Submitted to: Mycologia
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/20/2000
Publication Date: 8/20/2000
Citation: Martin, F.N. 2000. Phylogenetic relationships in the genus pythium. Mycologia. Interpretive Summary: The fungal genus Pythium is a complex genus containing over 120 described species that occupy a variety of terrestrial and aquatic ecological habitats. Perhaps the most economically important members of this genus are plant pathogens, many of which have a broad host range and cause losses by both pre- and post-emergence damping-off, as well as by reduction in plant growth and yield due to root pruning. Members of this genus can be pathogens on a range of organisms, including algae, fish, shrimp, or mosquito larvae. One species is a mammalian pathogen and can cause human disease. Some species in the genus can be beneficial, functioning as biological control agents protecting against pathogenic Pythium species or for production of polyunsaturated fatty acids for use as human dietary supplements. Historically speaking, a range of morphological criteria has been used to classify members of this genus, however, overlap of some of these features among different species complicates accurate species identification. In addition, these features have not been useful for determining the evolutionary relationships among the disparate species. Studies investigating the evolutionary relationships in the genus have been limited and inadequate to evaluate the genus as a whole. This investigation uses molecular data to determine evolutionary relationships among a group of economically important species and determine the relationships between shared morphological characteristics and evolutionary relatedness. This molecular data also is useful for taxonomic purposes and identification of unknown isolates with greater accuracy.
Technical Abstract: The phylogenetic relationships of 67 isolates representing 24 species of Pythium were assessed by sequence alignment of the mitochondrially encoded cytochrome oxidase II gene. The species grouped into three major clades that were in general sense reflective of zoosporangial or hyphal swelling morphology. Species with spherical to globose zoosporangia or hyphal swellings were in clade I or clade II while species with filamentous to lobulate zoosporangia were in clade III. Two species that produced globose zoosporangia grouped together separately from the other clades, as did P. nunn. 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 with the remaining species producing only spherical hyphal swellings. Pythium oligandrum, a species that produces subglobose zoosporangia with interconnecting filamentous parts was sintermediate between species with inflated to lobulate filamentous zoosporangia and species that produced spherical to globose zoosporangia (clades I and II). 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. Characteristics such as heterothallism, oogonial ornamentation, mycoparasitism and the presence of linear mitochondrial genomes were polyphyletic.