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 rD NAs. Restriction patterns of the internal transcribed spacer and porti ons of the nuclear large-subunit rDNA were diagnostic for the two spec ies. Isolates of P. graminicola shared identical restriction patterns. Most isolates of P. arrhenomanes exhibited identical restriction patt erns; however, two genetic subpopulations, each with multiple isolates from a single geographic location, were detected. A phylogenetic anal ysis of the molecular data, which included two additional species, ind icated that P. aphanidermatum was an outgroup and P. myriotylum was mo re closely related to P. arrhenomanes than was P. graminicola. No rest riction site variation was detected among the four species in the nucl ear small-subunit rDNA. Isolates of P. arrhenomanes, including both su bgroups, differed from the other three species by one restriction site in the mitochondrial large-subunit rDNA. A mean of less than five ant heridia per oogonium characterized isolates of P. graminicola, whereas five or more per oogonium characterized most isolates of P. arrhenoma nes. The ranges of isolate means for oogonium/oospore diameter overlap ped between the two species. Discriminant function analysis suggested that the number of antheridia per oogonium is more important than oogo nium/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, misid entifications of isolates of the two species have occurred.