|DIVERSITY OF LIFE|
The Ascomycota (as-ko-mi-KO-ta) is derived from two Greek roots that mean wineskin or bladder (aski -ασκί); and fungus (mykes -μύκης). The reference is to the structure (ascus) within which the sexual meiospores are formed.
INTRODUCTION TO THE ASCOMYCOTA
The ascus-bearing fungi include a very diverse and economically-important collection of organisms. Asci (Figure A) and ascocarps (Figure B), the structures that bear the asci, are among the important structural themes in this phylum. Asci contain the sexual meiospores, which may be agents of dispersal, but most taxa disperse themselves asexually by means of conidiospores contained on conidia (Figure C). The phylum itself is extraordinarily diverse formed of free-living, parasitic, and symbiotic taxa (Figures D-T). Many are parasites of agricultural plants and cause diseases like: apple scab, apple bitter rot, brown stone rot, strawberry stem rot, etc. Some, like Endothia parasitica, have by their introduction altered the Eastern Deciduous Forest in North America by the effective elimination of one of its dominant plants, the American Chestnut (Castanea dentata). Similarly, American Elms (Ulmus americana) have disappeared due to the introduction of another ascomycete that causes Dutch Elm Disease.
Ascomycete-caused diseases are not restricted to plants. For example, skin ailments (e.g. ringworm, athlete's foot), and histoplasmosis, a pneumonia-like disease, are caused by ascomytogenous fungi. Household molds (toxic molds, black molds, and green molds) tend to be from this phylum, though many have lost the ability to produce sexual spores. Ergot, a disease brought on by ingesting rye infected with Claviceps purpurea, causes hallucinations and uncontrolled contractions of certain muscles, especially the uterus. The active agent in ergotized grain seems to be a compound similar to LSD.
All ascomycetes are not dangerous or detrimental. Truffles and morels produce much-prized edible ascocarps. Yeast (Saccharomyces and related taxa) is perhaps the most economically-important fungus of all and is responsible for the alcoholic fermentation of beer, wine, etc. as well as the fermentation necessary for the production of leavened bread.
Some species of the Orbiliomycetes are associated with dry wood and are the causative agents of dry rot. These thrive in the xeric environments of dry dead wood on a tree (where they can be exposed to drying winds and sun) or the semi-arid soil associated with plants like Yucca. However, when the hyphae of their sparse mycelia come into contact with nematodes, they begin to elaborate hyphal loops, which function as nematode traps. When a nematode sticks its head into a snare, the hydrostatic pressure of the hyphal loop increases suddenly, and the worm is caught. The fungus then elaborates a feeding haustorium into the nematode and quickly digests the animal. The fungus, with the added nutrition from the nematode, elaborates conidia for dispersal. Not only do they lead a double life as wood eaters and nematode trappers, but some have lost the ability to make asci. The most well-known nematode-eating fungus, Arthrobotrys, is the anamorph (asexual form) of some taxa within the sexual genus Orbilia. Thus, these same fungi can consume the trim wood on my garden shed door, recycle wood and its elements in the brush pile at the bottom of my yard, and consume soil nematodes in the garden bed where I grow tomatoes. Clearly, the benefits to me far outweigh the costs.
Many species of the ascomycetes perform ecological functions that are quite valuable in the long run. Indeed, the environmental role of most ascomycetes cannot be overstated. Apart from their roles as "decomposers", many of them enter into symbiotic relationships with plants to form a fungus-plant mycorrhizal associations. Similar fungus chimeras include the lichens, most of which have an ascomycete as the mycobiont.
The typical ascomycete life cycle involves the association of haploid, monokaryotic branched filaments. In the case of morel (Morchella), hyphae of two compatible mating types associate and begin to weave the ascocarp. Then, in the hymenial layer, each filament has cells that enlarge. The functional female grows a long structure called a trichogyne that fuses with an enlarged cell in the compatible filament. The result is the emergence of a filament that remains haploid with two distinct nuclei (dikaryotic). As it divides, the terminal end makes a crook (called a crozier) that sequesters one of the nuclei to insure that each daughter cell has the full complement of haploid nuclei. This dikaryon is short-lived and after a few cell divisions leads to the development of the ascus, within which the haploid nuclei fuse and then undergo meiosis to form the ascospores on the surface of the ascocarp.
One of the oddest members of this phylum is Laboulbenia, an obligate parasite of insects, especially beetles, with a distinctive non-mycelial and determinate growth pattern. The fungus body, the receptacle, attaches to the host by a basal cellular holdfast and a single, simple haustorium penetrates the insect. Lateral filamentous appendages and one or more sessile or stalked perithecia arise on the receptacle after feeding on the insect. The ascus wall deliquesces (begins to gelatinize) prior to spore discharge.
SYSTEMATICS OF THE ASCOMYCOTA
The taxonomy of the Ascomycota has been in flux for some time ( e.g. Alexopoulos and Mims 1979, Bold et al. 1987, and Scagel et al. 1984). First, the practice of separating the lichens and imperfect fungi (those that do not exhibit sexual reproduction) was abandoned and more natural taxonomic systems began to appear. This trend can be seen in the systems of Margulis and Schwartz (1982, 1988, and 1998). Then, Nishida and Sugiyama (1994) discovered a distinct group that they called the Archiascomycetes according to their SSU rRNA analysis of fungi. Thus, they and others including Liu et al. (1999), defined the Ascomycota as having 3 classes: Archiascomycetes, Saccharomycetes, and the Euascomycetes. Both the Saccharomycetes and the Euascomycetes groups seemed to be well defined and monophyletic. The "Archiascomycetes" seemed to be paraphyletic and comprised the broad grouping from which the other two groups sprang. We feel that the diversity of the Ascomycota is too great to be reflected in a system of 3 classes. Thus, we have adopted the system of Eriksson et al. (2001) which has 3 subphyla and 14 classes. The analysis of Lutzoni et al. (2004) confirms the monophyly of the Ascomycota but calls into question the monophyly of some of the Taphrinomycotina. Adl et al. (2005) seem to separate the ascomycotes into four taxa at the level of Taphrinomycotina (which we interpret as 4 subphyla). Thus, the system of Eriksson et al. (2001) likely will be modified.
B. The types of ascocarps found in the Ascomycota.
C. An SEM micrograph illustrating the structure of an ascomycete conidium.
D. The ascocarps of Neolecta, a symbiont (parasite?) of spruce.
E. Spores of Pneumocystis from a lung tissue of a person who was immune compromised by HIV.
F. Schizosaccharomyces, a non-budding yeast.
H. An SEM image of Saccharomyces showing a developing bud and bud scars.
I. Arthonia, an ascomycete fungus in lichenized form.
J. Sooty mold caused by Chaetotherium.
K. Capnodium forming a perithecium on Pinus. This is the causative agent of sooty mold on pine.
L. Perithecia of Venturia in the leaf tissue of apple (causing apple scab).
M. An SEM micrograph of a Eurotium cleistothecium. This is the perfect stage of the mold that produces aflatoxins in peanuts and grain.
N. Cladonia, a common lichenized fungus known as British Soldier.
O. The cleistothecium of Microsphaera, the agent responsible for powdery mildew on lilac leaves.
P. Apothecia of Peziza.
Q. Ascocarps of Morchella, a prized delicacy for mushroom pickers.
R. Perithecia of Claviceps growing in rye. This is the agent responsible for ergot.
S. An SEM micrograph of the perithecium of Neurospora.
T. The receptacle of Laboulbenia attached to the body of an ant.
|Images taken from:
B: Bold et al. (1987)
SYNOPTIC DESCRIPTION OF THE ASCOMYCOTA
|The following description comes from Alexopoulos and Mims (1979), Alexopoulos et al (1996), Bold et al. (1987), and Scagel et al., (1984).|
I. SYNONYMS: ascomycetes, sac fungi.
II. NUMBER: >15,000 species.
III. PHYLUM CHARACTERISTICS:
A. ASEXUAL REPRODUCTION: Conidia.
B. SEXUAL REPRODUCTION: Ascospores produced within an ascus and often enclosed within an ascocarp. Nuclear fusion followed by meiosis (and usually a mitosis) to produce 8 ascospores in an ascus. Distinctive gametangia and stages of ascospore formation.
D. CELL WALLS: Chitin and glucan.
E. ECOLOGY: These are fungi that are free-living saprobes or parasites. Some of them make chimeroid entities like mycorrhizal associations and lichens.
HIERARCHICAL CLASSIFICATION OF THE ASCOMYCOTA
|This system is a modification of Eriksson et al (2001) which has 3 subphyla and 14 classes.|
SUBPHYLUM TAPHRINOMYCOTINA = CLASS ARCHIASCOMYCETES
This is the group that Nishida and Sugiyama (1994) called the class Archaeascomycetes. I have raised it to subphylum level according to the system of Ericksson (2000) who claims that they are the sisters to all of the other Ascomycota, and they appear to be the groups from which the other Ascomycotes arose. However, the following classes differ from each other structurally and, according to Ericksson (2000), on the basis of their SSU rRNA sequences. Thus, the diversity of the group of four classes really indicates that they are defined by exclusion from the well-defined and natural groupings: Saccharomycotina and Pezizomycotina. Also, I am troubled by their apparent primitiveness. All of these taxa (except the fission yeasts) are parasites and, therefore, may only appear to be primitive through reduction. Clearly, the book is not closed on the taxonomy of the Ascomycota.
CLASS SCHIZOSACCHAROMYCETES = OCTOSPOROMYCES
SUBPHYLUM SACCHAROMYCOTINA = HEMIASCOMYCETES
These are the budding yeasts. Vegetative phase unicellular yeast-like or filamentous; asci one-walled, naked, not borne on ascogenous hyphae, produced singly, following karyogamy; no ascocarps.
These comprise most of the ascomycota. The organisms form mycelia that make ascocarps (ascus-bearing structures also called ascomata) with hymenia. Some of the taxa are lichenized (enter into a symbiotic relationship with algae to form lichens). Some of the taxa have lost the ability to undergo meiosis and, although they might fuse, they can not produce ascospores or asci. Such taxa were once called the Fungi Imperfecti or Deuteromycota. Such a distinction is decidedly artificial. On the other hand, symbiotic entities like lichens do not easily fit into a natural system unless the fungal symbiont (mycobiont) is given complete preference. To be consistent with current fungal taxonomic systems, I will include the lichenized fungi in this system and describe the lichens in a separate page. This subphylum follows that of Ericksson (2000), but I have added a 9th class, Laboulbeiomycetes, a group of uncertain status in Ericksson's system.
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By Jack R. Holt. Last revised: 03/10/2013