Stenlid, J., Elfstrand, M., Cleary, M., Ihrmark, K., Karlsson, M., Davydenko, K. and Brandström Durling, M. 2017. Genomes of Hymenoscyphus fraxineus and Hymenoscyphus albidus Encode Surprisingly Large Cell Wall Degrading Potential, Balancing Saprotrophic

   In Europe, an epidemic is currently occurring on common ash (Fraxinus excelsior). The disease, commonly known as ash dieback, is the result of a biological invasion by the causal Helotialean fungus Hymenoscyphus fraxineus Baral, Queloz, Hosoya. This study describes the genomes of H. fraxineus and H. albidus, a native non-pathogenic sister species to H. fraxineus. The Hymenoscyphus sp. genomes harbour similar and extensive Cell Wall Active Enzyme (CAZYme) repertoires, they appear better at degrading cellulose than e.g. Botrytis but has similar pectin-degrading capacities. In planta, the pathogenic H. fraxineus showed higher gene expression than H. albidus of two of the pectin degrading enzymes, consistent with a higher disruption of primary cell walls and possibly leading to a stronger host reaction. Based on SignalP and Phobious annotations, we identified 2160 and 2006 secreted genes in H. fraxineus and H. albidus, respectively. This is almost twice as many as for most other Helotialean fun-gi. Two small secreted proteins were transcribed in H. fraxineus, one being a cerato-platinin like protein with a putative role in pathogenicity. No small secreted proteins were detected in the H. albidus transcriptome. It has been suggested that fungal metal-lopeptidases, can target and degrade non-structural defense proteins in planta. We found that the Hymenoscyphus genomes en-code more metallopeptidases than other Helotialean species. In conclusion, the prolonged saprotrophic growth phase on shed ash leaves of H. fraxineus and H. albidus has probably shaped the genomes. Both genomes are highly similar and have CAZYme profiles similar to saprotrophic fungi. The relatively small differences between the two Hymenoscyphus spp. in gene expression are likely indicative of their differential interaction patterns with the host tree F. excelsior.

Keywords: Ash dieback, Hymenoscyphus fraxineus, Hymenoscyphus albidus, Fraxinus excelsior, cell wall degrading enzymes, small secreted proteins, metalloproteinases, invasive species