Generic concepts in Drechslera sens. lat. and correlated teleomorphs

Abstract

There is divergence of opinion among mycologists concerning the generic classification of graminicolous hyphomycetes previously accommodated in Helminthosporium. It has been claimed that three supraspecific taxa can be recognized in the group, and that these taxa should be accorded generic rank as Drechslera, Bipolaris, and Exserohilum. Opponents of this view claim that only one genus is necessary, namely Drechslera sens. lat. In the work reported in this thesis, these hypotheses were tested experimentally.Morphological and biological attributes of conidia have been used as taxonomic criteria, and these were examined in the type species of Drechslera, Bipolaris, and Exserohilum. The number of cells producing germ tubes in each conidium was found to vary significantly with changes in origin of conidia and the substrates on which they were incubated for germination. Polarity per se was rejected as a generic criterion, but the point of origin of the germ tube from the basal cell and its direction of growth were found to be more constant and therefore suitable for use as differentiating characteristics. Primary septum ontogeny in developing conidia was also examined, and again useful taxonomic differences were found. Clear distinctions between the three genera were revealed by a study of hilum morphology, and previously unreported characteristics of the protruding hilum were demonstrated. In addition conidiophore morphology and conidial shape and colour were considered. It was concluded from this study of the type species for Drechslera, Bipolaris, and Exserohilum that recognition of three genera could be justified.The characteristics used to differentiate between the type species in the three genera were examined in a selection of other species and unnamed isolates of Drechslera sens. lat. Three groups were established as a result of this examination, in which the combinations of characteristics were similar to those exhibited by the type species of Drechslera, Bipolaris, and Exserohilum. The conidial hilum in species referable to Drechslera sens. str. was found to be more complex than reported previously, and the term 'atrium type' was coined to refer to it. In addition to primary septum ontogeny, it was found that the positions occupied by the second and third septa in maturing conidia also differed in the type species, and in species assigned to the groups corresponding with them. It is suggested that this characteristic be added to the criteria used to separate these genera.Some authors have claimed that the genera Drechslera, Bipolaris and Exserohilum are sufficiently distinctive to allow prediction of teleomorph correlations, and this hypothesis was tested. Nine previously undescribed teleomorphs were produced, eight referable to Cochliobolus and one to Setosphaeria. The anamorphs of these species are in Bipolaris and Exserohilum respectively. These results offer strong support for the hypothesis concerning anamorph characteristics and teleomorph correlations. In addition the relationship of Pseudocochliobolus to Cochliobolus was examined. The former genus was found not to be distinct on the grounds offered by its authors. Further studies on Cochliobolus and its correlated anamorphs, Bipolaris and Curvularia, led to the discovery that the anamorph of the type species for Pseudocochliobolus should be classified in Curvularia. An amended generic description of Pseudocochliobolus and segregation in that genus of Cochliobolus species with Curvularia anamorphs is suggested.A novel mechanism of ascospore discharge in Cochliobolus was detected in six species. An hypothesis concerning ascus structure and function in the genus was formulated as a result of these observations. It is suggested that the ascus in Cochliobolus may represent an intermediate between the unitunicate and bitunicate types of structure, which have been used as primary differentiating criteria at the rank of class in the ascomycetes.It was concluded that recognition of three anamorph genera in the fungi under discussion is justified and desirable. Thirteen new combinations in Bipolaris, and one in Curvularia, are proposed. In addition, eight new species of Bipolaris are recognized. An examination of ovariicolous species on Sporobolus showed that four Bipolaris species, two of which are new, occur in this specialized habitat. Helminthosporium crustaceum, previously considered a synonym of B. ravenelii by other authors, was shown to be distinct.The evidence accumulated during this study largely removes any impediment to the acceptance of the genera Drechslera, Bipolaris, and Exserohilum. Stability of generic nomenclature in this group should benefit users of the classification in many applied fields.