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

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.

This proposal is to research the ecological genetics and evolutionary potential of Hymenoscyphus pseudoalbidus (Hp), the ascomycete fungus which causes Chalara ash dieback (CAD) of the European ash (Fraxinus excelsior). This disease has destroyed large numbers of trees in continental Europe; it appeared in the UK in 2012 and has spread rapidly. The project will study four key aspects of Hp to provide a sound foundation for research on its population biology and disease management.

1. We will investigate the spatial distribution of vegetative compatibility (VC) groups in populations of Hp in the UK and will research the potential for dsRNA viruses to become established in Hp and thus attenuate the CAD epidemic.

2. We will study the spatial distribution of DNA marker variation in Hp, both within and between local populations and with developed lesions within trees. This will provide insights into the potential for natural selection to influence evolution of the fungus.

3. We will investigate variation in pathogenicity and life-cycle traits in Hp. This will provide insights into the potential for evolution of Hp by via natural selection, including responding to enhanced resistance in ash populations. Also, as coevolutionary theory makes the robust prediction that a higher cost of pathogenicity will lead to stronger selection for host resistance, studies of pathogen variation will allow predictions of the extent to which 'natural' resistance will become established in the ash population and how quickly this will happen.

4. We will investigate the genetic and ecological relationship between Hp and a closely related fungus, H. albidus (Ha), which is a non-pathogenic fungus indigenous to the UK. The research will compare pathogenicity traits in Ha and Hp and, through comparative study of their VC and mating sytems, will assess the potential for any dsRNA viruses to be transmitted between these fungal species.

Knowledge about populations of Hymenoscyphus pseudoalbidus (Hp) will inform (1) breeding strategies for resistance to Chalara ash dieback (CAD) in commercial ash trees in the UK and elsewhere and (2) management of natural populations of Fraxinus excelsior, the European ash, to promote evolution of resistance to CAD. All four objectives of this project will contribute to this goal.

Diversity in vegetative compatibility (VC) groups (VCG) is an indicator of the likely success of control methods using hypovirulent Hp infected by dsRNA viruses pathogenic to the fungus. The diversity of VCG in ascomycetes is an important determinant of the rate of transmission of deleterious dsRNA. If VC diversity in Hp is low, both across the UK and in local populations, and if each genet in a developed lesion occupies a large volume of wood, it may be possible to consider using hypovirulent Hp to control CAD by transmitting the virus to other genets. This project will therefore provide information about the likely success of attempts to use hypovirulent strains of Hp to limit the severity of outbreaks of CAD.

The project will also provide information relevant to genetic and pathogenic variation in Hp and processes by which new genotypes are generated and dispersed. Such data on pathogen variation are essential in breeding for disease resistance. If new outbreaks are established by genetically diverse populations of Hp originating from sites where CAD is already established, it will be relatively easy to set up trials to select CAD-resistant ash because each trial will be exposed naturally to a wide range of pathogen genotypes. By contrast, if new outbreaks are established by small sub-samples of established populations, it will be necessary to establish many trial sites because data from any one site may not be typical of responses to Hp in general.

Data on fitness costs and trade-offs will indicate the likely success of resistance breeding and releasing resistant germplasm into the natural environment. An important feature of host-parasite coevolution is that variation in the host influences parasite evolution and vice-versa. Specifically, a higher cost of pathogenicity leads to a higher frequency of resistance in the host. If costs of pathogenicity are high, we can rely on natural selection to re-establish resistance to CAD in the wild population of F. excelsior because resistance genes will be reassorted within genomes of the host, which reproduces sexually and disperses prolific amounts of seed. By contrast, if costs are low, it may be necessary to devise other strategies for reviving the native ash population, such as introduction of resistant germplasm from East Asia.

Research on Hymenoscyphus albidus (Ha), a fungus indigenous to the UK which is closely related to Hp, is more speculative but may in the long term be richly rewarding. Ha may be a source of dsRNA viruses capable of mitigating the damage caused by Hp The potential for using Ha as a source of hypovirulence to control Hp requires not only knowledge of dsRNA in Ha but also of ecological and biological interactions between Ha and Hp.

The project will also contribute to the quality of life in the UK (BBSRC Strategic Priority for Lifelong Health and Wellbeing). A public health study (Donovan et al. 2013, Am. J. Preventative Medicine 44:139-145) showed that access to the natural environment, especially woodland, has great benefits in increasing opportunities for exercise and thus for cardiovascular and respiratory health. Indeed, FR has found that one of the main public concerns about CAD and other tree epidemics is the loss of access to woodland for recreation, a common theme in the value that the general public in the UK places on natural forests. As ash represents 13% of broadleaf tree cover in the UK, recovery from this destructive disease will benefit the public by restoring access to woodland and reducing the risk of injury from falling, diseased trees.