From hot-dogs to salad greens, the tangy, spicy taste of mustard in its various forms has been enjoyed by people for thousands of years. But that tang we associate with enjoyment has a much longer and little-known history, dating back almost 90 million years.
The origins of mustard, cabbage, and related plants in the Brassicaceae family dates to the Cretaceous period and encompasses over 3,700 species, according to the Royal Botanic Gardens, Kew.
A new study explains how the spiciness of plants in the Brassicaceae family came about through an ongoing “arms race” between plants and insects that has been going on for millions of years, according to researchers at the University of Missouri.
The research uncovered an ancient arms race between butterflies and plants that is still going on in gardens today. About 90 million years ago, prompted by relentless chewing of the plants by caterpillars, ancestors of Brassica (mustard and cabbage) and related plants started making a chemical compound called glucosinolates as a repellent to butterflies.
Glucosinolates produce pungent mustard oils when crushed or chewed. But as the larvae of the butterflies evolved new ways to cut the taste of the mustard oils, the plants upped the ante, making spicier and zestier oils. The new study sheds some light on the genetics behind the co-evolution of butterflies and Brassicaceae.
“We found the genetic evidence for an arms race between plants like mustard, cabbage, and broccoli and insects like cabbage butterflies,” says co-author and University of Missouri biologist Chris Pires in a statement.
A look inside the arms race and co-evolution
When plants began evolving glucosinolates eons ago, they eventually diversified and were able to produce over 120 varieties of the compound. Even though these compounds are highly toxic to insects, certain species have also evolved ways to detoxify the plant’s chemical defenses.
This is an example of co-evolution. What happened? Two different species mutually influenced the way each species evolved. Scientists first described the process of co-evolution with the publishing of a 1964 study in the journal Society for the Study of Evolution, called Butterflies and Plants: A Study in Co-evolution.
The new research used the previous study and enhanced it by offering added details on how the plant and herbivore interaction came about, as well as suggesting how humans might leverage the relationship to our benefit. Researchers had to first create a family tree, using genomes from a number of Brassicaceae plants.
They in turn compared the genomes from an equal number of family trees of butterfly species. The researchers discovered three evolutionary spikes if you will, where plants created additional new 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.”
This back-and-forth evolutionary war led to more plant and insect diversity in this particular group, more than in any other group. It also gave us the spicy flavors we love so well. But there is also something else this arms race has given us, and it is worth considering.
The use of natural insect repellents instead of synthetic repellents to protect our crops is much more ecologically sound. 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.”
This study was published in the online journal Proceedings of the National Academy of Sciences (PNAS) in June 17, 2015, entitled: The butterfly plant arms race escalated by gene and genome duplications.