Phylogenetic relationships of Pythium and Phytophthora species based on ITS rDNA, cytochrome oxidase II and β-tubulin gene sequences

Abstract

Fifty-eight isolates representing 39 Pythium species and 17 isolates representing nine Phytophthora species were chosen to investigate intra- and intergeneric relationships with sequence analysis of three genomic areas. The internal transcribed spacer regions (ITS1 and ITS2), including the 5.8S gene of the ribosomal DNA were PCR amplified with the universal primers ITS1 and ITS4. On the other hand 563 bp of the cytochrome oxidase II (cox II) gene was amplified with the primer pair FM66 and FM58 for Pythium and FM75 and FM78 for Phytophthora. The 658 bp partial β-tubulin gene was amplified with the forward primer BT5 and reverse primer BT6. Maximum parsimony analysis of the three DNA regions revealed four major clades, reflective of sporangial morphology. Clade 1 was composed of Pythium isolates that bear filamentous to lobulate sporangia. Clade 2 represents Pythium isolates that bear globose to spherical zoosporangia or spherical hyphal swellings. Meanwhile Phytophthora isolates were lumped into Clade 3 wherein the papillate, semipapillate and nonpapillate species occupied separate subclades. Lastly, Clade 4 was composed of Pythium species that bear subglobose sporangia resembling the papillate sporangia observed in Phytophthora. Hence a number of species (Ph. undulata, P. helicoides, P. ostracodes, P. oedochilum and P. vexans) have been proposed to be the elusive intermediate species in the Pythium-to-Phytophthora evolutionary line.

• molecular phylogeny
• oomycetes
• Pythiales

We thank Dr J.W. Kim, Seoul National University, Dr M. Tojo, Osaka Prefecture University, Dr S. Uematsu, Chiba Prefecture Agricultural Research Center, Dr E.B. Nelson, Cornell University, Mr. H. Watanabe, Gifu Research Institute for Agricultural Sciences, and Dr A. Chikuo, Japanese National Institute of Floricultural Science, for providing the isolates. This work was supported in part by Grant-in-Aid for Scientific Research (B) (15310024) from the Ministry of Education, Science, Sports and Culture of Japan.