Mustard is of course not a simple creation of nature. This spicy ingredient of our kitchen not only pushes our recipes to the sensory limits, mustard has some military purpose actually out there. Actually it has earned the title “mustard oil bomb” considering its defensive potential. This gorgeous invention wasmade out of necessity to win a race long time ago.
The milleniums-long evolutionary arms race
The time goes back to about 80 million years ago. It was the Cretaceous period right after the Jurassic age. Dinosaurs were happily roaming around on earth. In the plant community something remarkable was going on as well. The newly arrived flowering plants were testing their existential capacity among the then omnipresent gymnosperms, the great great grandparents of today’s pines, firs, cedars. However, the impressive appearance of attractive flower-bearing plants of course drew attention and made them famous among the insects. The following interactions led mostly to longstanding beneficial and mutual relationships. However, it is not that every visitor was nice or desired. Plants soon decided to draw some barrier from them. They started to produce toxic products that will deter those intruders. The insects did not wean off silently. They started to retaliate. Some were successful and some were not. Thus began an evolutionary arms race in the gardens of earth. And that has been never-ending and the outcome is awe-inspiring.
Brassicales vs. Pierinaes
The Brassicales plants ended up with a compound called glucosinolate to discourage the Pierinae butterflies. However, this product itself was not toxic.The punch was given on demand. Whenever there was a wound, they secreted a protein named myrosinase which will catalyze the process of breaking harmless glucosinolate into something that was really shocking to those butterflies. The glucosinolate-myrosinase cooperation is known as the mustard oil bomb. This innovation was helpful no doubt, however, for probably the next 10 million years as Pierinae butterflies were experimenting hard not to mention. By that time they struck back with a smart solution, a catalytic protein named nitrile specifier protein that can break glucosinolate, leaving no scope for generation of toxic products downstream, to bamboozle them at lunch. This is the so-called coevolution where healthy (you have the right to disagree!) competition can produce wonderful results. However, as I told before, it was a forever story. As both Brassicales and Pierinae kept evolving to keep up the standard of their respective arms-kit in order to survive and thrive all the following 80 million years till date. The repeated innovation and selection led to bursts of diversification in both Brassicales and Pierinae.
Evolution is not blind
Like most other flowering plants Brassicales has gone through ancient whole genome duplication events. The first one happened around 77.5 million years ago by the time glucosinolate was invented already. Each genome duplication events are followed by gene losses. The resultant birth-death dynamics determines the evolutionary fates and nature of the resultant bursts of diversification. Interestingly at this point, the glucosinate-myrosinase system related genes have been specifically preserved when most other gene duplicates lost over time. This points out that ancient evolution was not a random event, rather a guided process where the selection process was based on preservation of key innovations for better adaptations. Around 32 million years ago, specifically Brassicaceae plants went through another round of whole genome duplication event and following bursts of diversification using the same principle. All these duplication also involve gene duplication and that is not mere duplication. As in this way the glucosinolate biosynthesis pathway has diversified and so their defensive versatility. Slowly slowly thus expanded the repertoire of plants producing mustard. Although now we have an idea about the origin of mustard oil, nobody knows yet, exactly when we discovered the bombing power of mustard to enlighten our curries!