Symposium: “Convergently evolved toxic secondary metabolites in plants drive the parallel molecular evolution of insect resistance”

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Georg Petschenka, Vera Wagschal, Michael von Tschirnhaus, Alexander Donath, and Susanne Dobler

Phytomyza hellebori females probe leaves of Helleborus foetidus with their ovipositor (small dots) before egglaying and imbibe the extruding plant juices. Both females and their leaf mining larvae (in brown areas) survive the diet of toxic cardiac glycosides unharmed thanks to amino acid substitutions in the target site of their sodium pump.
(Credit: Samuel Waldron)

Plants have to defend themselves against a plethora of herbivorous insects. Over evolutionary times, reciprocal interactions between plants and insects resulted in convergence on all levels, reaching from toxic compounds on the plants’ side and behavioral countermeasures on the herbivores’, down to the molecular level causing specific mechanisms of resistance. Based on a molecular phylogeny of leaf mining flies (family Agromyzidae), the authors report five independent colonization events of cardiac glycoside containing host plants from four botanical families. These toxins owe their name to the therapeutic effect they exert – at very low concentration – in human patients with heart conditions. However, at the concentrations present in the host plants, cardiac glycosides block a ubiquitous animal ion carrier, the sodium pump or Na,K-ATPase. In all of the fly species that are specialized on cardiac glycoside–containing plants, the authors detected amino acid substitutions in the sodium pump that may render the enzyme less sensitive to the detrimental effect of the toxins. This five-fold convergent evolution of toxin resistance adds to the authors’ previous evidence that target site insensitivity of the sodium pump is a common response to the challenge presented by dietary cardiac glycosides and leads to highly predictable evolution at the level of amino acids. Read the Article