Differential processing of host plant toxins by insect herbivores as a driver of multi-trophic interactions

植物毒素在昆虫与植物的共同进化中起着重要作用，因为它们是食草昆虫必须通过适应来克服的障碍。一般认为，昆虫对某些植物毒素产生抗性是为了利用植物作为食物资源。此外，许多昆虫储存植物毒素是为了防止捕食者和寄生蜂（隔离）。值得注意的是，某些抗性机制与植物毒素的吸收直接相关，即捕食者和寄生蜂也可以选择抗性机制。我们发现乳草虫（异翅目：乳草科）预先存在的抗性机制和隔离能力有利于与某些植物产生与该群体祖先寄主家族相同的植物毒素的进化新关联。因此，防御物质的获取，而不仅仅是新的饮食资源的定植，可以介导植物与昆虫的特殊相互作用。乳草虫Spilostethus saxatilis从秋藏红花（Colchicum autumn）中吸收了大量有毒的微管蛋白抑制剂秋水仙碱。这是特别有趣的，因为它已经能够隔离心脏糖苷，并且像所有乳草虫一样具有抗心脏糖苷的Na+/K+- atp酶。此外，saxatilis对秋水仙碱具有抗性机制，而这一机制在其近亲pandurus中是缺失的。在这里，我们将研究秋水仙碱抗性的基础，并讨论秋水仙碱封存和秋水仙碱抗性的演变。我们将测试假设a)在分子水平，b)在功能水平，以及c)在系统发育和有机体背景下。利用现有的转录组和基因组数据，我们将研究saxatilis是否已经进化出对秋水仙碱不敏感的靶点。为此，我们将分析S. saxatilis的α-和β-微管蛋白编码基因，用于秋水仙碱结合位点的氨基酸交换和基因复制，并将其与其他乳草虫进行比较。此外，我们将在体外研究saxatilis微管蛋白，以检测其对秋水仙碱的亲和力是否降低。同时，我们将进行竞争性秋水仙碱拮抗剂的注射实验，以检测沙氏葡萄球菌是否对这些物质产生交叉抗性。最后，我们将在系统发育的背景下进行比较研究，以了解秋水仙碱的吸收、口服摄入（如在S. pandurus中观察到的）和吸收的秋水仙碱的进化。我们的研究将允许的机制基础和演变的一种新的隔离综合征得到全面了解。

Plant toxins play a major role for insect-plant coevolution as they represent a barrier that herbivorous insects must overcome through adaptations. It is generally assumed that insects develop resistance to certain plant toxins for exploiting plants as a dietary resource. In addition, however, many insects store plant toxins for protection against predators and parasitoids (sequestration). Notably, certain resistance mechanisms are directly related to the sequestration of plant toxins, i.e. predators and parasitoids can also select for resistance mechanisms. We showed that in the milkweed bugs (Heteroptera: Lygaeinae) pre-existing resistance mechanisms and the ability to sequester favored evolutionarily novel associations with certain plants that produce the same plant toxins as the ancestral host family of this group. Accordingly, the acquisition of defense substances and not just the colonization of novel dietary resources can mediate specialized plant-insect interactions. The milkweed bug Spilostethus saxatilis sequesters large amounts of the toxic tubulin inhibitor colchicine from autumn crocus (Colchicum autumnale). This is of special interest since it is already able to sequester cardiac glycosides and has a cardiac glycoside-resistant Na+/K+-ATPase like all milkweed bugs. In addition, S. saxatilis possesses a resistance mechanism to colchicine, which is missing in the closely related S. pandurus. Here, we will examine the basis of colchicine resistance in S. saxatilis and address the evolution of colchicine sequestration and colchicine resistance. We will test hypotheses a) at the molecular level, b) at the functional level, and c) in a phylogenetic and organismic context. Using existing transcriptome and genome data, we will investigate whether S. saxatilis has evolved target site insensitivity to colchicine. For this purpose, we will anaylze the α- and β-tubulin encoding genes of S. saxatilis for amino acid exchanges in the colchicine binding site as well as for gene duplications and compare them with other milkweed bug species. Furthermore, we will study S. saxatilis tubulin in vitro to test if it shows reduced affinity to colchicine. At the same time, we will carry out injection experiments with competitive colchicine antagonists to test if S. saxatilis shows cross-resistance to these substances. Finally, we will conduct comparative studies in a phylogenetic context to understand the evolution of colchicine sequestration, resistance to orally ingested (as observed in S. pandurus) and sequestered colchicine. Our investigations will allow the mechanistic basis and the evolution of a novel sequestration syndrome to be understood comprehensively.