Are sterols landscape limiting nutrients for wild bees in the UK?

甾醇是昆虫必需的营养物质(例如，膜流动性、激素和生理功能)。植物营养组织中的甾醇主要由两种甾醇组成：β-谷甾醇和豆甾醇。然而，在蜜蜂提供营养的花粉中，甾醇结构多样，变异性很大。我们的试点数据显示，来自英国皇家植物园全球采集的代表50多个家庭的100多种植物的25种花粉甾醇之间存在显着差异，特定的甾醇组存在于密切相关的植物中。对蜜蜂的营养需求的研究表明，蜜蜂不能合成类固醇，其组织类固醇的组成由其饮食中的特定类固醇决定。因此，就类固醇而言，蜜蜂就是它们吃的东西。我们还从现有的人工饲料研究中了解到，蜜蜂幼虫的生长和存活对其饮食中的类固醇高度敏感。例如，蜜蜂需要24-亚甲基胆固醇酯(24MC)来饲养幼蜂，当饲料中添加这种类固醇时，它们的发育会得到显著改善。我们的初步数据显示，24MC在提供给幼虫的蜂王浆和一系列受蜜蜂欢迎的植物花粉中含量丰富，例如在蔷薇科中，但在包括菊科在内的许多植物家族中都不存在，这表明蜜蜂避免使用花粉，这表明甾醇组成可能影响蜜蜂对花粉的选择。然而，对野生蜜蜂物种的营养需求几乎一无所知，特别是类固醇，但我们的试点工作表明，不同物种蜜蜂的类固醇谱差异很大，但与这些蜜蜂收集的用作食物的花粉的类固醇谱非常匹配。例如，常春藤花花粉(Hedera Helix)中的甾醇与常春藤蜜蜂的类固醇高度相似，常春藤蜜蜂只在常春藤上觅食花粉。此外，虽然常春藤蜜蜂和常春藤花粉的甾醇是匹配的，但它们与大黄蜂的甾醇有很大的不同，熊蜂的固醇与它们采集的花粉相匹配。这些反过来又不同于红石蜂(双角薇菜)中的甾醇。花粉和蜜蜂甾醇化学之间的这种密切联系强烈地表明，这些其他蜜蜂使用它们收集的花粉中的特定甾醇。我们目前还不知道这些差异是反映了类固醇耐量的差异还是反映了类固醇需求的差异。这个项目将首次测试这一点。无论如何，这些差异将为蜜蜂种群和群落动态提供一个机械基础，因此也是保护多样化的蜜蜂群落及其提供的生态系统服务的宝贵工具。我们将对英国植物花粉和英国野生蜜蜂中的甾醇营养物质进行首次全面评估。利用这些数据，我们将确定能够支持野生蜜蜂物种最大多样性的关键植物物种。这些数据将被整合到花粉资源的景观尺度模型中，以告知英国花粉甾醇的供应情况。将使用基于植物分布测量的景观规模建模来估计景观支持传粉者种群的能力，在现有的花蜜和碳水化合物评估的基础上向前迈进了一大步。我们将把这些数据与对野生蜜蜂物种的评估相匹配，以确定特定的花粉甾醇对特定蜜蜂的重要性。我们将进行有史以来第一次评估类固醇如何影响野生蜜蜂的发育，以充分了解在景观中限制类固醇的潜在成本，蜜蜂物种对特定类固醇的依赖程度，以及这如何影响野生蜜蜂的专门化。我们的数据将用于英国政府25年环境计划和国家传粉者战略的实施，以及更广泛的更新花卉景观以支持蜜蜂的倡议。

Sterols are essential nutrients for insects (e.g., for membrane fluidity, hormones and physiological functions). Sterols in plant vegetative tissues predominantly comprise two sterols; beta-sitosterol and stigmasterol. However, in pollen - from which bees obtain their nutrient provision - sterols are structurally diverse and highly variable. Our pilot data show striking variations among 25 pollen sterols across more than 100 plants representing over 50 families sampled from a global collection at Kew with specific sterol groups occurring in closely related plants. Research on the nutritional requirements of honey bees - the only well-studied bee species in this respect - indicates that they cannot synthesise sterols and that their tissue sterol composition is determined by the specific sterols in their diet. Thus, in terms of sterols bees are what they eat. We also know from existing research on artificial diets that honey bee larval growth and survival is highly sensitive to their dietary sterols. For example, honey bees require 24-methylenecholesterol (24MC) for brood rearing and their development is improved significantly when diets are supplemented with this sterol. Our pilot data shows that 24MC is abundant in royal jelly provisioned to larvae and in a range of plant pollens popular with honey bees e.g., in the Rosaceae but is absent from many plant families including Asteraceae which is avoided for pollen by honey bees suggesting sterol composition could influence pollen choice in bees. However, almost nothing is known about the nutritional needs of wild bee species, especially sterols, but our pilot work shows that the sterol profiles of bees vary dramatically across different species but closely match the sterol profiles of the pollen collected by these bees for food. For example, the sterols from ivy flower pollen (Hedera helix) are highly similar to those of the ivy bee (Colletes hederae), which forages exclusively on ivy for pollen. Furthermore, while the sterols of ivy bees and ivy pollen match they differ dramatically from sterols of bumble bees which have sterols matching the pollen they collect. These in turn differ from the sterols of red mason bees (Osmia bicornis). This close association between pollen and bee sterol chemistry strongly suggests that these other bees use the specific sterols in the pollen they collect. We do not currently know whether these differences reflect differences in sterol tolerances or differences in sterol requirements. This project will test this for the first time. Either way, these differences would provide a mechanistic basis for bee population and community dynamics, and thus also a valuable tool in conserving diverse bee communities and the ecosystem services they provide. We will undertake the first comprehensive assessment of sterol nutrients in pollen of UK plants and in UK wild bees. Using these data, we will identify key plant species that can support the greatest diversity of wild bee species. These data will be integrated into landscape scale models of pollen resources to inform pollen sterol provision in the UK. Landscape-scale modelling based on measurements of plant distributions will be used to estimate the capacity of the landscape to support pollinator populations, taking a major step forward from existing assessments on nectar and carbohydrates. We will match these data with assessments of wild bee species to determine how important specific pollen sterols are for specific bees. We will conduct the first ever assessment of how sterols influence development in wild bees to understand fully the potential cost of sterol limitations in the landscape, how dependent bee species are on specific sterols and how this influences specialisation in wild bees. Our data will inform delivery of the UK government's 25 Year Environment Plan and National Pollinator Strategy and wider initiatives to renew floral landscapes in support of bees.