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对它们所吃的植物组织的化学防御施加了自然选择。这里进行的研究是针对马利筋植物的，马利筋植物含有一种强效毒素，labriformin，这种毒素在几个物种中已经进化到高浓度。当高表达时，labriformin抑制了帝王蝶和大马利筋虫的一种必需的运输酶（钠钾泵）。这些特殊的害虫通过生物化学方法将labformin转化为毒性较低的化合物，并将这些产物隔离起来以供自己防御。黑脉金斑蝶和种虫的高耐受性钠泵和生化修饰表明，乳草草和它的食草动物之间的互惠进化的中心是乳草甲素。这项工作旨在通过生理和进化分析揭示这种相互作用的机制。这项研究还为理解植物和害虫之间的自然和农业相互作用提供了一个模型。最后，马利筋-帝王蝶系统由于其标志性的地位，保护关注的数量下降，以及在学校课程中的使用，使其非常适合公众参与。作为该项目的一部分，研究人员正在为K-12学校开发模块，并通过演讲和博客进行公众宣传。马利筋植物以几种特殊的昆虫食草动物为食，共同进化的相互作用已经得到了很好的研究，并阐明了新的机制。研究人员最近发现一种马利筋含有高水平的labriformin，这是一种剧毒的菊石内酯，它的形式很不寻常(含有氮（N)和硫(S)的环），这很可能是一种昂贵的升级防御形式。目前的工作旨在研究马利筋系统发育中额外的含氮硬核素，并试图破译它们在独立草食动物谱系中的生理影响和解毒作用。在植物-食草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)确定两种专业马利筋昆虫，君主毛虫和种虫，处理，解毒和储存含氮硬核内酯防御的机制。这将使用分离化合物的喂养试验、生理分析和行为研究，使用专家和crispr编辑的果蝇，这些果蝇具有基因替代耐受性。2)利用定量遗传学方法，结合对帝王蝶和种虫在含氮和不含氮的马利筋植物上的摄食和化学分析，评估解毒和封存的成本和效益。遗传相关性将揭示与差异封存相关的生活史成本和特征。3)研究乳草在系统发育过程中的含氮防御进化模式，验证关于n限制、防御分配和硬核内酯浓度与效力权衡的假设。有一种假说认为，后来分化的乳草进化出了效力更强但浓度更低的硬核，这种假说曾被理论预测过，并由一种系统发育模式提出，但从未得到严格的验证。总的来说，这些目标将通过描述新的植物防御和特殊食草动物的反适应生理生态学，毒素解毒和隔离的遗传变异以及宏观进化尺度上的植物防御进化来促进对共同进化机制的理解。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.