In this article we will discuss about the classification of ascomycetes into various orders.
The classification of the Ascomycetes constitutes perhaps the most difficult taxonomic problem in mycology. The large number of species and the morphological complexities and variations in the Ascomycetes have often led to an understandable hesitancy on the part of the majority of mycologists to accept revisions of the class which attempt to incorporate the data that have been obtained into the system of classification.
The system of Lindau presented in the Naturlichen Pflanzenfamilien in 1897, with various modifications is still in general use today despite the fact that the knowledge of the morphology, cytology and genetics of the Ascomycetes has developed since its publication.
In the widely accepted age-old classification system, the Ascomycetes were subdivided into three major groups: Plectomycetes, Pyrenomycetes, and Disco- mycetes, based on the gross morphologolical structure of the ascocarp being cleistothecium, perithecium, and apothecium respectively.
This above classification system has, however, been modified by different mycologists. One of such modified classification systems is presented below. Here the Ascomycetes are classified into two subclasses which are again subdivided into orders and families. In one of the subclasses the taxonomic category series is introduced.
The names—form-genera Aspergillus and Penicillium are introduced for convenience, instead of introducing the names of their perfect stages.
A. Asci are naked and are formed singly or in loose clusters, absence of ascogenous hyphae or ascocarp
B. Assimilatory phase uninucleate cells or mycelioid, ascus formed directly from zygote which is the product of copulation of two cells or part of hyphae
C. Asci bearing more than eight ascospores
BB. Asci bearing one to eight ascospores
D. Mycelium scanty or lacking
DD. Mycelium abundant
BB. Assimilatory phase mycelioid, asci arising from ascogenous cells derived from the binucleate cells of septate mycelium
AA. Asci produced from ascogenous hyphae and enclosed in well-developed ascocarps
B. Asci scattered at various levels within the ascocarp, ascocarp lacking any opening—a cleistothecium
C. Interior of ascocarp filled with evanescent asci, ascocarp without appendages.
BB. Ascocarp wide open—an apothecium or a modified form thereof
C. Ascocarps above ground
D. Asci operculate or suboperculate
E. Receptacle stalked, ascospores elliptical
EE. Receptacle usually sessile, cup-shaped or button-shaped
F. Asci narrowly cylindrical; turn blue, at least at the tip with Melzer’s reagent; asci not protruding beyond the general level of the hymenium at maturity; ascospores uniseriate
Family Pezizaceae Genus Peziza
FF. Asci broad, not blued at the tip by Melzer’s reagent, asci protruding beyond the general level of the hymenium at maturity, ascospores lie in two to three irregular rows.
DD. Asci inoperculate
CC. Ascocarps below the ground
BBB. Ascocarp with differentiated wall and with or without a regular ostiole, asci arising as a fascicle at a common level in the ascocarp
C. Ascocarp without an ostiole
Order Erysiphales Family Erysiphaceae
CC. Ascocarp with an ostiole—a perithecium D. Ascospores thread-like
Order Clavicipitales Family Clavicipitaceae
DD. Ascospores not thread-like
E. Ascocarp borne singly or in a stroma, dark membranous or carbonous.
F. Perithecia free, ostiole without long hairs
FF. Perithecia embedded in stroma, stroma free from substratum
BBBB. Ascocarp with differentiated wall lacking, asci borne in stromatic locules
Luttrell in 1955 put more emphasis on the structure of asci—unitunicate or bitunicate, and combined this character with that of ascocarp and similar other ascus-bearing structure. He recognized two subclasses: Euascomycetes and Loculoascomycetes.
In recent years there has been a tendency to follow the line of approach indicated by Luttrell. The main subdivisions of the Ascomycetes are based on the structure of the asci themselves and of the ascocarps within or upon which they assemble. The primary division depends on the nature of the ascus wall, which may be composed either of one or two layers, i.e., ascus may be unitunicate or bitunicate.
The Ascomycetes may thus be classified as follows:
A. Asci unitunicate, arranged in hymenium borne in an ascocarp or otherwise
B. Asci produced singly
CC. Developed from thick-walled chlamydospores produced within the tissue of vascular plants.
CG. Developed directly from gametangial copulation
BB. Asci in continuous layer but not in ascocarp
BBB. Asci in hymenium of an ascocarp
C. Asci operculate
D. Receptacle stalked, ascospores elliptical
DD. Receptacle usually sessile, cup-shaped or button-shaped
E. Asci narrowly cylindrical; turn blue, at least the tip with Melzer’s reagent; asci not protruding beyond the general level of the hymenium at maturity; ascospores uniseriate
Family Pezizaceae Genus Peziza
EE. Asci broad, not blued at the tip by Melzer’s reagent, asci protruding beyond the general level of the hymenium at maturity, ascospores lie in two to three irregular rows
Family Ascobolaceae Genus Ascobolus
GG. Asci inoperculate
D. Asci indehiscent or at least without a definite apical dehiscence mechanism
E. Hymenium continuous, ascocarps subterranean
EE. Hymenium not continuous
F. Ascocarp more or less globose, without ostiole, opening by splitting
G. Saprophytes, mostly
H. Ascocarp sessile
Ascocarp globose or ellipsoidal, minute fungi on plant debris above ground
Form-Genus Aspergillus Form-Genus Penicillium
HH. Ascocarp stalked
GG. Parasites, especially on leaves and other green parts ol flowering plants
H. Mycelium without hyphopodia, conidia abundant
HH. Mycelium bearing hyphopodia, conidia absent Family Meliolaceae
FF. Ascocarp with a long beak, asci diffluent and ascospores emerging in a drop of mucilage at the apex of the beak
DD. Asci with an apical pore
E. Asci very long, narrowly cylindrical with thread-like ascospores F. Asci in perithecia borne in stroma
Order Clavicipitales Family Glavicipitaceae Genus Claviceps
FF. Asci in perithecia without stroma EE. Asci not cylindrical, ascospores narrow but not thread-like F. Asci in apothecia
Order Helotiales Family Geoglossaceae
FF. Asci in perithecia
G. Paraphyses usually present, perithecia with or without stroma
H. Perithecia free, ostiole without long hairs Family Sordariaceae Genus Neurospora HH. Perithedia embedded in stroma, stroma free from substratum
AA. Asci bitunicate, borne in locule
Talbot (1969) and Webester (1970) categorized Ascomycetes as Subdivision Ascomycotina. The chief distinguishing features of the classes of Ascomycotina and outlined by them are shown below.
The classification adopted by them is a compromise between older systems in which the nature of the ascocarp was emphasized to the virtual exclusion of other characters, and newer ones in which microscope characters are prominent.
Asci unitunicate, or if bitunicate then in an exposed hymenium of an apothecium Asci naked, i.e., formed as discrete free cells or in a hymenium of indefinite extent, not bounded by a stroma or by ascocarp tissue; asci indehiscent
Asci formed in ascocarps
Asci scattered at various levels within a cleistothecium or a beaked perithecium; asci indehiscent
Asci forming a hymenium or arising as a fascicle at a common level in the ascocarp, or rarely single
Ascocarp usually a perithecium, less often a cleistothecium with fasciculate asci or an ascostroma with unitunicate asci; asci inoperculate, with an apical pore or slit; not minute external parasites of insects and arachnida
Ascocarp a perithecium with inoperculate asci whose walls soon disintegrate; minute external parasites of insects and arachnida
Ascocarp an apothecium or its hypogean derivative; asci operculate, inoperculate or indehiscent
Asci bitunicate, formed in an ascostroma but not in an apothecium
But Elizabeth-Moore-Landecker (1972) classified Ascomycetes based on gross morphology, anatomy, and life history of the individual organisms into subclasses.