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

灰顶枯病自1992年以来一直是欧洲灰的破坏性疾病，2012年首次在英国出现。导致这种疾病的真菌原产于东亚，据信是在20世纪80年代抵达东北欧的。鉴于白蜡树作为自然界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舒适价值以及其在景观和生态系统功能中的服务价值，特别是减少洪水，顶枯病流行对英国产生了严重影响。通过减少农田径流和巩固河流和运河河岸。虽然在受影响严重的地区，大多数树木受到严重损害，少数人显然不太容易受到影响，提出了一个重大的希望，这种遗传变异可能会导致长期恢复灰在英国和欧洲普遍。这种树种是高度可变的，在英国的人口已经从欧洲大陆的大幅度分化。我们的假设是，某些化学物质（次生代谢物）赋予抵抗某些昆虫的损害，但增加了灰的敏感性。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顶枯易感性低的品系。这个名为JENNIFER的标本集将用于研究多个性状之间的关系，以了解白蜡树枯梢病的生态背景。对枯梢病敏感性低的白蜡树可能对枯梢病有部分抗性，因此它们的叶子不太容易被真菌感染，或者具有一种生长和发育的形式，导致真菌孢子的暴露量较低，从而促进疾病的逃避。该项目的目标1是确定英国白蜡树对枯梢病的低敏感性的多样性。我们将测量疾病的严重程度，在复制的田间试验暴露于自然感染，调查电阻的组成部分，估计贡献的电阻和逃避低敏感性，并测试，如果在英国树木的枯梢病的遗传抗性已经偏离了以前在丹麦的研究。由项目合作伙伴开发的小规模研究枯梢病真菌感染的方法大大提高了研究抗性变化的能力。在早期的研究中，发现低水平的环烯醚萜苷（一种次级代谢产物）与抗枯梢病有关，但白蜡树还含有许多其他次级代谢产物。目的二是检验白蜡中次生代谢产物的多样性与抗枯梢病性的差异有关的假设。我们将使用非靶向代谢物谱来鉴定最强烈区分抗性和敏感克隆的感染和未感染叶片的化学特征，联合收割机和转录组分析来确定响应于感染的次生代谢物变化的遗传基础，鉴别分子特征水平的时空变化，目的3是检验这样一个假设，即次生代谢产物控制白蜡树对顶梢枯病和对不同植食性昆虫的不同反应。这将通过测试枯梢病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.