Ash Dieback - the genome, invasion and evolution of the pathogen

We are using a two-pronged approach to understand Ash Dieback:.

Understanding it’s evolution and population genomics:

A fungal invasion presents just a snapshot of the diversity present in the native range and this makes it difficult to understand a disease using this diversity alone. Further introductions of genetic diversity can be an important determinant for an increase in severity of invasions and this means it is important to understand the native diversity and the pathogen’s propensity to invade and evolve.

Our work on the population genomics of the ash dieback fungus shows us that the European population passed through an extreme bottleneck and quickly expanded into Europe. Analysis of genetic diversity present in Europe suggests that diversity present in the range from which invaders arrived was high. Preliminary estimates show that native genetic diversity is high and is present as non-synonymous diversity at putative effectors. However, this diversity is estimated based on a single wood in Japan whereas the native range also encoumpases large parts of China and Eastern Russia.

Furthermore, in Europe there is a signal of preservation of genetic diversity which indicates that structural genomic variation may operate to facilitate invasion. Fungi are known to employ numerous mechanisms which can preserve genetic variation through a bottleneck and also generate novel diversity. The implications of this structural genetic variation have generally been associated with rapid adaptation to new hosts. However, this variation may also be important in preserving and generating genetic variation through a bottleneck. It is important that we understand these processes in greater detail because as our climate changes and global trade increases we might have to deal with more fungal invasions.

Understanding its association with its Ash Host:

Despite the gloomy prognosis for European ash and the devastation that has already occurred, surveyance of Norfolk ash has provided reason for cautious optimism:

We have identified a large number of adult trees in areas known to contain ash dieback that appear to be significantly healthier than their neighbours. We intend to investigate exactly how these adult trees are surviving in such areas by comparing them with their diseased neighbours: are their natural defence mechanisms resisting infection or can they better tolerate it? In the native range, the fungus can complete its life cycle asymptomatically, i.e., without causing disease in its host – is something similar occurring here?

Furthermore, we have also identified populations of juveniles that seem to be thriving despite local susceptibility to ash dieback. Juvenile trees are usually more vulnerable to pathogens such as ash dieback than adults, so this has obvious implications for conservation because recovery of ash will require juveniles that can survive such an environment until reproductive age. Looking for differences in the genomes of these juveniles compared to local adults may elucidate any genetic component of juvenile survival.