Tissue-specific plant toxins and adaptation in a specialist root herbivore

Abstract

In coevolution between plants and insects, reciprocal selection often leads to phenotype matching between chemical defense and herbivore offense. Nonetheless, it is not well understood whether distinct plant parts are differentially defended and how herbivores adapted to those parts cope with tissue-specific defense. Milkweed plants produce a diversity of cardenolide toxins and specialist herbivores have substitutions in their target enzyme (Na+/K+-ATPase), each playing a central role in milkweed-insect coevolution. The four-eyed milkweed beetle (Tetraopes tetrophthalmus) is an abundant toxin-sequestering herbivore that feeds exclusively on milkweed roots as larvae and less so on milkweed leaves as adults. Accordingly, we tested the tolerance of this beetle's Na+/K+-ATPase to cardenolide extracts from roots versus leaves of its main host (Asclepias syriaca), along with sequestered cardenolides from beetle tissues. We additionally purified and tested the inhibitory activity of dominant cardenolides from roots (syrioside) and leaves (glycosylated aspecioside). Tetraopes' enzyme was threefold more tolerant of root extracts and syrioside than leaf cardenolides. Nonetheless, beetle-sequestered cardenolides were more potent than those in roots, suggesting selective uptake or dependence on compartmentalization of toxins away from the beetle's enzymatic target. Because Tetraopes has two functionally validated amino acid substitutions in its Na+/K+-ATPase compared to the ancestral form in other insects, we compared its cardenolide tolerance to that of wild-type Drosophila and CRISPR-edited Drosophila with Tetraopes' Na+/K+-ATPase genotype. Those two amino acid substitutions accounted for >50% of Tetraopes' enhanced enzymatic tolerance of cardenolides. Thus, milkweed's tissue-specific expression of root toxins is matched by physiological adaptations in its specialist root herbivore.