Wallflowers independently evolved a pair of complementary chemical defenses, according to new research.

To deter herbivores, flowers must evolve defenses. Some species, including wallflowers belonging to the mustard family Brassicaceae, evolve chemical defenses.

All Brassicaceae plants produce chemicals called glucosinolates. When herbivores take a bite of a wallflower, the chemical is activated, producing an intensely bitter taste.

Many wallflower-eating herbivores have evolved defenses against the pungency of glucosinolates. In response, some Brassicaceae plants have evolved a secondary defense, chemicals called cardenolides.

"Studying how these two defences evolved in this large genus can help scientists understand the trade-offs that the plants face as they try to defend themselves against multiple enemies," Tobias Züst, research associate at the Institute of Plant Sciences at the University of Bern in Switzerland, said in a news release.

For the study, Züst and his colleagues sequenced the genome of the wormseed wallflower, a wild mustard species native to Europe and Asia. Using the genomic data, researchers plotted a family tree, detailing the evolutionary relationships among wormseed wallflower and 47 other wallflower species. Next, scientists analyzed the variety of glucosinolates and cardenolides produced by the family's different species.

The analysis -- detailed in the journal eLife -- showed the two chemical defenses evolved independently of one another.

Scientists found closely related wallflower species growing in the same areas shared similar cardenolide traits, but not glucosinolate traits. It's likely the evolution of glucosinolates and cardenolides were triggered by different selective pressures.

"Even though most species co-expressed two different types of potentially costly chemical defenses, there was no evidence of a trade-off between glucosinolates and cardenolides," Züst said. "Instead, these two types of chemicals appear to complement each other and do not serve redundant functions."

The data genomic and chemical defenses data showed wallflower species began to diversify more rapidly after the development of cardenolides, the secondary chemical defense. The evolution of cardenolides likely helped Brassicaceae plants thrive.

"Further analysis of the wormseed wallflower genome will be needed to help scientists identify glucosinolate and cardenolide-producing genes in this species, as well as aid our understanding of the function of these chemicals in the evolution of Brassicaceae defences," said senior study author Georg Jander, professor at the Boyce Thompson Institute in New York.

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