Molecular convergence analyses identify candidate genes for low susceptibility to the ash dieback pathogen

The detection of molecular convergence can be an effective approach for uncovering loci underlying adaptive traits that have evolved independently between lineages. Within the genus Fraxinus (ash trees) low susceptibility to the fungal pathogen Hymenoscyphus fraxineus, the causative agent of the ash dieback disease (ADB) epidemic, is observed in three independent lineages of known or plausible natural hosts. Here, we seek to identify the genetic basis of this trait, which is key to the future survival of Fraxinus excelsior populations in Europe. We analysed 4,300 protein-coding loci for amino acid convergence between lineages with low susceptibility. After filtering for potential false positives, we find 62 genes that have a signal of excess convergence between Fraxinus lineages with low ADB susceptibility. Eleven of these loci have additional evidence for a role in defence against fungal pathogens, with a further 17 linked to more general immunity or defence, or to other functions relevant to the response against ADB such as cell wall biogenesis. Two of the loci we detect have previously been identified as differentially expressed in F. excelsior in response to infection by H. fraxineus in controlled inoculations. The set of candidate loci discovered here complements those reported by previous genomic studies of F. excelsior, and can be targeted in efforts to mitigate the devastation caused by this deadly disease by informing breeding programmes involving hybridisation and marker-assisted back-crossing.