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

以植物为食的昆虫被认为是对它们所吃的植物组织中的化学防御进行自然选择。这里正在进行的研究是在乳草植物上进行的，乳草植物含有一种非常有效的毒素，labrivaline，它在几个物种中已经进化到高浓度。当高表达时，labrivaline抑制帝王蝶和大型马利筋虫的必需运输酶（钠-钾泵）。这些专门的昆虫害虫生物化学修改labrivaline毒性较低的化合物和螯合这些产品为自己的防御。帝王蝶和种子虫对钠泵的高度耐受性和生化修饰表明，金龟子是马利筋和其食草动物之间相互进化的中心。这项工作旨在通过生理和进化分析来揭示这种相互作用的机制。这项研究也是了解植物和害虫之间自然和农业相互作用的模型。最后，马利筋-帝王蝶系统很适合公众参与，因为它的标志性地位，保护关注的人口下降，并在学校课程中使用。作为该项目的一部分，研究人员正在为K-12学校开发模块，并通过演讲和博客参与公共宣传。马利筋属植物是几种专门的食草性昆虫类群的食物来源，它们之间的共同进化相互作用已被深入研究，并阐明了新的机制。研究人员最近发现了一种乳草，其中含有高水平的labrivaline，这是一种剧毒的cardenaline，其形式不寻常（含有氮（N）和硫（S）的环），这很可能是一种升级形式的昂贵防御。目前的工作旨在调查更多的含N cardenolides在马利筋的发育，并试图破译其生理影响和解毒独立的食草动物谱系。在植物-草食动物谱系中反复进化的进攻-防御机制可能会揭示共同进化的普遍性。三个目标将解决共同进化的两个方面：1）确定含N的cardenoprotein防御被两种专业马利筋昆虫，君主毛虫和种子错误处理，解毒和储存的机制。这将采用分离化合物的喂养试验，生理测定和行为研究，使用专家以及CRISPR编辑的果蝇，这些果蝇具有耐受cardenolides的基因替代。2)评估解毒和螯合的成本和效益，使用定量遗传学加上饲养和化学测定的君主和种子错误的马利筋植物含氮和不含强心内酯。遗传相关性将揭示生活史成本和与差异螯合相关的特征。3)检查模式的含N的防御进化马利筋在整个生育期，测试有关N-限制，防御分配的假设，和cardenopropylene浓度和效力之间的权衡。有一种假说认为，后来分化的乳草已经进化出了更有效但浓度更低的强心内酯，这是理论预测和系统发育模式的建议，但从未经过严格的检验。总的来说，这些目标将通过表征新的植物防御和专业食草动物的反适应生理生态学，解毒和螯合毒素的遗传变异，该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响进行评估，被认为值得支持审查标准。

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.