Molecular Distinctions Between Phytophthora capsici and Ph. tropicalis Based on ITS Sequences of Ribosomal DNA

Among four species of Phytophthora tested, only Ph. capsici and Ph. tropicalis showed the same length for DNA sequence for both internal transcribed spacer (ITS)1 and ITS2 of ribosomal DNA. Phytophthora palmivora and P. nicotianae have lengths different from each other, and from the other two species. Although A1 and A2 types of Ph. capsici differ from each other by only one nucleotide, there are 10 different nucleotides between A1 and A2 types of Ph. tropicalis. Phylogenetic analysis of combined ITS sequences identified four clades each consisting A1 and A2 mating types of same species. The neighbor-joining and maximum parsimony trees show that Ph. tropicalis (A2) is clustered with the clade of two isolates of Ph. capsici before joining the clade of A1 and two other isolates of Ph. tropicalis from GenBank. Our results support the separation of Ph. tropicalis and demonstrate the need to sequence more than a single isolate of a species in the study of molecular phylogeny of Phytophthora. The phylogenetic trees also suggest that Ph. tropicalis (A2) may represent a transitional isolate in the process of species evolution