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

自1992年以来，白蜡树枯萎病一直是欧洲白蜡树的一种破坏性疾病，2012年首次在英国出现。导致这种疾病的真菌原产于东亚，被认为是在20世纪80年代到达欧洲东北部的。鉴于白蜡树在自然界中的关键物种的生态作用、其作为木材的经济价值、其在城市和公园中的宜人价值以及其在景观美化和生态系统功能方面的服务价值，尤其是通过减少农田径流和巩固河流和运河堤岸来减少洪水，枯萎病疫情对英国具有严重的影响。虽然受影响严重地区的大多数树木都受到了严重破坏，但一小部分树木显然不那么容易受到影响，这让人们燃起了巨大的希望，即这种基因变异可能会导致英国和欧洲的灰树长期恢复。该项目旨在了解为什么这种抗枯病的变异会在欧洲白蜡树中进化。这种树种变异很大，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)对某些昆虫的损害具有抵抗力，但增加了白蜡树对枯萎病的易感性。作为这个项目和未来研究的资源，我们从英国收集了328个不同的ASH基因类型，其中包括许多即使在强烈的疾病压力下也对枯萎病具有低敏感性的品系。这个被称为珍妮弗的收集将被用来研究多个性状之间的关系，以了解白蜡树枯萎病的生态背景。对白蜡树枯萎病敏感性较低的白蜡树可能对枯萎病具有部分抗性，因此它们的叶片不太容易被真菌感染，或者具有某种生长发育形式，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萎病低敏感性的多样性。我们将在暴露在自然感染下的重复田间试验中测量疾病严重程度，调查抗性成分，估计抗性的贡献和对低敏感性的逃逸，并测试英国树木对枯萎病的遗传抗性是否与以前在丹麦的研究有所不同。由项目合作伙伴开发的研究枯萎病菌感染的小规模方法极大地提高了研究抗药性变异的能力。在早期的研究中，低水平的环烯醚萜糖苷(一种次生代谢物)被发现与对枯萎病的抗性有关，但灰分还含有许多其他次生代谢物。目的2是验证灰分中不同的次生代谢物与抗枯病的变异有关的假说。我们将使用非靶向代谢物图谱来确定最强区分抗病和感病无性系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次生代谢物在感染反应中变化的遗传基础，表征鉴别分子特征水平的时间变化，并在Jennifer小组中将它们与抗性联系起来。目标3是测试次生代谢物控制白蜡树对枯萎和对不同草食性昆虫的对比反应的假设。这将通过测试抗枯病能力和关键代谢物水平的变化是否与昆虫取食、生长和繁殖的变化有关，以及通过评估珍妮弗白蜡树克隆在自然条件下对无脊椎动物攻击的敏感性来实现这一点。

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.