The ash dieback invasion of Europe was founded by two genetically divergent individuals

Abstract

Accelerating international trade and climate change make pathogen spread an increasing concern. Hymenoscyphus fraxineus, the causal agent of ash dieback, is a fungal pathogen that has been moving across continents and hosts from Asian to European ash. Most European common ash trees (Fraxinus excelsior) are highly susceptible to H. fraxineus, although a minority (~5%) have partial resistance to dieback. Here, we assemble and annotate a H. fraxineus draft genome which approaches chromosome scale. Pathogen genetic diversity across Europe and in Japan, reveals a strong bottleneck in Europe, though a signal of adaptive diversity remains in key host interaction genes. We find that the European population was founded by two divergent haploid individuals. Divergence between these haplotypes represents the ancestral polymorphism within a large source population. Subsequent introduction from this source would greatly increase adaptive potential of the pathogen. Thus, further introgression of H. fraxineus into Europe represents a potential threat and Europe-wide biological security measures are needed to manage this disease.

Fungal colonization patterns in necrotic rootstocks and stem bases of dieback‐affected Fraxinus excelsior L.

“…This bi‐allelic pattern has already been found and described in more detailed population genetic studies of H. fraxineus (Burokiene et al, ; Gross, Hosoya, & Queloz, ; Schoebel, Botella, Lygis, & Rigling, ). It points to a strong founder effect due to the limited initial number of two‐parent genotypes upon the introduction of H. fraxineus in Europe (McMullan et al, ).…”

“…Junker, Vries, Eickhorst, and Schulz () demonstrated that each investigated H. fraxineus isolate has its own exoenzyme profile. The European population of H. fraxineus went through a genetical bottleneck and virulence of co‐infecting pathogens can increase with increasing genetic relatedness (Buckling & Brockhurst, ; McMullan et al, ).…”

“…The European population of H. fraxineus went through a genetical bottleneck and virulence of co-infecting pathogens can increase with increasing genetic relatedness (Buckling & Brockhurst, 2008;McMullan et al, 2018).…”

Life at the edge – the cytology and physiology of the biotroph to necrotroph transition in Hymenoscyphus fraxineus during lesion formation in ash

“…Like other hemibiotrophs, including P. infestans , the H. fraxineus genome encodes an impressive array of plant cell wall‐degrading enzymes (Stenlid et al ., ; McMullan et al ., ). Apart from the highly localized degradation of primary and secondary cell walls at penetration sites, there was very little sign of cell wall‐degrading enzyme activity during the stages of biotrophic and necrotrophic invasion examined, despite the formation of large lesions.…”

“…Numerous genes encoding potential virulence factors have been identified. Both Stenlid et al (2017) and McMullan et al (2018) pointed out the high frequency of cell wall-degrading hydrolases and cytochrome P450s possibly involved in the metabolism of toxic by-products of lignin degradation. Genes encoding enzymes in pathways involved in the production of potentially phytotoxic metabolites are also present.…”