Phylogenetic relationships among some Pythium species inferred from sequence analysis of the mitochondrially encoded cytochrome oxidase II gene

Abstract

The phylogenetic relationships of 67 isolates representing 24 species of Pythium were assessed by sequence alignment of 684 bp of the mitochondrially-encoded cytochrome oxidase II gene. Sequence differences among species ranged 1.6–14.7% substitutions. The species grouped into three major clades that were, in a general sense, reflective of zoosporangial or hyphal swelling morphology. Clade I contained species with globose to spherical zoosporangia or spherical hyphal swellings. Clade II was comprised of four species, only one of which produced zoosporangia (P. ultimum var. sporangiiferum) with the remaining species producing only spherical hyphal swellings. Species with filamentous to lobulate zoosporangia were in clade III. Pythium oligandrum, a species that produces subglobose zoosporangia with interconnecting filamentous parts was intermediate between species with inflated to lobulate filamentous zoosporangia and species that produced spherical to globose zoosporangia (clades I and II). Two species that produced globose zoosporangia (P. pulchrum and P. rostratum) grouped together separately from the other clades, as did P. nunn. The evolutionary relationships among species obtained by analysis of cox II DNA sequence data corresponds well with the genomic location of this mitochondrially encoded gene as well as the location of the nuclear encoded 5S rRNA gene for a subset of species examined. Characteristics such as heterothallism, oogonial ornamentation, mycoparasitism and the presence of linear mitochondrial genomes were polyphyletic. The only species that contained isolates that did not group together were P. ultimum and P. irregulare, possible reasons for this are discussed.