Non-parent of Origin Expression of Numerous Effector Genes Indicates a Role of Gene Regulation in Host Adaption of the Hybrid Triticale Powdery Mildew Pathogen.

Powdery mildew is an important disease of cereals. It is caused by one species, , which is divided into each of which is highly specialized to one host. Recently, a new form capable of growing on triticale () has emerged through hybridization between wheat and rye mildews ( and , respectively). In this work, we used RNA sequencing to study the molecular basis of host adaptation in . We analyzed gene expression in three isolates, two isolates and two isolates and identified a core set of putative effector genes that are highly expressed in all . We also found that the genes differentially expressed between isolates of the same form as well as between different were enriched in putative effectors. Their coding genes belong to several families including some which contain known members of mildew avirulence () and suppressor () genes. Based on these findings we propose that effectors play an important role in host adaptation that is mechanistically based on interactions. We also found that gene expression in the hybrid is mostly conserved with the parent-of-origin, but some genes inherited from showed a -like expression. Finally, we identified 11 unambiguous cases of putative effector genes with hybrid-specific, non-parent of origin gene expression, and we propose that they are possible determinants of host specialization in triticale mildew. These data suggest that altered expression of multiple effector genes, in particular and related factors, might play a role in mildew host adaptation based on hybridization.