Isozyme variation in heterothallic species and related asexual isolates of Pythium

A collection of 63 heterothallic and asexual (sexually sterile) isolates were compared by isozyme analysis, vegetative characters, and their ability to produce oospores in paired cultures. The collection included isolates identified as Pythium heterothallicum, Pythium intermedium, Pythium macrosporum, Pythium splendens, Pythium sylvaticum, and Pythium tracheiphilum, and several unidentified isolates. Isolates were separated into 41 multilocus genotypes as defined by 11 isozyme loci. The data were analyzed with previously published data for Pythium ultimum, and dendrograms were obtained from unweighted pair-group method, arithmetic average cluster analysis. Forty-six genotypes (including nine of P. ultimum) grouped into seven clusters and three solitary genotypes. Six clusters contained known isolates of the aforementioned species and the seventh contained P. ultimum. Isolates within both the P. heterothallicum and P. sylvaticum clusters produced oospores when grown in paired cultures. Two of three isolates in the P. splendens cluster produced oospores, but none of the isolates in the P. intermedium and P. macrosporum clusters produced oospores when grown in pairs. Genotypes belonging to the P. intermedium cluster separated into two sister groups that had morphologically different vegetative bodies. There were no consistent vegetative characteristics to distinguish the species P. macrosporum, P. heterothallicum, and P. sylvaticum. Isozyme analysis provides an effective means of identifying Pythium species that do not readily produce oospores in culture.