GENETIC-VARIATION IN HOMOTHALLIC AND HYPHAL SWELLING ISOLATES OF PYTHIUM-ULTIMUM VAR ULTIMUM AND P-ULTIMUM VAR SPORANGIFERUM

Genetic variation in a collection of 22 Pythium ultimum isolates was a nalyzed using restriction fragment length polymorphisms (RFLPs), rando m amplified polymorphic DNA (RAPD), and sequence characterized amplifi ed regions (SCARs) as genetic markers. Qualitative evidence for the oc currence of sexual outcrossing in the field, asexual mechanisms affect ing variation, and differences in aggressiveness between isolates was found. Codominant SCAR and RFLP markers detected multiple alleles in s everal isolates. Genetic analysis of F-1 progeny from a cross indicate s that heterozygosity is one cause of multiple alleles and contributes to genetic variation. Segregation analysis of F-2 progeny fit diploid expectations and supported the use of the molecular markers for phene tic analysis. One isolate contained three alleles at one locus suggest ing that polyploidy, aneuploidy or heterokaryosis may also contribute to genetic variation. Phenetic analysis using UPGMA clustering of Nei' s distance calculated from RFLP data, UPGMA clustering of similarity m atrixes calculated from RAPD data, and principle component analysis of RAPD data revealed no clustering of the three morphological types of Pythium ultimum (var, ultimum, var, sporangiferum, and group HS). Our results suggest that the three morphological variants of this homothal lic oomycete are not genetically distinct.