Escalation in coevolution: Characterization of novel plant toxins, how specialist herbivores cope, and tests of phylogenetic intensification in plant defense potency

Cornell University, Ithaca NY

Investigators

Abstract

Insects that feed on plants are thought to impose natural selection for chemical defenses in the plant tissues they eat. The research being conducted here is on milkweed plants, which contains a highly potent toxin, labriformin, which has evolved to high concentrations in in several species. When highly expressed, labriformin inhibits an essential transport enzyme (the sodium-potassium pump) of the monarch butterfly and the large milkweed bug. These specialized insect pests biochemically modify labriformin to less toxic compounds and sequester these-products for their own defense. The monarch and seed bug’s highly tolerant sodium pump and biochemical modifications point to labriformin being at the center of reciprocal evolution between milkweed and its herbivores. This work serves to unravel the mechanisms of the interaction through physiological and evolutionary analyses. The research also serves as a model for understanding both natural and agricultural interactions between plants and pests. Finally, the milkweed - monarch system lends itself well to public engagement because of its iconic stature, declining populations of conservation concern, and use in school curricula. As part of this project, the researchers are developing modules for K-12 schools and engaging in public outreach through presentations and blogs. Among milkweed plants which are fed upon by several specialized lineages of insect herbivores, coevolutionary interactions have been well-studied and novel mechanisms have been elucidated. Researchers recently discovered a milkweed that contains high levels of labriformin, a highly toxic cardenolide which is unusual in form (containing a ring with nitrogen (N) and sulfur (S)), which could well be an escalated form of costly defense. The current work seeks to investigate additional N-containing cardenolides across the milkweed phylogeny and seek to decipher their physiological impacts and detoxification in independent herbivores lineages. Repeatedly evolving offense-defense mechanisms in plant-herbivore lineages will likely reveal generalities of coevolution. Three objectives will address both sides of coevolution: 1) Determine the mechanisms by which N-containing cardenolide defenses are processed, detoxified, and stored by two specialist milkweed insects, monarch caterpillars and seed bugs. This will employ feeding trials with isolated compounds, physiological assays, and studies of behavior, using both specialists as well as CRISPR-edited Drosophila that have genetic substitutions for tolerating cardenolides. 2) Assess the costs and benefits of detoxification and sequestration using quantitative genetics coupled with feeding and chemical assays of monarchs and seed bugs on milkweed plants with and without N-containing cardenolides. Genetic correlations will reveal life-history costs and traits associated with differential sequestration. 3) Examine the pattern of N-containing defense evolution among milkweeds across the phylogeny, testing hypotheses about N-limitation, defense allocation, and trade-offs between cardenolide concentration and potency. One hypothesis, that later diverging milkweeds have evolved more potent but less concentrated cardenolides, was predicted by theory and suggested by a phylogenetic pattern, but never rigorously tested. Overall these objectives will advance understanding of coevolutionary mechanisms by characterizing novel plant defenses and the physiological ecology of counter-adaptation in specialist herbivores, genetic variation in detoxification and sequestration of toxins, and plant defense evolution at the macroevolutionary scale. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.