GENETIC-RELATIONSHIPS AMONG AUSTRALIAN AND NORTH-AMERICAN ISOLATES OF PHYTOPHTHORA-MEGASPERMA F-SP GLYCINEA ASSESSED BY MULTICOPY DNA PROBES

Genetic relationships among five Australian and five North American isolates of Phytophthora megasperma f. sp. glycinea were explored using 10 random multicopy clones from a genomic DNA library of P. m. glycinea. Total DNA was cut with eight restriction endonucleases, giving 80 probe X enzyme combinations for each P. m. glycinea isolate. Analysis of restriction fragment length polymorphisms (RFLPs) generated by each probe revealed two size classes of RFLP difference between isolates. A large class was observed only with two probes, each distinguishing a single isolate of apparent genetic distance, d (expected frequency of nucleotide substitution), of about 0.05 from the other nine isolates. These two probes provide highly diagnostic DNA fingerprints for particular isolates and should be useful to track further closely related isolates in population studies. A small class, detected by the remaining eight probes, was characterized by values of d of about 0.01 or less between isolates. Cluster analysis using polymorphisms pooled from these eight probes generated a phenogram that clearly separated a U.S. isolate of race 7 from the other nine isolates (average d of 0.0066). These other isolates formed a discrete cluster of four U.S. isolates (average d within 0.0026) and a discrete cluster of five Australian isolates (average d within 0.0022), with these two clusters being separated by an average d of 0.0031. The close relationships between these geographically separate groups suggest that Australian P. m. glycinea isolates are derived from recent ancestors common to the U.S. isolates.