DIVERSITY OF LIFE

DESCRIPTION OF THE PHYLUM ASCOMYCOTA (Cavalier-Smith 1998)

EUKARYA>OPISTHOKONTA>UNIKONTA>FUNGI>DIKARYA>ASCOMYCOTA

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 1) and ascocarps (Figure 2, see examples below), the structures that bear the asci, are among the important structural themes in this phylum.  Asci contain the sexual meiospores, the ascospores, which may be agents of dispersal, but most taxa disperse themselves asexually by means of conidiospores contained on conidia (Figure 3).  The phylum itself is extraordinarily diverse formed of free-living, parasitic, and symbiotic taxa. Furthermore, they may form mycelia or live in the unicellular state as yeasts.

When they make hyphae, ascomycetes typically produce crosswalls that have at least one pore allowing cytoplasmic communication from cell to cell.  The crosswall separating the last vegetative cell and a ascus does not have a pore.  Furthermore, the cells have have Woronin bodies, specialized proteinaceous structures that seem to function as stoppers for the pores to prevent hyphal hemorrhage (Taylor 2011).   Cell walls are made of glucans and chitin.

FIGURE 2.  TYPES OF ASCOCARPS.  Upper Left:  Naked asci of Taphrina are scattered over the host tissue rather than being united into an ascocarp.  Upper Right: A cleistothecium (asci contained in an enclosed ascocarp without an opening). This is an SEM micrograph of a Eurotium cleistothecium.  This is the perfect stage of the mold that produces aflatoxins in peanuts and grain.  Lower Left: Perithecia, asci mostly enclosed in an ascocarp with a single opening.  These are perithecia of Venturia in the leaf tissue of apple (causing apple scab).  Lower Right: An apothecium, asci on exposed surface of mushroom-like structure.  This is the apothecium of Peziza, a common cup fungus.
Images taken from:
upper left: http://botit.botany.wisc.edu/images/332/Ascomycota/Hemiascomycetes/
upper right: http://schimmel-schimmelpilze.de/download-1/eurotium-herbariorum.gif
lower left: http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm
lower right: http://www.uni-greifswald.de/~mycology/gallery/Seiten/Peziza%20micropus.htm

The typical ascomycete life cycle (see Figures 5 and 6) 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.  

FIGURE 5. Generalized Lifecycle of an ascogenous fungus.  Haploid filaments of compatible mating types fuse forming a brief dikaryotic phase (N+N).  The nuclei fuse in the ascus and then undergo meiosis.  The ascospore (a meiospore) is shed and germinates to form a haploid, monokaryotic mycelium in the asexual phase.  During this phase, asexual spores, usually conidia (see Figure 3) further disperse the organism.  If it associates with another compatible haploid filament, the sexual cycle can begin again.

The image from Taylor et al. Tree of Life Web. 

FIGURE 6. Steps in the development of the ascus from the formation of the dikaryotic cell to the ascus.  Note that the crozier serves to marshal each of the haploid nuclei to the appropriate daughter cells. 
This is modified from Figure 1 of Berteaux-Lecellier et al. 1998.

The ascomycete fungi are divided into three broad groupings: Taphrinomycotina, Saccharomycotina, and Pezizomycotina.  These are defined by molecular means, but they each have a few general features like the mycelium and ascogenous structures.  Taphrinomycotina includes basal taxa that form small mycelia or are unicellular (yeasts).  The asci, which may be associated with a specialized ascocarp, never have sterile filaments, paraphyses, associated with them.  Furthermore, they never make croziers. 

The name sake of the subphylum is Taphrina, a plant parasite that causes leaf-curl and witch's broom diseases (see a photomicrograph of Taphrina in Figure 2).  Peach leaf curl is caused by Taphrina deformans, which can attack peach and other members of the plant genus, Prunus.  During part of the life history, Taphrina grows as a yeast on the leaf surface and begins to form typical hyphae as it invades the leaf and feeds on plant cells.  Ascogenous hyphae then erupt to the leaf surface and produce asci.  The ascopores then germinate as yeasts.

Pneumocystis jirovecii is a yeast that occurs in the lungs (Figure 7).  The yeasts are quite common and usually occur in the lungs of healthy people, but can cause serious pneumonia in those who are weakened, especially with compromised immune systems.  The CDC reports that 9% of hospitalized patients with HIV/AIDS have Pneumocystis pneumonia (CDC Pneumocyctis pneumonia 2012).  Mortality can be 100% if untreated.

Schizosaccharomyces (Figure 8) is a yeast that superficially resembles common bread yeast.  The cells are cylindrical and divide with crosswalls (bread yeast divides by producing buds) and can make very short hyphae.  These live as saprobes.

Saccharomycotina has taxa that live primarily as yeasts, but when they make hyphae, the crosswalls have multiple pores.  In this group, the vegetative cell divides as a bud.  That is, the cell wall weakens and new cytoplasm plus a daughter nucleus move into an aneurism that grows out of the wall.  In general, this is not different from how conidiospores forms; though, in this case, the conidium is from a a single cell.  The walls of these yeasts are primarily glucan with a little chitin around the scar left by the bud.  The sexual cycle is somewhat reduced to the minimum.  Vegetative cells fuse and from that a zygote is formed.  The zygote wall serves as the ascus, and the liberated ascospores become vegetative cells.

Saccharomyces (Figure 9) 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.  Yeast commonly occurs in the wild on fruits like grapes and produce the 'bloom' or hazy covering.  It is not surprising that in the storage of grapes, yeasts began to grow in the juice and form the fermented product that we know of as wine.  Yeasts are unicellular taxa that evolved from multicellular ancestors.

Some can be parasitic.  Candida, a genus with about 30 different species that infect humans (CDC. Candidiasis. 2012).  They grow on the skin and mucus membranes where they normally do not become invasive.  In a weakened state, a human can develop an infection of the mouth (thrush), vagina (yeast infection).  The most dangerous form is a systemic infection of the bloodstream (invasive candidiasis) and is the fourth most commonly-acquired blood infection in hospitals (CDC).

The largest group of Ascomycota is the Pezizomycotina, all of which are mycelial with hyphae having a single pore and Woronin bodies.  The life cycle has a very brief dikaryotic stage with croziers prior to the development of asci.  They may have any of the four types of ascocarps illustrated in Figure 2.  They can be free-living, symbionts (as lichens), and pathogens of plants and animals.

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 Figure 10, 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 pneumonia-like diseases (e.g. histoplasmosis) 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 (Figures 11 and 12), 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.  The coincidence of the symptoms of ergotism and the testimony in the Salem Witch trials suggested to Caporael (1976) that the physical foundation for the accusations were the affects of eating ergotized rye in their grain stores. It had occurred through Europe commonly through the Medieval Period when it was called St. Anthony's Fire and may have been the causative agent of the Plague of Athens.

 All ascomycetes are not dangerous or detrimental.  Truffles (Tuber many species) produce large cleistothecial ascocarps that are entirely subterranean.  They have a strong odor and are collected and eaten by fungivorous animals, mainly rodents and boars, which then disperse the spores.  Morels (Morchella, Figure 13) also produce much-prized edible ascocarps that emerge in a large, mushroom-like structure of many apothecia.  

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 (Figure 14).  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.  Indeed, both truffles and morels tend to be associated with certain woody pants and likely enter into mycorrhizal associations with them.  Similar fungus chimeras include the lichens like Cladonia (Figure 15), most of which have an ascomycete as the mycobiont.

One of the oddest members of this phylum is Laboulbenia (Figure 16), 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. 


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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), but Adl et al. (2012) then reduced the number of taxa at the subphylum rank to three.  The topology of three subphyla (see Figure 17) was confirmed by a phylogenomic analysis by Robbertse et al. (2006), and a 6-gene X 420 species analysis by Schoch et al. (2009). 

 

 

FIGURE 17. A cladogram showing the relationships between classes of the Ascomycota (taxa in the shaded box).  The dikaryotic clade is indicated by D. The topology is supported by Liu et al. (1999), Lutzoni et al. (2004), Robbertse et al. (2006), and Schoch et al. (2009). 

 

 

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By Jack R. Holt and Carlos A. Iudica.  Last revised: 03/28/2017