Genetic Variation in Homothallic and Hyphal Swelling Isolates of Pythium ultimum var. ultimum and P. utlimum var. sporangiferum. David M. Francis. Department of Vegetable Crops, University of California, Davis 95616-8746 U.S.A. Mary F. Gehlen, and Dina A. St.Clair. Department of Vegetable Crops, University of California, Davis 95616-8746 U.S.A. MPMI 7:766-775. Accepted 15 August 1994. Copyright 1994 The American Phytopathological Society.

Genetic variation in a collection of 22 Pythium ultimum isolates was analyzed using restriction fragment length polymorphisms (RFLPs), random amplified polymorphic DNA (RAPD), and sequence characterized amplified regions (SCARs) as genetic markers. Qualitative evidence for the occurrence of sexual outcrossing in the field, asexual mechanisms affecting variation, and differences in aggressiveness between isolates was found. Codominant SCAR and RFLP markers detected multiple alleles in several isolates. Genetic analysis of F1 progeny from a cross indicates that heterozygosity is one cause of multiple alleles and contributes to genetic variation. Segregation analysis of F2 progeny fit diploid expectations and supported the use of the molecular markers for phenetic analysis. One isolate contained three alleles at one locus suggesting 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 matrixes 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 homothallic oomycete are not genetically distinct.