Molecular taxonomy of Phytophthora megasperma based on mitochondrial and nuclear DNA polymorphisms

Abstract

One hundred and ninety-four isolates of P. megasperma from a wide range of host plants and of worldwide origin were subjected to molecular characterization based on numerical analyses of their mitochondrial DNA (mtDNA) restriction fragment length polymorphisms (RFLPs). Of the nine distinct molecular groups identified, four represented groups of isolates from single host plants. In addition to the previously recognized host-specific taxa isolated from alfalfa, clover and soybean, a previously unidentified, fourth group contained only isolates from asparagus. An additional mtDNA group included isolates recovered primarily from Douglas fir. Most of the remaining isolates, from a wide range of woody and herbaceous hosts, were placed into another four groups. Of these, groups A, B and C were closely related; group D was not. A number of additional isolates had unique mtDNA patterns. An intergroup comparison confirmed the genetic distinctness of the nine groups and intergroup genetic distances were sometimes large. In addition, isolates representing specific molecular groups of the P. cryptogea/P. drechsleri ‘species complex’ ( Mills, Forster & Coffey, 1991 ) were interspersed between those of the P. megasperma groups. The mtDNA RFLP findings were largely supported by an analysis based on chromosomal DNA RFLPs. However, amongst the three mtDNA groups A, B and C containing isolates from a broad host range (BHR protein group; Hansen et al., 1986 ), only group A was resolved.