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High Genetic Similarity Among Populations of Phaeosphaeria nodorum Across Wheat Cultivars and Regions in Switzerland

First and second authors: Institute for Plant Sciences, Phytopathology Group, Federal Institute of Technology, Zürich, Switzerland; third author: TerraGen Diversity Inc., Suite 300, 2386 East Mall, University of British Columbia, Vancouver B.C. V6T 1Z3 Canada; and fourth author: Department of Plant Pathology and Microbiology, Texas A&M University, College Station 77845-2132

Phaeosphaeria nodorum was sampled from nine wheat fields across a 30-km transect representing three geographical regions in Switzerland to determine the scale of genetic differentiation among subpopulations. Three different wheat cultivars were sampled three times to determine whether differences in host genotype correlated with differences among corresponding pathogen populations. Seven restriction fragment length polymorphism (RFLP) loci and one DNA fingerprint were assayed for each of the 432 isolates in the collection. DNA fingerprints differentiated 426 unique genotypes. Though absolute differences were small, five RFLP loci exhibited significant differences in allele frequencies across the nine sub-populations. Gene diversity within all subpopulations was high (H_T = 0.51), but only 3% of the total genetic variation was distributed among the nine subpopulations. When subpopulations were grouped according to geographical region or host cultivar, less than 1% of the genetic variation was distributed among groups, suggesting widespread gene flow and the absence of pathogen adaptation to specific wheat cultivars. Tests for gametic equilibrium within subpopulations and across the entire Swiss population supported the hypothesis of random mating.

Additional keywords: gametic disequilibrium, genetic structure, population genetics, Septoria nodorum, Stagonospora nodorum.