Evolution and Chemical Ecology of Buzz-Pollinated Flowers and Pollen-Collecting Bees

超过70%的开花植物是由动物授粉的，其中许多植物主要依靠蜜蜂授粉。蜜蜂在自然系统中提供基本的生态系统服务，在几种农作物的生产中发挥着重要作用。然而，蜜蜂种群最近受到了一些环境压力的负面影响，导致对它们的健康和继续提供授粉服务的能力产生了严重的担忧。花和它们的传粉者之间的关系是通过进化时间形成的，有时导致花特征与它们的访客的形态和行为之间的复杂关联。一个这样的例子是蜂鸣授粉的进化，在这种进化中，具有特殊形态的无花蜜的花由蜜蜂授粉，蜜蜂使用振动来提取花粉粒(图1)。令人费解的是，蜜蜂用来提取花粉的振动行为在一些物种(如大黄蜂)中存在，但在其他物种(如蜜蜂)中没有。从植物的角度来看，不相关的蜂花授粉植物在花形上的汇聚是汇聚进化的一个显著例子。蜂鸣授粉的进化还没有被很好地理解。与蜂鸣授粉相关的花形态包括花药将其花粉锁在花成熟后保持关闭的花药壁(不开裂)，花粉仅通过微小的孔口或气孔释放(杀孔的花药)。以前的工作表明，这种锁定机制用于排除贪婪的花卉访客，并为能够解锁这种类型的花药的传粉者子集进行选择，例如通过产生高频振动。因此，蜂鸣授粉进化的一个主要假设是，它代表了奖励花粉的花朵和收集花粉的昆虫之间不断升级的军备竞赛的解决方案。然而，到目前为止，很少有实证研究解决这一假设。从嗡嗡传粉的花中收集花粉提供了一个独特的机会，可以更全面地了解蜜蜂是如何评估花粉奖励的，而与花蜜奖励相比，这一领域是不发达的。嗡嗡传粉的花药将奖赏水平“隐藏”在不透明的花药后面，因此蜜蜂不能在视觉上确定奖赏水平(花中剩余的花粉量)。因此，为了评估奖励水平，蜜蜂可能会依赖于其他线索，包括花粉去除率，以及潜在的化学线索，如气味。化学线索在产蜜物种中的作用已经得到了很好的研究，但我们现在对化学线索在仅有花粉的花中的作用知之甚少。了解花粉信号如何调节传粉者的访问不仅与数千种只开花粉的物种有关，而且可能对改善作物的授粉有实际应用，如番茄，它具有无蜜的、嗡嗡传粉的花。本项目的主要目的是调查蜂传粉花及其采粉访客的进化和化学生态。具体来说，我们将解决以下问题：1.是什么推动了蜂鸣授粉的进化？2.低效传粉者和窃取花粉的人的造访如何影响植物的繁殖？3.蜜蜂如何评估花粉奖励？4.视觉和化学线索在调节传粉者访问无蜜蜂传粉花朵中的作用是什么？5.花如何利用这些视觉和化学线索来操纵蜜蜂的行为？最终，我们的目标是在生态和进化水平上了解奖励花粉的花朵和它们的蜜蜂传粉者之间的联系，以及潜在的共同进化。

More than 70% of flowering plants are animal-pollinated, with many of these plant species relying mainly on bee pollinators. Bees provide essential ecosystem services in natural systems, and play a significant role in the production of several agricultural crops. However, bee populations have recently been negatively affected by a number of environmental stressors, causing serious concern for their health and capacity to continue providing pollination services.The relationship between flowers and their bee pollinators has been shaped through evolutionary time resulting in, sometimes, complex associations between floral traits and the morphology and behaviour of their visitors. One such case is the evolution of buzz pollination, in which nectar-less flowers with specialised morphologies are pollinated by bees that use vibrations to extract pollen grains (Fig. 1). Puzzlingly, the vibrational behaviour used by bees to extract pollen is present in some (e.g., bumblebees) but not in other species (e.g., honeybees). From the plant perspective, the convergence in floral form in unrelated buzz-pollinated plants is a striking example of convergent evolution. The evolution of buzz pollination is not well understood. Floral morphologies associated with buzz pollination include anthers that "lock" their pollen behind anther walls that remain closed after the flower matures (non-dehiscent), and from which pollen is released only through microscopic apertures or pores (poricidal anthers). Previous work has suggested that such a locking mechanism serves to exclude greedy floral visitors and selects for a subset of pollinators that can unlock this type of anthers, for example by producing high frequency vibrations. Therefore one of the main hypothesis for the evolution of buzz pollination is that it represents the escalating solution of an arms race between pollen-rewarding flowers and pollen-collecting insects. However, few empirical studies to date have addressed this hypothesis. The collection of pollen from buzz-pollinated flowers presents a unique opportunity to understand more generally how bees assess pollen rewards, and area that compared to nectar rewards is poorly developed. Anthers of buzz-pollinated flowers "hide" reward levels behind opaque anthers, and therefore bees cannot visually establish reward level (amount of pollen left in the flower). In order to assess reward level, the bee may thus rely on other clues including pollen removal rates, and potentially, chemical cues such as scent. The role of chemical cues has been well studied in nectar-producing species, but we now very little about the role of chemical cues in pollen-only flowers. Understanding how pollen cues mediate pollinator visitation is not only relevant for the thousands of species with pollen-only flowers, but may also have practical applications to improve the pollination of crops such as tomatoes which possess nectarless, buzz-pollinated flowers. The main aim of this project is to investigate the evolution and chemical ecology of buzz-pollinated flowers and their pollen collecting visitors. Specifically, we will address the following questions: 1. What drives the evolution of buzz pollination?2. How does visitation by inefficient pollinators and pollen thieves affect plant reproduction?3. How do bees assess pollen rewards?4. What is the role of visual vs. chemical cues in mediating pollinator visitation of nectarless, buzz-pollinated flowers?5. How can flowers use these visual and chemical cues to manipulate bee behaviour?Ultimately our goal is to understand at both ecological and evolutionary levels the association, and potentially co-evolution, between pollen-rewarding flowers and their bee pollinators.