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

开花植物会产生花蜜来吸引传粉者。因此，鲜花花蜜经常含有次生代谢物，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“有毒”花蜜的假说包括：保护花朵免受掠夺花蜜的动物的侵袭，鼓励特殊授粉和防止微生物生长，但花蜜次生代谢物也可能是叶片化学防御的意外后果。花蜜次生代谢物可以对传粉者的健康和行为产生重大影响。动物可以避免观赏含有“有毒”花蜜的花朵，或者可以减少观赏的次数或每朵花所花费的时间。食用花蜜次生代谢物会减少传粉者的营养同化和移动性，在某些情况下，还会导致死亡。考虑到它们对传粉者的影响，花蜜次生代谢物被预测会对授粉和植物繁殖产生重大影响，然而这一点很少被检验。 我提议的研究将讨论花蜜化学在调节植物-传粉者相互作用和植物繁殖成功中的作用。研究重点将放在已经成为研究植物化学生态学的模式系统的乳草(Asclepias Spp)上。乳草产生的次生代谢物称为卡登内酯，对食草动物有很强的毒性。在马利筋花蜜中也发现了这些化合物，但它们对传粉者和授粉的影响目前尚不清楚。我的研究计划的目标将是确定黄花菜中的花蜜香石竹内酯是否会通过阻止传粉者、抑制授粉和减少种子产量而对植物造成生态成本。 为了研究乳草花蜜化学对授粉的影响，我的研究小组将结合实验室分析、温室试验和田间操纵实验。首先，我们讨论了花蜜锦鸡儿内酯是如何改变大黄蜂的觅食行为的，熊蜂是常见的乳草传粉者。在实验室里，我们将测试花蜜锦鸡儿内酯如何影响传粉者的喜好，以及花蜜糖浓度或花蜜体积的变化是否会影响这种喜好。然后，我们将研究草食对花蜜化学和授粉的直接影响，使用帝王毛虫。我们将允许毛虫以温室中的马利筋植物为食，随后将释放大黄蜂工作人员访问马利筋花，以寻找由于草食性而导致的花蜜化学、传粉者行为和授粉成功的变化。最后，我们将在田野中种植马利筋，并观察毛虫捕食草食如何影响野生传粉者的行为和授粉。我们将使用分子技术来跟踪这些地块中种子的父子关系，以评估草食对花蜜化学的影响是否会改变基因在植物种群中的移动方式。 授粉对野生生态系统和农业生态系统都是至关重要的，了解植物和传粉者之间的关系对于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.