Phylogeny of five fungus-like protoctistan Phytophthora species, inferred from the internal transcribed spacers of ribosomal DNA.
Ribosomal DNA variation was used to study evolutionary relationships among 5 Phytophthora spp. On the basis of morphological and ecological characteristics, 4 of these species, P. palmivora, P. megakarya, P. capsici and P. citrophthora were once thought to be related, while variation within a species was extensively studied in a fifth, outgroup species, P. cinnamomi known, on the basis of ecological, isoenzyme, and mitochondrial DNA studies, to be variable. Internal transcribed spacer regions (ITS I, between the 18S and 5.8S rDNAs; and ITS II, between the 5.8S and 25S rDNAs) from 27 isolates of these 5 species were analysed by polymerase chain reaction amplification and direct sequencing. Intraspecific variability was undetected or low. Interspecific nucleotide difference was 0.3-14.6%, and comparisons of variable regions permitted the evaluation of phylogenetic relationships among species. Both neighbour-joining and parsimony analysis of ITS variability supported a close relationship between cocoa isolates of P. capsici and P. citrophthora and a common lineage for P. palmivora and P. megakarya. Large distance values were estimated between P. cinnamomi and the other species. Inferred relationships based on ITS variability are compared with those based on other characters.