Submitted to: Fungal Biology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/23/2011
Publication Date: 11/25/2011
Citation: Spies, C.F., Mazzola, M., Botha, W.J., Langenhoven, S., Mostert, L., Mcleod, A. 2011. Molecular analyses of Pythium irregulare isolates from grapevines in South Africa suggest a single variable species. Fungal Biology. 115:1210-1224. Interpretive Summary: The genus Pythium currently comprises more than 120 species, most of which are associated with plant roots in interactions ranging from beneficial to highly virulent. This range of interactions makes accurate species identification paramount in attempting to discern the causal agents of certain plant diseases and the risks involved in planting crops in soils infested with pathogenic Pythium species. Pythium irregulare is a genetically diverse species complex and typically exhibits high virulence, and it is an important pathogen of grapevine in South Africa. The goal of the current study was to determine whether P. irregulare isolates from grapevine belong to more than one phylogenetic species within the P. irregulare species complex. Altogether, molecular and morphological analyses indicated that the P. irregulare isolates from grapevines in South Africa consist of a genetically diverse group of isolates, but that these represent only one phylogenetic species. Our studies indicate that a previously described species P. cylindrosporum may not be a valid species. Furthermore, also considering the reference isolates that were used, our data were unable to support the presence of the two previously described phylogenetically distinct species P. irregulare s.s. and P. cryptoirregulare within the P. irregulare species complex.
Technical Abstract: The Pythium irregulare species complex is the most common and widespread Pythium spp. associated with grapevines in South Africa. This species complex can be subdivided into several morphological and phylogenetic species that are all highly similar at the sequence level. The complex includes P. regulare and P. cylindrosporum, which are morphologically distinct, and P. irregulare sensu stricto (s.s) and P. cryptoirregulare, which are morphologically similar. The aim of the current study was to determine whether South African grapevine P. irregulare isolates represented more than one phylogenetically distinct species. The isolates were characterised using nuclear (ITS and ß-tubulin) and mitochondrial (cox I and II) gene region phylogenies and two isozyme loci (glucose-6-phosphate isomerase [Gpi] and malate dehydrogenase [Mdh-1]). Altogether, the molecular data only revealed the presence of one phylogenetic species, P. irregulare. Morphological analyses of a subset of the isolates confirmed that the isolates were P. irregulare, and further showed that the P. cylindrosporum type strain formed typical P. irregulare oogonia, and not the previously reported distinct elongated oogonia. Some of the molecular analyses suggested the occurrence of outcrossing events and the formation of aneuploids or polyploids since (i) the nuclear- and mitochondrial gene data-sets were incongruent, (ii) polymorphic ITS copies were present within the same isolate (iii) heterozygosities were observed in the ß-tubulin gene and Gpi and Mdh-1 loci in some isolates and (iv) at least two copies of the ß-tubulin region were identified in some isolates.