TAXONOMIC AND PHYLOGENETIC ANALYSES OF 10 PYTHIUM SPECIES USING ISOZYME POLYMORPHISMS

Isozyme analysis of 13 enzymes was used to compare 204 isolates of the following 10 Pythium species from diverse geographic locations: P. aphanidermatum, P. arrhenomanes, P. deliense, P. graminicola, P. heterothallicum, P. irregulare, P. myriotylum, P. spinosum, P. torulosum, and P. ultimum. Morphologically distinct species could be differentiated by banding patterns. However, morphologically similar species, such as P. aphanidermatum and P. deliense or P. arrhenomanes and P. graminicola, showed similar or identical banding patterns. Cluster and principal components analyses showed that isolates within morphological species generally clustered together, but considerable variation existed within certain species. Intraspecific variation could not be attributed to the host or habitat from which the isolates were originally obtained. With some species, intraspecific clusters and geographic origin were closely correlated. It was concluded that Pythium species could not be characterized by isozyme banding patterns. These results are interpreted in light of current theories in population genetics and evolutionary biology with regard to life histories and host-parasite interactions.