Dramatic recent changes in the population genetic diversity of avirulence gene AvrStb6 in the pathogenic fungus Zymoseptoria tritici

Zymoseptoria tritici, the causal agent of Septoria tritici blotch (STB), is among the most important global pathogens of wheat. Recently, the genetic factors of a previously characterised wheat-Z. tritici gene-for-gene interaction have been identified: the wheat receptor-like kinase Stb6 and the Z. tritici secreted protein AvrStb6. Although historic, global collections of Z. tritici have been used to analyse the genetic diversity of AvrStb6, it remains unknown which of the identified AvrStb6 haplotypes confer virulence on wheat possessing Stb6 and the frequency of virulence in contemporary populations. Here, we re-sequenced AvrStb6 from recent field populations of Z. tritici collected between 2014 and 2017 from Western Europe, Turkey, USA, South America, and Australia. AvrStb6 was present in all isolates tested, with a small number of haplotypes encoding the same protein isoform conditioning virulence on Stb6-containing wheat, predominating in many parts of the world. The avirulence isoform of AvrStb6 was not detected. These findings contrast with those from previous studies of global Z. tritici populations (Brunner & McDonald, 2018; Zhong et al. 2017), suggesting a significant shift in the global Z. tritici populations, towards a single Stb6 resistance-breaking isoform of Avrstb6, has taken place in recent years. We hypothesise that selection pressure imposed by Stb6 haplotypes in many modern commercial wheat cultivars may be responsible for this shift. Brunner P & McDonald B (2018) Molecular Plant Pathology 19: 1836-46; Saintenac C et al. (2018) Nature Genetics 50: 368-74; Zhong Z et al. (2017) New Phytologist 214: 619-31