Retaining the Ashes: The potential for ash populations to be restored following the dieback epidemic

自1992年以来，白蜡树枯梢病一直是欧洲白蜡树的一种破坏性疾病，2012年在英国首次发现。导致这种疾病的真菌原产于东亚，据信是在20世纪80年代传入欧洲东北部的。鉴于白蜡树作为自然中重要物种的生态作用、其作为木材的经济价值、其在城市和公园中的美化价值，以及其在景观美化和生态系统功能方面的服务价值，特别是通过减少农田径流和巩固河流和运河堤防来减少洪水，白蜡树的流行对英国产生了严重影响。虽然受影响严重地区的大多数树木都受到了严重破坏，但有一小部分树木显然不那么容易受到影响，这给人们带来了很大的希望，即这种基因变异可能会导致英国和欧洲灰烬的长期恢复。这个项目的目的是了解为什么这种变异在欧洲白蜡树中发生了进化。这种树种是高度可变的，英国的种群与欧洲大陆的种群有很大的差异。我们的假设是，某些化学物质（次生代谢物）赋予白蜡树抵抗某些昆虫损害的能力，但增加了白蜡树对枯死病的易感性。作为本项目和未来研究的资源，我们从英国收集了328种不同的白蜡树基因型，包括许多即使在强烈的疾病压力下也对枯死病具有低易感性的品种。这个被称为JENNIFER的集合将用于研究多种性状之间的关系，以了解白蜡树枯死的生态环境。对枯死病易感性低的白蜡树可能对枯死病有部分抗性，因此它们的叶子不容易被真菌感染，或者有一种生长发育形式，导致较少接触真菌孢子，从而促进疾病逃逸。该项目的目标1是描述英国白蜡树对枯死病低易感性的多样性。我们将在暴露于自然感染的重复田间试验中测量疾病的严重程度，调查抗性的组成部分，估计抗性和逃避低易感性的贡献，并测试英国树木对枯死病的遗传抗性是否与先前在丹麦研究的遗传抗性不同。由项目合作伙伴开发的研究枯死真菌感染的小规模方法，大大提高了研究抗性变异的能力。在早期的研究中，发现低水平的环烯醚萜苷（一类次生代谢物）与抗枯死有关，但白蜡树还含有许多其他次生代谢物。目的2是验证灰分中多种次生代谢物与枯梢抗性变异相关的假设。我们将使用非靶向代谢物分析来识别最强烈区分抗性和易感克隆的感染和未感染叶片的化学特征，结合代谢物和转录组分析来确定响应感染的次级代谢物变化的遗传基础，表征鉴别分子特征水平的时间变化，并将其与JENNIFER小组中的抗性联系起来。目的3是验证次生代谢物控制白蜡树对枯死病和多种草食性昆虫的不同反应的假设。这将通过测试枯死抗性和关键代谢物水平的变化是否与昆虫摄食、生长和繁殖的变化有关，以及通过评估JENNIFER ash克隆在自然条件下对无脊椎动物攻击的敏感性来完成。

Ash dieback has been a destructive disease of European ash since 1992 and was first seen in the UK in 2012. The fungus which causes this disease is native to East Asia and is thought to have arrived in north-east Europe during the 1980s. The dieback epidemic has serious implications for the UK given the ecological role of ash as a keystone species in nature, its economic value as timber, its amenity value in cities and parks, and its service value in landscaping and ecosystem functions, notably reduction of flooding by reducing water run-off from fields and consolidating river and canal banks. While most trees in heavily affected areas are severely damaged, a small minority are clearly less susceptible, raising a significant hope that this genetic variation might lead to the long-term recovery of ash in the UK and Europe generally.This project aims to understand why this variation in dieback-resistance has evolved in European ash. This tree species is highly variable and the population in the UK has diverged substantially from that in continental Europe. Our hypothesis is that certain chemicals (secondary metabolites) confer resistance to damage by certain insects but increase the susceptibility of ash to dieback. As a resource for this project and for future research, we have collected 328 diverse ash genotypes from the UK, including numerous lines which have had low susceptibility to dieback even under intense disease pressure. This collection, known as JENNIFER, will be used to research the relationship between multiple traits to understand the ecological context of ash dieback.Ash trees with low susceptibility to dieback may either have partial resistance to dieback, so their leaves are less easily infected by the fungus, or have a form of growth and development which leads to lower exposure to spores of the fungus, thus promoting disease escape. Objective 1 of the project is to characterise the diversity of low susceptibility to dieback in UK ash. We will measure disease severity in replicated field trials exposed to natural infection, investigate components of resistance, estimate contributions of resistance and escape to low susceptibility, and test if genetic resistance to dieback in UK trees has diverged from that studied previously in Denmark. Small-scale methods of studying infection by the dieback fungus, developed by the project partners, greatly increase the capacity to study variation in resistance.In earlier research, low levels of iridoid glycosides, a class of secondary metabolite, were found to be associated with resistance to dieback but ash also contains many other secondary metabolites. Objective 2 is to test the hypothesis that diverse secondary metabolites in ash are associated with variation in dieback-resistance. We will use untargeted metabolite profiling to identify chemical features which most strongly discriminate infected and uninfected leaves of resistant and susceptible clones, combine metabolite and transcriptome analysis to determine the genetic basis of changes in secondary metabolites in response to infection, characterise temporal variation in levels of discriminant molecular features, and relate them to resistance in the JENNIFER panel.Objective 3 is to test the hypothesis that secondary metabolites control contrasting responses of ash to dieback and to diverse herbivorous insects. This will be done by testing if variation in dieback-resistance and levels of key metabolites are associated with variation in insect feeding, growth and reproduction, and by assessing the susceptibility of the JENNIFER ash clones to attack by invertebrates in natural conditions.