HIGH GENETIC SIMILARITY AMONG POPULATIONS OF PHAEOSPHAERIA-NODORUM ACROSS WHEAT CULTIVARS AND REGIONS IN SWITZERLAND

Phaeosphaeria nodorum was sampled from nine wheat fields across a 30-k m transect representing three geographical regions in Switzerland to d etermine the scale of genetic differentiation among subpopulations. Th ree different wheat cultivars were sampled three times to determine wh ether differences in host genotype correlated with differences among c orresponding pathogen populations. Seven restriction fragment length p olymorphism (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 RF LP loci exhibited significant differences in allele frequencies across the nine subpopulations. Gene diversity within all subpopulations was high (H-T = 0.51), but only 3% of the total genetic variation was dis tributed among the nine subpopulations. When subpopulations were group ed according to geographical region or host cultivar, less than 1% of the genetic variation was distributed among groups, suggesting widespr ead gene flow and the absence of pathogen adaptation to specific wheat cultivars. Tests for gametic equilibrium within subpopulations and ac ross the entire Swiss population supported the hypothesis of random ma ting.