Phylogenetic relationships of Pythium species based on ITS and 5.8S sequences of the ribosomal DNA

The sequences of ITS regions in 30 species and two groups of the genus Pythium were resolved. In the phylogenetic trees, the species were generally divided into two clusters, referred to here as the F and S groups. The species in the two groups correspond in terms of their sporangial morphology, with the F group being filamentous/lobulate and the S group being spherical. Genetic divergence within the F group was lower than that within the S group. Other morphological characteristics such as oogonial structure and sexual nature appeared to be unrelated to the groupings in these trees. An alignment analysis revealed common sequences to all the species and arrangements specific to each F or S group. It was found that the ITS region was a good target in designing species-specific primers for the identification and detection of Pythium species. In the tree based on 5.8S rDNA sequences, oomycetes are distantly related to other fungi but separated from algae in Chromista.