3.7 Ascomycota

With about 64,000 species known, Ascomycota (informally, ascomycetes) is the largest group in Kingdom of Fungi. Like the fungi that belong to its sister group, the Basidiomycota, the majority of species within the Ascomycota are filamentous fungi that produce a mycelium in which the hyphae are regularly septate. What characterised the Ascomycota is that their sexual spores (ascospores) are formed within a sac-like structure hyphal cell which is called an ascus (from the Greek askos which means ‘bag’).

Ascomycota includes many species of great importance. Names to look out for include the plant pathogens Fusarium, Magnaporthe, and Cryphonectria, as well as medically important genera like Candida and Pneumocystis that cause human disease, and Penicillium chrysogenum, the producer of penicillin, the first antibiotic to be discovered, and Penicillium citrinum and Aspergillus terreus which were among the first moulds shown to produce precursors of today’s ‘wonder drugs’ the statins, which are currently crucial to cholesterol-management by many millions of patients.

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Fungi in medicine

For some outline discussions of the origins of drugs in current use in medicine CLICK HERE

The majority of the lichenised fungi belong to the Ascomycota, as does the yeast used in our baking and brewing industries, Saccharomyces cerevisiae. As we’ll see later (Chapter 5) this yeast, in particular, has been used as a model organism for science experimentation for well over a century, and is the best characterised of all eukaryotic cells.

Ascomycota contribute to other foods: Penicillium camembertii and P. roquefortii condition cheese, Fusarium venenatum provides the myco-protein used to make the vegetarian meat substitute Quorn, and two celebrated edible fungi are the morel, Morchella esculenta, and truffle Tuber magnatum (white truffle of Northern Italy) and T. melanosporum (black truffle of the Périgord region in France). Some of these moulds (especially Aspergillus spp.) produce metabolites that spoil food and, like aflatoxins, can be extremely toxic.

Ascomycota also includes the majority of fungi for which sexual reproduction has not been observed. In traditional classifications (based largely on morphological features) these asexual fungi have been placed in a separate group, variously called Deuteromycetes, Deuteromycotina or Deuteromycota (according to whether the taxonomist concerned though they deserved the status of Class, Subphylum or Phylum). Molecular techniques now permit them to be classified among their sexually reproducing relatives from comparison of nucleic acid sequences, which makes this separation redundant.

The recent AFTOL review of the phylogeny of Ascomycota divides this phylum into three major evolutionary lineages (Blackwell et al., 2006), given the rank of Subphylum:

  • Taphrinomycotina, also known, informally, as the archiascomycetes.
  • Saccharomycotina, also known, informally, as the hemiascomycetes.
  • Pezizomycotina, also known, informally, as the euascomycetes.

Members of the Taphrinomycotina do not make fruit bodies (called ascomata in this phylum). The group includes a varied collection of organisms, including the filamentous plant pathogen Taphrina. Taphrina deformans causes peach leaf curl, and witches’ broom disease of birch is caused by T. betulina; other species attack, oak, poplar, maple and many others). Taphrina normally grows in a yeast form until it infects plant tissues in which typical filamentous hyphae are formed. Ultimately the fungus forms a naked layer of asci on the deformed, frequently brightly pigmented surfaces of their hosts. CLICK HERE to see a page of illustrations.

The fission yeast Schizosaccharomyces pombe is also placed in the Taphrinomycotina. This species, first isolated in 1893 from East African millet beer (‘pombe’ is the Swahili word for beer), has been used as a model organism in molecular and cell biology for over 50 years. These yeast cells grow by apical extension, and then divide by medial fission through a new, centrally-placed, septum to produce two daughter cells of equal sizes. The regularity of this process, coupled with ease of cultivation, makes them a powerful tool in cell cycle research.

The fission yeast researcher Paul Nurse won the 2001 Nobel Prize in Physiology or Medicine for his work on cyclin dependent kinases in cell cycle regulation, together with Lee Hartwell (who developed the genetic analysis of the cell cycle in budding yeast, Saccharomyces cerevisiae, and introduced the concept of checkpoints) and Tim Hunt (who discovered cyclins in sea urchins).

Other, even more enigmatic inclusions in the Taphrinomycotina are the human pathogen Pneumocystis and the widely-distributed genus Neolecta. Long considered to be a protozoan, Pneumocystis is now clearly accepted as a yeast-like fungus. Pneumocystis pneumonia (PCP) is caused by Pneumocystis carinii (now called P. jirovecii in honour of the Czech parasitologist Otto Jirovec).  Pneumocystosis is one of the most common infections in immunosuppressed patients because AIDS and other immunity impairments such as immunodeficiencies, steroid treatment, organ transplantation medication and cancers predispose the patient to Pneumocyctis infection.

Neolecta has been found in Asia, North and South America, and Northern Europe, in association with trees. The fungus produces club-shaped and brightly coloured fruiting bodies, known as ‘earth tongues’, which are a few to several cm tall. N. vitellina grows from rootlets of its host, but it is not known whether the fungus is parasitic, saprotrophic, or symbiotic.

The Saccharomycotina contains a single order, the Saccharomycetales that includes the majority of the ascomycetous yeasts, including the economically important genera Saccharomyces and Candida.

Historical records from ancient Egypt and China depict brewing and baking 8,000 to 10,000 years ago although it was Louis Pasteur, in 1857, who demonstrated that yeasts caused the fermentation of grape juice to wine. Eventually, yeasts were recognised as fungi, and the name Saccharomyces means ‘sugar fungus’. Yeasts usually grow as single cells that reproduce by budding or less frequently by fission. Asci and ascospores are not enclosed in the fruiting bodies (ascomata) commonly found in the filamentous ascomycetes.

Yeast cellular morphology is rather simple (CLICK HERE to see a page of illustrations), and yeast genomes tend to be smaller than those of filamentous fungi, but it is a highly adapted growth form. Not all yeasts are ascomycetes; there are members of the Basidiomycota that adopt a yeast form and the term ‘yeast-like’ also has been applied to dimorphic members of the zygomycete genus Mucor. Ascomycete yeasts are usually found in specialised habitats, which tend to be small volumes of liquid rich in organic carbon (e.g. flower nectaries). Basidiomycete yeasts, in contrast, seem to be adapted to colonising solid surfaces that are poor in nutrients.

Candida albicans is commensal on humans, its success being apparently the combination of an extracellular lipase activity, the ability to form invasive hyphae, and the ability to grow at 37°C.

C. albicans lives on the skin, in the mouth and gut and other mucous membranes of about 80% of the human population, and for most of the time with no harmful effect. When the balance between the normal microorganisms is lost, for example as a result of antibiotic treatment, hormonal disturbance or immunocompromise, overgrowth of C. albicans results in candidiasis, or ‘thrush’. This common condition is usually easily cured, but in immunocompromised patients, such as HIV-positive individuals, the yeast form of Candida reacts to environmental cues by switching into an invasive filamentous growth form and a systemic and very serious infection can result.

The Pezizomycotina or euascomycetes (= ‘true-ascomycetes) contains about 90% of the Ascomycota. The characteristic feature of this group is that sexually reproducing species produce ascomata. Inevitably with such a large range of organisms, its members can be found in all aquatic and terrestrial habitats and participating in all ecosystems, including wood and litter decay, animal and plant pathogens, mycorrhizas and lichens (with only a few exceptions, all lichenised fungi belong to this group).

Prior to the application of molecular phylogenetics, classification of Pezizomycotina was based on the morphology and development of ascomata and asci. The four main ascoma morphologies are:

  • Apothecia.
  • Perithecia.
  • Cleistothecia.
  • Ascostromata.

Organisms that produced these fruit bodies are said to be apothecial, perithecial, cleistothecial or ascostromatic, as appropriate.

Apothecia are typically disk- to cup-shaped to spoon-shaped (spathulate) and produce their asci in a well defined tissue layer, a hymenium, which is exposed to the air.

Perithecia and cleistothecia are, respectively, partially or completely closed ascomata; their asci are formed in the central cavity (‘centrum’) of the ascoma. In perithecia, which are considered to be ‘true’ ascomata, the inner wall of the fruit body forms at the same time as the ascogenous hyphae develop. Asci are formed in a defined hymenium and are frequently mixed with sterile paraphyses arising from the subhymenial tissue although paraphyses are absent in some lineages (e.g. Hypocreales). The term hamathecium or hamathecial tissue is a general term applied to whatever tissue separates asci within the ascoma (it’s derived from the Greek háma, meaning ‘all together’). It may originate from different parts of the fruit body, be composed of paraphyses or other sterile cells, be generally-distributed or localised; it may even be absent (e.g. Dothidea).

In acostromata, the asci develop in preformed spaces, called locules, and the stroma often forms a flask-shaped (pseudothecia) or open, cup-shaped (hysterothecia and thyriothecia) structure that resembles the gross morphology of perithecia or apothecia.

Ascus walls appear to be multilayered in transmission electron micrographs, so a classification of asci has developed based on the number and thickness of wall layers as well as on the mechanism by which the ascospores are released (= dehiscence). The descriptive names for the major ascus types include:

  • unitunicate, ascus with relatively thin walls; encompassing operculate, inoperculate and prototunicate asci.
    • prototunicate, produced by apothecial, cleistothecial and perithecial fungi; thin-walled, globose to broadly club-shaped, ascospores released passively by disintegration of the ascus wall.
    • operculate, found in apothecial fungi; release ascospores through a defined opening with a ‘lid’ (operculum) that is formed either at the ascus apex or just below it.
    • inoperculate, produced by apothecial, cleistothecial and perithecial fungi; typically thin-walled, the tip of the ascus usually has a small pore filled with loose wall material; the spores are discharged through this pore, or if there is no pore, dehiscence by rupture of the ascus apex.
  • bitunicate, conspicuously thick-walled with two walls, called the exotunica and endotunica; produced by ascostromatic lichenised and nonlichenised species and ascohymenial lichens. In the traditional definition of bitunicate asci, fissitunicate dehiscence occurs (a fissitunicate ascus is a double-walled ascus where the inner wall pops completely out of the outer wall during dehiscence in a jack-in-the-box manner; it happens when the endotunica ruptures through the exotunica). Other dehiscence mechanisms exist among ‘bitunicate’ ascus morphologies involving little to no wall separation; these occur especially in lichenised taxa.

Some of these morphologies most likely represent ancestral traits for the Pezizomycotina (e.g. apothecium), while others have occurred several times through convergent evolution (e.g. cleistothecium, prototunicate asci). CLICK HERE to see a page of illustrations.

The current classification, based mainly on DNA phylogenies of Ascomycota divides Pezizomycotina into ten Classes (in brackets we show their ascoma and ascus morphologies and we also show some names to look out for):

  • Arthoniomycetes (apothecia; bitunicate asci); includes the lichen Lecanactis abietina.
  • Dothideomycetes (ascostromata; bitunicate asci); contains Dothidia, Aureobasidium,
    Pleospora, Tyrannosorus (yes, honestly) and Tubeufia.
  • Eurotiomycetes (perithecia, cleistothecia, ascostromata; bitunicate or prototunicate asci); contains Aspergillus, Penicillium, Histoplasma and Coccidioides.
  • Laboulbeniomycetes (perithecia; prototunicate asci); comprises ectoparasites of insects and other arthropods (Laboulbeniales), e.g. Herpomyces, and mycoparasites and coprophiles (Pyxidiophorales), e.g. Pyxidiophora and Rhynchonectria; ascospores characterised by holdfasts.
  • Sordariomycetes (perithecia, cleistothecia; inoperculate, prototunicate asci); contains the bulk of the traditional ‘pyrenomycetes’, including Sordaria, Cordyceps, Neurospora, Hypocrea, Verticillium, Bombardia, Xylaria and Diaporthe.
  • Lecanoromycetes (apothecia, perithecia; bitunicate, inoperculate, prototunicate asci); composed exclusively of lichen-forming ascomycetes like Lecanora, Cladonia, Usnea, Peltigera, and Lobaria.
  • Leotiomycetes (apothecia, cleistothecia; inoperculate, prototunicate asci); contains Leotia, Sclerotinia, Monilinia, Mitrula, Hymenoscyphus, Microglossum and Cudonia.
  • Lichinomycetes (apothecia; bitunicate, inoperculate, prototunicate asci); includes the lichen Lempholemma, Peltula, Geoglossum and Trichoglossum.
  • Orbiliomycetes (apothecia; inoperculate asci); contains Orbilia.
  • Pezizomycetes (apothecia; operculate asci); includes the ‘discomycetes’ Peziza, Aleuria, Morchella, Gyromitra, Tuber and Pyronema.

The AFTOL data (Spatafora et al., 2006) strongly indicate that Pezizomycotina, Arthoniomycetes, Eurotiomycetes, Orbiliomycetes and Sordariomycetes are all monophyletic. Pezizomycetes and Dothideomycetes are also probably monophyletic though the statistical support was not strong, and the outcome was uncertain for Lecanoromycetes. Leotiomycetes was polyphyletic. The study showed Orbiliomycetes and Pezizomycetes to be the most basal classes of Pezizomycotina; both of these consist of species that produce apothecia.

Updated December 16, 2016