Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/24/2006
Publication Date: 3/20/2007
Citation: Bowers, J.H., Martin, F.N., Tooley, P.W., Luz, E. 2007. Genetic and Morphological Diversity of Temperate and Tropical Isolates of Phytophthora capsici. Phytopathology 97:492-503. DOI:10.1094/PHYTO-97-4-0492 Interpretive Summary: This objective of this study was to evaluate a range of isolates to clarify the taxonomic relationship between Phytophthora capsici and the closely related Phytophthora tropicalis. This included conducting traditional morphological analysis and molecular techniques (amplified fragment length polymorphisms and DNA sequence analysis of several transcribed regions). The results indicated the two species were closely related but were not able to determine if they were variants of the same species.
Technical Abstract: Phytophthora capsici is a diverse species causing disease on a broad range of both temperate and tropical plants. It was proposed that temperate and tropical isolates represent different species based on morphological observations. In this study we used cultural characteristics, amplified fragment length polymorphism (AFLP) and DNA sequence analyses of the ribosomal internal transcribed spacer region (ITS) and mitochondrial cytochrome oxidase II (cox II) genes to characterize temperate and tropical isolates from a wide range of host species. All but one temperate isolate grew at 35'C, while all tropical isolates did not. All but two tropical isolates formed chlamydospores, while temperate isolates did not. There was strong bootstrap support for separation of temperate and tropical isolates using AFLP analysis. The majority of temperate isolates clustered within a single clade without regard to host or geographical origin, while the tropical isolates grouped into three distinct clades. Phylogenetic analysis of the ITS regions resulted in a similar separation of temperate and tropical isolates as the AFLP results with 1.0 - 1.6% sequence divergence between the groups. Analysis including other species revealed similar branch lengths separating temperate and tropical isolates as were observed in comparisons among other phylogenetically closely related species in the genus. Analysis of cox II sequence data was less clear. Although the temperate and tropical isolates grouped together apart from other species, there was no bootstrap support for separating these isolates. The level of sequence divergence between these groups was similar to comparisons among P. infestans, P. mirabilis, and P. phaseoli (0.6 to 0.9%). RFLP analysis of the ITS regions separated the temperate and tropical isolates as in the AFLP and ITS phylogenetic analyses, however, RFLP analysis of the cox I and II gene cluster did not distinguish between temperate and tropical isolates. The differences in grouping of isolates in these two RFLP studies should be helpful in identifying isolate subgroups. Our data is equivocal and does not fully clarify whether or not temperate and tropical isolates should be separated into different species.