An Integrated approach to the taxonomy of some mitosporic fungi of the Bipolaris complex

Abstract

Previous taxonomic studies culminated in the current differentiation of the mitosporic fungal genera Drechslera, Curvularia, Bipolaris and Exserohilum. The circumscription of Drechslera and Exserohilum are generally accepted and supported by the association with the teleomorphic ascomycetous genera Pyrenophora and Setosphaeria respectively. The separation of Curvularia and Bipolaris, both anamorphs of Cochliobolus, has been disputed. Many highly similar species have been described that share features of more than one genus. In this study 45 strains representing 14 species of Bipolaris, Curvularia and Exserohilum were characterised with regard to spore morphology, growth characteristics in culture and host preferences. Nucleic acid-based methods were used to evaluate the status of current morphological species. Integration of all this information represents a new approach to the taxonomy of this group. A list of Bipolaris, Curvularia and Exserohilum species known to occur in South Africa is provided and confrrmed their host relations presently known from the literature and includes eight new records. The stability of various spore dimensions and physiological reactions of cultures was analysed by using First Order Inductive Logic (FOIL) and a Decision Tree (DT) algorithms. Both tools proved spore shape to be more consistent than dimensions, and that colony diameter on days three to five, provided measurements with the best diagnostic value. Colony growth rates can be diagnostic within the long-spored species (fusoid to cylindrical spores) but not between the short-spored (clavate to navicular spores) species of the Bipolaris-complex. Differences in colony diameters from five of 25 culture-based tests enabled FOIL to differentiate between species with 79% certainty. Partial sequencing of the 28S rRNA encoding gene differentiated between Exserohilum species and a second group including Bipolaris and Curvularia species. The molecular marker type, Inter-simple sequence repeats (ISSR) using four anchored primers DBD-(AC)7, BDB-(CAC) 5, DHB-(CGA) 5 and VHV -(GT) 7G, were used with the similarity coefficients of Dice and Pearson and the clustering method of Ward to reveal various levels of intra-specific variability. ISSR data indicated greater similarity between Cochliobolus anamorph species with three to four septate conidia than the species with multiseptated conidia. All short-spored species investigated are plurivorous while the long-spored species seem to have a definite graminicolous host preference. Based on these observations it is proposed that the genera Curvularia and Bipolaris be retained as distinct taxa but amended: plurivorous mitosporic Cochliobolus species with short, clavate or navicular spores should be assigned to Curvularia and mitosporic Cochliobolus species with fusoid to cylindrical spores with gramicolous host preferences should be assigned to Bipolaris. The study confrrmed Exserohilum to be a well-defmed genus. Exserohilum inaequale was found to be synonymous with C. cymbopogonis. Two species, separated only on spore length, E. longirostratum and E. rostratum should be retained since they are also distinct in physiology and ISSR patterns. Integration of physiological traits, spore dimensions, host relationships and ISSR fingerprint patterns can therefore enhance the taxonomy of the Bipolaris complex.