Doomed by popularity: The broad use of the Pm8 resistance gene in wheat resulted in hypermutation of the AvrPm8 gene in the powdery mildew pathogen

Summary The Pm8 resistance gene against the powdery mildew disease has been introgressed from rye into wheat as part of a large 1BL.1RS chromosomal translocation. Due to its high agronomic value, this translocation has seen continuous global use since the 1960’s on large growth areas, even after Pm8 resistance was overcome. This allows studying the effect of long and widespread resistance gene use on a pathogen population. Using genome wide association studies in a global population of wheat mildew isolates, we identified the avirulence effector AvrPm8 specifically recognized by Pm8 . Haplovariant mining in the global population revealed 17 virulent haplotypes of the AvrPm8 gene that grouped into two categories. The first one from geographically diverse regions comprised two single amino acid polymorphisms at the same position in the AvrPm8 protein, which we confirmed to be crucial for recognition by Pm8. The second category consisted of numerous destructive mutations to the AvrPm8 open reading frame. Most individual gain-of-virulence mutations were found in geographically restricted regions, indicating they occurred recently as a consequence of the frequent Pm8 use. We conclude that both standing genetic variation as well as locally occurring new mutations contributed to the global breakdown of the Pm8 resistance gene introgression.