mapping a family tree that spans millennia, the team was able to figure out where major defense changes occurred. Then they compared this family tree to that of nine Pieridae butterfly species (pictured above and to the right). They identified three key steps of escalation since glucosinolates evolved about 90 million years ago. 

“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,” Pires explains in a statement. “The origin of brand-new chemicals in the plant arose through gene duplications that encode novel functions rather than single mutations,” adds lead author Pat Edger, also of MU.

Always trying to one-up each other has resulted in diversification into many more species of plants and butterflies than in other groups without these same glucosinolate pressures. More than 120 different types of this compound are being synthesized by plants today.

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