Genomic basis of European ash tree resistance to ash dieback fungus

Populations of European ash trees (Fraxinus excelsior) are being devastated by the invasive alien fungus Hymenoscyphus fraxineus, which causes ash dieback (ADB). We sequenced whole genomic DNA from 1250 ash trees in 31 DNA pools, each pool containing trees with the same ADB damage status in a screening trial and from the same seed-source zone. A genome-wide association study (GWAS) identified 3,149 single nucleotide polymorphisms (SNPs) associated with low versus high ADB damage. Sixty-one of the 192 most significant SNPs were in, or close to, genes with putative homologs already known to be involved in pathogen responses in other plant species. We also used the pooled sequence data to train a genomic prediction model, cross-validated using individual whole genome sequence data generated for 75 healthy and 75 damaged trees from a single seed source. Using the top 20% of our genomic estimated breeding values from 200 SNPs, we could predict tree health with over 90% accuracy. We infer that ash dieback resistance in F. excelsior is a polygenic trait that should respond well to both natural selection and breeding, which could be accelerated using genomic prediction.