Population structure and natural selection in the Chalara ash dieback fungus, Hymenoscyphus pseudoalbidus

查拉拉白蜡树枯病是欧洲白蜡树的一种毁灭性疾病，在过去20年中摧毁了欧洲大陆和斯堪的纳维亚半岛的大量树木。它是由一种真菌引起的，Hymenoscyphus pseudobidus，其无性阶段是Chalara fraxobacterium，因此该疾病的俗称。它于2012年在英国首次被发现，此后在英国和爱尔兰的数百个地点被发现。这种真菌通过风和病树的进口传播有性孢子。白蜡树是一种有性物种，通过多产种子繁殖，因此随着时间的推移，很可能通过自然选择在英国种群中进化出对Chalara的抗性。森林科学家面临的一个重要挑战是加快这一进程，以便火山灰数量能够更快地恢复，最好在几十年内恢复。由于引入的森林病原体的行为可能是不可预测的，因此了解真菌的进化潜力是非常重要的。pseudoalbidus，即真菌中遗传变异与自然环境及其响应自然选择而进化的能力有关的分布方式。我们将获得关于该真菌种群生物学的四个关键方面的信息，这些信息可以应用于商业和天然白蜡种群的育种和管理。pseudoalbidus在英国许多真菌使用VC作为自我/非自我识别系统，因此当遗传上不同的个体彼此相遇时，它们在它们之间形成障碍，这在很大程度上防止每个个体入侵另一个个体所占据的领土。VC屏障的另一个重要特征是它们抑制真菌相互传播双链RNA病毒。研究VC组和两种交配类型的空间分布将使我们能够评估dsRNA病毒在H.其次，我们将研究拟白姑鱼种群遗传变异的空间分布。pseudoalbidus，由两种类型的DNA标记确定。由于真菌似乎是一种生态专性病原体，完全依赖其宿主来完成其生命周期，自然选择最有可能发生在宿主组织内。我们将估计当地人群的遗传多样性水平的H。pseudoalbidus和种群间的变异。这将使我们能够了解通过风和进口树木传播的真菌种群的多样性。然后，我们将调查树木在生命周期的不同阶段的遗传多样性。第三，我们将研究与致病性和生活史相关的性状变异。假白纹鲨这些信息对于理解自然选择导致真菌在自然环境中进化的方式至关重要。这对于培育抗性白蜡树也很重要，因为如果较高水平的致病性涉及真菌在其生殖适合度方面的成本，那么抗性应该在白蜡树种群中更广泛地建立。pseudoalbidus与一种近缘真菌H. albidus，自19世纪世纪以来就在英国被发现，并不被认为是有害的病原体。特别是，我们的研究将旨在了解为什么H。伪白纹伊蚊是一种比H.以及双链RNA病毒从H. albidus转移的潜力。albidus到H. pseudoalbidus，可能有助于Chalara流行的下降。

Chalara ash dieback is a devastating disease of the European ash and has destroyed large numbers of trees in continental Europe and Scandinavia over the last 20 years. It is caused by a fungus, Hymenoscyphus pseudoalbidus, of which the asexual stage is Chalara fraxinea, hence the common name of the disease. It was first identified in the UK in 2012 and has since been found at hundreds of sites throughout Britain and Ireland. The fungus spreads by dispersal of the sexual spores by the wind and by imports of diseased trees.Ash is a sexual species which reproduces by prolific seed production, so over the course of time, it is very likely that resistance to Chalara will evolve in the UK population by natural selection. An important challenge for forest scientists is to accelerate this process so that the ash population can recover more rapidly, ideally within a few decades. As the behaviour of introduced forest pathogens can be unpredictable, it is important to understand the evolutionary potential of the fungus.This project will investigate the ecological genetics and evolutionary potential of H. pseudoalbidus, i.e. the way that genetic variation in the fungus is distributed in relation to the natural environment and its capacity to evolve in response to natural selection. We will obtain information about four key aspects of the population biology of the fungus which can be applied to breeding and management of commercial and natural ash populations.First, we will investigate the distribution of vegetative compatibility (VC) groups in populations of H. pseudoalbidus in the UK. Many fungi use VC as a system of self/non-self recognition so that when genetically different individuals encounter each other, they form barriers between them which largely prevent each individual from invading the territory occupied by the other. Another important feature of VC barriers is that they inhibit fungi from transmitting doubled-stranded RNA viruses to each other. Study of the spatial distribution of VC groups and of the two mating types will allow us to assess the potential for dsRNA viruses to become established in the H. pseudoalbidus population and thus contribute to attenuating the Chalara epidemic.Second, we will study the spatial distribution of genetic variation in H. pseudoalbidus, as determined by two types of DNA marker. As the fungus appears to be an ecologically obligate pathogen which depends entirely on its host to complete its life cycle, natural selection is most likely to takes place within host tissue. We will estimate levels of genetic diversity in local populations of H. pseudoalbidus and variation between populations. This will enable us to understand how diverse are the populations of the fungus which are dispersed by the wind and on imported trees. We will then investigate genetic diversity within trees at different stages of the life cycle. This will provide insights into the operation of natural selection on the fungus within its host.Third, we will investigate variation in traits related to pathogenicity and the life-cycle of H. pseudoalbidus. This information is fundamental to understanding the way that natural selection can cause the fungus to evolve in the natural environment. It is also important for breeding resistant ash trees because if a higher level of pathogenicity involves a cost to the fungus in terms of its reproductive fitness, resistance should become established more widely in the population of ash trees.Lastly, we will investigate the relationship of H. pseudoalbidus to a closely-related fungus, H. albidus, which has known in the UK since the 19th century and is not considered a harmful pathogen. In particular, our research will aim to understand why H. pseudoalbidus is a much more aggressive parasite than H. albidus and what potential there is for dsRNA viruses to be transferred from H. albidus to H. pseudoalbidus, possibly contributing to a decline in the epidemic of Chalara.