Molecular and morphological comparison of Pythium arrhenomanes and P. graminicola

Worldwide collections of Pythium arrhenomanes and P. graminicola were studied morphologically and by examining variation in PCR-amplified rDNAs. Restriction patterns of the internal transcribed spacer and portions of the nuclear large-subunit rDNA were diagnostic for the two species. Isolates of P. graminicola shared identical restriction patterns. Most isolates of P. arrhenomanes exhibited identical restriction patterns; however, two genetic subpopulations, each with multiple isolates from a single geographic location, were detected. A phylogenetic analysis of the molecular data, which included two additional species, indicated that P. aphanidermatum was an outgroup and P. myriotylum was more closely related to P. arrhenomanes than was P. graminicola. No restriction site variation was detected among the four species in the nuclear small-subunit rDNA. Isolates of P. arrhenomanes, including both subgroups, differed from the other three species by one restriction site in the mitochondrial large-subunit rDNA. A mean of less than five antheridia per oogonium characterized isolates of P. graminicola, whereas five or more per oogonium characterized most isolates of P. arrhenomanes. The ranges of isolate means for oogonium/oospore diameter overlapped between the two species. Discriminant function analysis suggested that the number of antheridia per oogonium is more important than oogonium/oospore diameter in separating the two species. The rDNA results indicate that P. arrhenomanes and P. graminicola are distinct species. However, due to similar morphology and intraspecific variation, misidentifications of isolates of the two species have occurred.