Evolution of azole resistance and thermal adaptation in global populations of Parastagonospora nodorum

Organisms become adapted to their local environment through natural selection. This process is easily observed in agroecosystems where human activities result in rapid evolution of fungal plant pathogens towards higher levels of virulence and reduced sensitivity to fungicides. Different pathogen populations may respond differently to the same human-imposed selection. A sustainable management of crop diseases requires improved knowledge of the genetic basis of the observed phenotypes in pathogen populations. A first step to obtain this knowledge is to differentiate the evolutionary forces acting on quantitative traits by using a Q_ST /F_ST comparison. Q_ST describes the distribution of the phenotypic variation in quantitative traits within and among populations while F_ST describes the distribution of neutral genetic variation within and among populations. The joint comparison of Q_ST and F_ST indexes enables evaluation of the causes of adaptive divergence in quantitative traits that reflect local adaptation of the organism. Parastagonospora nodorum is a globally distributed necrotrophic pathogen causing the Stagonospora nodorum leaf and glume blotch (SNB) disease on wheat. We conducted a Q_ST /F_ST analysis using 176 isolates sampled from field populations in Switzerland, South Africa, Australia, China, Oregon, New York, Texas and Iran. Q_ST values were obtained by phenotyping all isolates for azole sensitivity (EC₅₀) and thermal adaptation (growth rate at different temperatures). Full genome sequences of all isolates were used to calculate F_ST based on neutral single nucleotide polymorphisms (SNPs). We also conducted a genome-wide association study (GWAS) for the same isolates to identify genomic regions and candidate genes associated with the observed variation for fungicide sensitivity and thermal adaptation.