How wallflowers evolved a complementary pair of plant defenses

Flower of the Rhaetian wallflower (Erysimum rhaeticum) under attack by a caterpi

Flower of the Rhaetian wallflower (Erysimum rhaeticum) under attack by a caterpillar. Image: Tobias Züst

A new study led by Tobias Züst from the Institute of Plant Sciences of the University of Bern shows that a pair of complementary chemical defenses evolved independently in wallflowers, shaped by co-evolution with local insects.

A pair of chemicals used by wallflowers and their kin to ward off predators have evolved to complement each other, with one targeting generalist herbivores and the other targeting specialized herbivores that have become resistant to the generalist defense.

Plants are engaged in an ongoing arms race with the creatures that eat them. They evolve defenses to deter plant eaters, while their herbivores evolve counter-defenses. The new study, published today in eLife, reveals details of the evolutionary chemical arms race in the wallflower genus Erysimum, a group of flowering plants in the mustard family Brassicaceae.

The defense lines of the wallflowers

The first line of defense in all Brassicaceae plants are chemicals called glucosinolates, which are activated when predatory creatures nibble on the plants. More recently, wallflowers have developed a second line of defense by producing chemicals called cardenolides to deter plant-eating creatures that evolved defenses to glucosinolates.

"Studying how these two defenses 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," explains lead author Tobias Züst, PhD, Research Associate at the Institute of Plant Sciences, University of Bern.

A detailed family tree for 48 wallflower species

To do this, Züst and his team sequenced the genome of the wormseed wallflower, a short-lived wild mustard. They next created a detailed family tree for this plant and 47 other wallflower species and compared the diversity and abundance of glucosinolates and cardenolides across these species. They found that the two defenses varied independently of each other, and that closely related, geographically co-occurring species shared similar cardenolide traits, but not glucosinolate traits. This is likely a result of separate selective pressures acting on each defense.

"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 says. "Instead, these two types of chemicals appear to complement each other and do not serve redundant functions." Züst adds that the emergence of cardenolides corresponds with an explosion in the number of wallflower species, which suggests this second complementary defense may have allowed these plants to succeed and diversify into new habitats.

"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 defenses," concludes senior author Georg Jander, Professor at the Boyce Thompson Institute in Ithaca, New York, US.