Collaborative Research: Macroevolution of a group of plant secondary defense compounds (pyrrolizidine alkaloids) in the dogbane and milkweed flowering plant family (Apocynaceae)

已知植物可以合成大约20万种不同的次级代谢物，这些化合物对它们的细胞代谢不是必需的。这些植物次生化学物质是药物（如阿司匹林）、杀虫剂（如除虫菊酯）、工业化合物（如乳胶）、香料（如薄荷）和毒药（如士的宁）的重要来源。对于植物来说，这些化学物质是抵御食草动物，特别是昆虫的基本防御手段。然而，随着时间的推移，这些防御的功效并不是静态的，因为食草动物适应了它们的宿主植物防御，进化出了解毒机制，甚至选择它们来防御捕食者。因此，一种曾经是有价值的防御手段的化学物质可能会成为一种负担，其生产也会损失。本项目将研究夹竹桃科植物夹竹桃属和马利筋属中一组植物防御次生化合物（吡咯里西啶生物碱）的进化。夹竹桃科植物次生化合物种类繁多，其中许多具有药用价值。夹竹桃科植物中的吡咯里西啶生物碱对非适应性食草动物具有高度毒性，但对将其储存在组织中以防御捕食者的专业昆虫几乎没有影响。研究人员将在夹竹桃科中广泛取样，并使用DNA序列数据重建该科内部的进化关系。由此产生的进化树将被用作一个框架，以重建组内吡咯里西啶生物碱的进化，并更好地了解这些化学物质，产生它们的植物和它们的食草动物之间的关系。本科生将接受各种分子，进化和生物化学方法的培训。该项目的成果也将被纳入三个机构的本科课程，其中包括两个主要的本科机构。这项研究将为医学上重要的夹竹桃科植物家族内的进化关系提供新的见解，可以用作该家族内更广泛问题的框架。费城自然科学院的一个展览将向公众广泛传播这项研究的结果，以及植物和昆虫防御、化学和共同进化等更广泛主题的信息。在这个研究项目中要测试的总体假设提出，吡咯里西啶生物碱的多样性将在高度利用昆虫免疫这些化合物的影响的植物谱系中较低。为了验证这一假设，研究人员将使用靶向富集和基因组略读（Hyb-Seq）的组合来测序ca。850个低拷贝核基因座、全叶绿体基因组和核核糖体基因，来自APSA进化枝（夹竹桃科内的谱系，包括大多数已知的马利筋蝴蝶宿主植物）的287个属中的每一个的2个物种，以产生所需的良好解析和支持的家族的进化史和年表。然后，研究人员将对整个APSA进化枝中叶片中的吡咯里西啶生物碱进行系统调查，并重建吡咯里西啶生物碱表型和潜在生物合成途径的进化，以及物种子集中的表达和功能分析。然后将夹竹桃科中吡咯里西啶生物碱生物合成的起源与Danainae的吡咯里西啶生物碱共选择的年龄进行比较，并测试吡咯里西啶生物碱的损失是否与宿主植物状态相关。

Plants are known to synthesize about 200,000 distinct secondary metabolites, chemical compounds that are not essential for their cellular metabolism. These plant secondary chemicals are important sources of medicines (e.g. aspirin), pesticides (e.g. pyrethrin), industrial compounds (e.g. latex), flavors (e.g. mint), and poisons (e.g. strychnine). To plants, these chemicals are essential defenses against herbivores, particularly insects. The efficacy of these defenses is not static over time, however, as herbivores adapt to their host plants defenses, evolving mechanisms to detoxify them or even co-opt them for their own defense against their predators. Thus a chemical that was once a valuable defense may become a liability, and its production lost. This project will investigate the evolution of a group of plant defense secondary compounds (pyrrolizidine alkaloids) in the dogbane and milkweed family (Apocynaceae). Plant secondary compounds in the Apocynaceae are diverse, with many having medicinal properties. Pyrrolizidine alkaloids in members of the Apocynaceae are highly toxic to non-adapted herbivores, but have little effect on specialist insects that store them within their tissues for defense against predators. Researchers will sample broadly across the Apocynaceae and reconstruct evolutionary relationships within the family using DNA sequence data. The resulting evolutionary tree will then be used as a framework to reconstruct the evolution of pyrrolizidine alkaloids within the group and better understand the relationship between these chemicals, the plants that produce them, and their herbivores. Undergraduate students will be trained in diverse molecular, evolutionary and biochemistry methods. Results from this project will also be integrated into undergraduate classes at three institutions, including two primarily undergraduate institutions. The research will provide new insights into evolutionary relationships within the medically important Apocynaceae plant family that can be used as a framework for broader questions within the family. An exhibit at the Academy of Natural Sciences in Philadelphia will broadly disseminate results from this research, and information on the broader topics of plant and insect defense, chemistry, and co-evolution, to the public. The overarching hypothesis to be tested in this research project proposes that the diversity of pyrrolizidine alkaloids will be lower in plant lineages that are highly exploited by insects immune to the effects of these compounds. To test this hypothesis researchers will use a combination of targeted enrichment and genome skimming (Hyb-Seq) to sequence ca. 850 low-copy nuclear loci, whole chloroplast genomes, and nuclear ribosomal genes, from 2 species of each of the 287 genera of the APSA clade (the lineage within Apocynaceae that includes most known milkweed butterfly host plants) to generate the required well-resolved and supported phylogeny and chronogram of the family. Researchers will then conduct a systematic survey of pyrrolizidine alkaloids occurrence in leaves across the APSA clade and reconstruct the evolution of both the pyrrolizidine alkaloids phenotype and of the underlying biosynthetic pathway, as well as expression and functional analyses in a subset of species. The origin of pyrrolizidine alkaloid biosynthesis in Apocynaceae will then be compared to the age of pyrrolizidine alkaloid co-option by Danainae, and test if the loss of pyrrolizidine alkaloids is correlated with host plant status.