WGS of pooled and individual samples for a GWAS and GP study of Ash susceptibility to Ash Dieback

Ash dieback (ADB), caused by Hymenoscyphus fraxineus, has severely damaged a large proportion of ash trees (Fraxinus excelsior) in continental Europe. In Britain, the disease was found only six years ago in the southeast, and is still spreading. A large-scale screening trial to evaluate ADB damage to provenances of F. excelsior sourced from throughout the British Isles was planted in 2013 in the southeast of England. Trees were scored in 2015 and 2016 for their level of ADB damage observed in field and all provenances contained some healthy trees, so a breeding programme to produce genetically variable native ash tree populations with lower ADB susceptibility may be feasible. Here, we assess the potential of a Genome wide association study (GWAS) and Genomic Prediction (GP) to detect genetic variants associated with resistance to ADB and to predict susceptibility of trees, using a whole genome sequencing pooled DNA approach (pool-seq). We have sequenced 1400 individuals sampled across all provenances: 700 high-susceptibility and 700 low-susceptibility trees. Of these, 1250 are placed in 31 pools with 80x coverage per pool, and 150 are being sequenced as individuals with 20x coverage each. We have identified genes associated with susceptibility to ash dieback and several of these have homologs with known functions relating to disease resistance. The results of this work, with further analysis and testing, will help inform breeding programmes and aid in the selection of resistant provenances around the UK.