Phylogenetic relationships of Phytophthora species based on ribosomal ITS I DNA sequence analysis with emphasis on Waterhouse groups V and VI

Phylogenetic relationships among Phytophthora species were investigated by sequence analysis of the internal transcribed spacer region I of the riboso mal DNA repeat unit. The extensive collection of isolates included taxa fro m all six morphological groups recognized by Waterhouse (1963) including mo lecular groups previously identified using isozymes and mtDNA restriction f ragment length polymorphisms. Similar to previous studies, the inferred rel ationships indicated that molecular groups of P. cryptooea/drechsleri-like and P. megasperma-like taxa are polyphyletic. Morphological groups V and VI , which are differentiated by the presence of amphigynous or paragynous ant heridia, are not monophyletic: species of the two groups are interspersed i n the tree. Species with papillate and semi-papillate sporangia (groups I-I V) clustered together and this cluster was distinct from those of species w ith non-papillate sporangia. There was no congruence between the mode of an theridial attachment, sporangial caducity, or homo- or heterothallic habit and the molecular grouping of the species. Our study provides evidence that the antheridial position together with homo- or heterothallic habit does n ot reflect phylogenetic relationships within Phytophthora. Consequently, co nfirming studies done previously (Cooke & Duncan 1997), this study provides evidence that the morphological characters used in Phytophthora taxonomy a re of limited value for deducing phylogenetic relationships, because they e xhibit convergent evolution.