The chemical ecology of multispecies interactions: how secondary metabolites mediate pollination and herbivory

开花植物产生花蜜来吸引传粉者。因此，花蜜经常含有次级代谢物，这是一种通常用于保护植物免受食草动物侵害的有毒化合物，这是不寻常的。解释这种所谓的“有毒”花蜜的假说包括保护花朵免受掠夺花蜜的动物的侵害，鼓励专门的授粉和防止微生物的生长，但花蜜的次级代谢物也可能是叶子中化学防御的意外后果。花蜜次生代谢物对传粉者的健康和行为有显著影响。动物可以避免访问含有“有毒”花蜜的花朵，或者可能减少访问次数或花在每朵花上的时间。食用花蜜次生代谢物会减少传粉媒介的营养吸收和流动性，在某些情况下会导致死亡。鉴于其对传粉媒介的影响，花蜜次生代谢物被预测对传粉和植物繁殖有重要影响，但这一点很少被研究。我提出的研究将解决花蜜化学在植物-传粉者相互作用和植物繁殖成功中的作用。研究重点为乳草（Asclepias spp），它已成为研究植物化学生态学的典范系统。乳草产生次生代谢物，被称为硬核内酯，这对食草动物来说是非常有毒的。这些化合物也在马利筋花蜜中发现，但它们对传粉者和传粉者的影响目前尚不清楚。我的研究计划的目标是确定乳草中的花蜜果仁内酯是否会通过阻止传粉者、抑制传粉和减少种子产量而给植物带来生态成本。为了研究马利筋花蜜化学对授粉的影响，我的研究小组将采用实验室分析、温室试验和操作田间试验相结合的方法。首先，我们讨论花蜜果仁如何改变大黄蜂的觅食行为，大黄蜂是乳草的常见传粉者。在实验室中，我们将测试花蜜硬核如何影响传粉者的偏好，以及花蜜糖浓度或花蜜体积的变化是否会影响这种偏好。然后，我们将利用帝王蝶毛虫来研究草食对花蜜化学和授粉的直接影响。我们将允许毛毛虫以温室里的马利筋植物为食，随后将释放大黄蜂工蜂去拜访马利筋花，寻找花蜜化学的变化，传粉者的行为和传粉成功率。最后，我们将在田间种植乳草，并观察毛毛虫的食草性如何影响野生传粉媒介的行为和授粉。我们将使用分子技术来跟踪这些地块内种子的父系关系，以评估草食对花蜜化学的影响是否会改变基因在植物群体中的移动方式。授粉对野生和农业生态系统都至关重要，了解植物与传粉者之间的关系对保护生态系统功能和生物多样性至关重要。综上所述，这些实验将为花蜜化学在构建陆地生态系统中复杂的多物种相互作用中的作用提供新的见解。

Flowering plants produce nectar to attract pollinators. It is therefore unusual that floral nectar frequently contains secondary metabolites, noxious compounds commonly used to defend plants from herbivores. Hypotheses to explain this so-called “toxic” nectar include protecting flowers from nectar robbing animals, encouraging specialized pollination and preventing microbial growth, but nectar secondary metabolites may also be an unintended consequence of chemical defenses in leaves. Nectar secondary metabolites can have significant effects on pollinator health and behaviour. Animals can avoid visiting flowers with “toxic” nectar or may reduce the number of visits or length of time spent per flower. Consuming nectar secondary metabolites can reduce nutrient assimilation and mobility in pollinators and, in some cases, cause death. Given their effects on pollinators, nectar secondary metabolites are predicted to have significant consequences for pollination and plant reproduction, however this has rarely been examined. My proposed research will address the role of nectar chemistry in mediating plant-pollinator interactions and plant reproductive success. Research will focus on milkweeds (Asclepias spp), which have become a model system for studying plant chemical ecology. Milkweeds produce secondary metabolites called cardenolides that can be very toxic to herbivores. These compounds are also be found in milkweed nectar, but their effects on pollinators and pollination are currently unknown. The goal of my research program will be to determine whether nectar cardenolides in milkweeds incur an ecological cost to plants by deterring pollinators, inhibiting pollination and reducing seed production. To investigate the effects of milkweed nectar chemistry on pollination, my research group will use a combination of laboratory assays, greenhouse trials and manipulative field experiments. First, we address how nectar cardenolides alter the foraging behaviour of bumble bees, common pollinators of milkweeds. In the laboratory, we will test how nectar cardenolides affect pollinator preference and whether variation in nectar sugar concentration or nectar volume influence this preference. We will then examine the direct effects of herbivory on nectar chemistry and pollination using monarch caterpillars. We will allow caterpillars to feed on milkweed plants in the greenhouse and will subsequently release bumble bee workers to visit milkweed flowers, looking for changes in nectar chemistry, pollinator behaviour and pollination success due to herbivory. Finally, we will plant milkweeds in field plots and look at how herbivory by caterpillars affects wild pollinator behaviour and pollination. We will use molecular techniques to track paternity of seeds within these plots to assess whether the effects of herbivory on nectar chemistry change how genes move within the plant population. Pollination is essential to both wild and agricultural ecosystems and understanding the relationships between plants and pollinators is critical to preserving ecosystem function and biodiversity. Taken together, these experiments will provide new insight into the role of nectar chemistry in structuring complex multispecies interactions in terrestrial ecosystems.