To Modes of Conidiogenesis in Some Onygenalean Fungi
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Mycopathologia 106: 155-161, 1989. 1989 Kluwer Academic Publishers. Printed in Belgium. 155 Problems in application of the terms 'blastic' and 'thallic' to modes of conidiogenesis in some onygenalean fungi Lynne Sigler University of Alberta, Microfungus Collection and Herbarium, Devonian Botanic Garden, Edmonton, Alberta, Canada T6G 2El Accepted 21 November 1988 Key words: conidiogenesis, blastic, thallic, ontogeny, Onygenales, aleuriospores Abstract In 1969, specialists at the Kananaskis hyphomycete workshop coined the terms 'blastic' and 'thallic' to describe two distinct modes of conidiogenesis. Since then, the original concepts have been slightly modified and redefined, and the terms have been widely adopted in taxonomic descriptions of conidial fungi. Problems arise in the application of these terms to conidial development in fungi which demonstrate morphological plasticity ranging from fragmentation of a hypha to extrusion of a portion of a hypha or cell. A number of fungi, such as anamorphs of Onygenales which includes many of the fungi pathogenic to man, demonstrate intergradations between blastic and thallic development. Because development in this group of fungi is difficult to categorize, it has led to an inconsistent treatment of taxa which share many other developmental features in common. In using these terms, it should be remembered that they represent extremes in a developmental spectrum. Historical background cognized [ 14]. Since then, the terminology has been widely adopted in descriptions of conidial The processes by which fungi form conidia have fungi, and there has been a proliferation of a new been subjected to a great deal of scrutiny during taxa of Hyphomycetes, often distinguished on a the past 35 years. The development of a conidium single ontogenic character. depends on some modification of a hypha or part Despite optimism that conidiogenesis might of a hypha (cell) to form a propagule which can provide the basis on which new classifications be dispersed. The degree of elaboration of the could be made, recent work has called for a hyphal modification formed the basis of the eight reevaluation of the degree of distinction among types of development recognized by Hughes [ 11 ] the different types of development heretofore re- and modifications of his scheme adopted by other cognized [18, 19, 20]. Further it has been workers. questioned whether some of the terms are ade- In 1969, specialists at the first Kananaskis con- quate to describe the processes of development in ference attempted to provide a nomenclatural morphologically plastic fungi. basis for developmental processes then re- Cole and Samson's [6] elegant study of This paper was presented at the X t~' congress of the International Society for Human and Animal Mycology at Barcelona, Spain from June 27 to July 1, 1988. 156 conidial fungi using both scanning and trans- Origin of the terms blastic and thallic mission electron microscopy provided much in- formation on the mechanisms by which conidia The 1969 Kananaskis conference established the are formed. As more taxa have been critically terms blastic and thallic to distinguish two dis- examined, however, our concepts of development tinct modes of development. Since then, the origi- no longer appear as rigid as they once did, and we nal concepts have been slightly modified and re- begin to question the basis on which generic cir- defined, notably by Cole and Samson [6] who cumscriptions can be made. Indeed, this reassess- provided these definitions: 'Blastic - ...the ment is now leading to the combination of some conidium differentiates from a fertile hypha (i.e. genera formerly separated on the basis of conidiogenous cell) by the blowing-out and apparent ontogenic differences. As stated by de novo growth of part of the hyphal element and Kendrick [ 16], '...we have come to realize that is delimited by a septum.' 'Thallic - ... the ontogeny is often much less conservative, and conidium differentiates by conversion of a seg- much more plastic, than we had hoped .... and it ment of a fertile hypha which may involve some is being relegated to the status of just another enlargement and secondary wall growth; the character, rather than the character.' conidium is delimited by a septum or septa.' To some degree this reassessment has arisen At the Kananaskis meeting, Carmichael [5] because of the movement toward a natural, i.e. noted the difficulty in evaluating developmental phylogenetic, classification of anamorph taxa ac- processes and concluded that 'the various cording to their teleomorph connections. Genera methods of spore production described .... are not of conidial fungi are now often treated as true separate and distinct kinds, but outstanding parts genera comprised of anamorphs of related teleo- of a continuously intergrading and overlapping morphs rather than form taxa based on mor- spectrum of methods for releasing propagative phological similarity of anatomic stages of the life elements.' He [5] recognized only three basic pro- cycle. It has been argued that anamorph relation- cesses by which fungi form conidia: 'fragmen- ships may provide criteria to achieve a better clas- tation' in which the cytoplasm becomes concen- sification of ascomycetes [ 1 ], and that relation- trated in some cells of the hypha while remaining ships to teleomorphs provide characters on which ones are exhausted; 'fission' in which the filament anamorph taxa can be circumscribed [21 ]. Others breaks apart at double septa, or 'extrusion' from [Carmichael, in 17] have argued that a strictly the ends or sides of the hypha (cell). The process morphological classification of anamorphs serves of extrusion describes development now generally a more useful function for identification. These accepted as blastic. The processes of fragmen- topics were the subject of the second Kananaskis tation and fission describes development conference [ 15 ] and symposia following the Inter- now generally accepted as thallic, but Carmichael national Mycological Congress in 1983 [24] and recognized that even such broad categories during the 1988 Congress of the International foundered on the evidence of intermediate types Society for Human and Animal Mycoses [25]. of development. The present work was a contribution to the In 1981, Ingold [13] provided examples of latter symposium, and represents an attempt to some aquatic Hyphomycetes with solitary tetra- show the difficulties that I have encountered in radiate conidia in which development of a single applying the terms blastic and thallic, which sup- conidium could have both blastic and thallic as- posedly represent fundamentally different forms pects. While he recognized several different types of ontogeny, to a group of fungi which demon- of development, he voiced concern about the 'wis- strate all manner of gradation, from the develop- dom of trying to impose a blastic-thallic dichoto- ment of solitary, terminal or lateral conidia to my on the whole system.' The problem in inter- intercalary arthroconidia. preting conidial development in morphologically plastic fungi has been recognized by other workers [2, 6, 18, 19]. 157 Application of the terms to fungi with solitary Conidia attached to the supporting hypha by a conidia narrow base were interpreted as blastic in origin whereas thallic conidia had a broad attachment Inconsistencies are evident in the treatment of a base. Likewise, the process of development of group of fungi which produce solitary conidia. macroconidia of Trichophyton, Microsporum and The conidiogenous cell consists of a portion of a Epidermophyton has been variously described as hypha which is usually not well differentiated holothallie [6] or holoblastic [9, 10]. from other adjacent cells. Conidia are single celled Part of the reason that conidium development or multicellular, solitary, arising terminally or is difficult to assess is because mature conidia laterally, and may intergrade with alternate frequently as sume a cylindrical shape only slightly arthroconidia. A number of these fungi are broader in dimension than the conidiogenous cell anamorphs of Onygenales, an order of the asco- (hypha) and their development is then considered mycetes which includes the teleomorphs of agents to occur by modification of an existing hyphal of dermatophytosis (in the Arthrodermataceae) element. Blastic ontogeny is assumed where the and systemic infection [in the Onygenaceae, see conidia enlarge to a more rounded shape.Whether 7]. Fungi with single celled conidia are placed in such distinctions represent the most accurate in- the genera Chrysosporium Cda., Myceliophthora terpretation of conidium development in this Cost., Histoplasma Darling, Emmonsia Cif. & group of fungi will be assessed by means of some Mont., Malbranchea Sacc. and others. Fungi with examples. multi-celled conidia are placed in Epidermophyton Sab., Microsporum Gruby and Trichophyton Malmsten, but the latter two genera are linked to Examples of some inconsistencies the others by development of unicellular conidia. Cole and Samson [6] noted the problems in In the In'st three species, conidia are borne singly assessing development in onygenalean fungi as from swollen conidiogenous cells or are sessile on either blastic or thallic since 'The anamorphs of the side of the hypha. All three are currently several genera of Gymnoascaceae, e.g. Arthroder- accommodated in Myceliophthora. Fig. 1 shows ma Currey, Gymnoascus Baran.) often referred to the anamorph of Ctenomyces serratus Eidam, Chrysosporium Corda,