Ascomycetes

The name ascomycete is derived from the presence of an ascus (plural asci), a tiny saclike structure formed during the life cycle of the fungi. Ascomycetes are a large class of fungi which contains over 46,000 species (lichens included), including Neurospora, Penicillium (producer of penicillin), Aspergillus (producer of citric acid, soy sauce, and vinegar), true yeasts (e.g. Saccharomyces used in fermentation and baking), and dermatophytes. A. niger is used to produce Beano, the trade name for an enzyme preparation that breaks down galactose, a type of sugar in cabbage, broccoli, and Brussels sprouts. Blue-green, pink, and brown Ascomycetes molds cause food to spoil. Ascomycetes also include edible cup fungi, morels and truffles. Some Ascomycetes cause serious plant diseases such as Dutch elm disease, chestnut blight, ergot disease on rye, and powdery mildew on fruits and ornamental plants, brown rot of stone fruits, and peach leaf curl disease,. Other Ascomycetes form symbiotic associations with certain algae and/or cyanobacteria to form lichens. Many new species, some undoubtedly of great economic importance, await discovery and scientific description.

Aspergillus fruiting bodies (20 X objectives)

Morels

Growth and Reproduction

are thought to have evolved from zygomacete-like ancestor. Although Ascomycetes are generally more complicated than zygomycetes, some Ascomycetes, such as yeasts, are single-celled organisms with no mycelium. Their individual cells, however, still perform most of the functions of a large mycelium. 2,4

The hyphae (filaments) of the mycelium usually have cross walls known as septa. One of the characteristic features of these cross walls in Ascomycetes is that they are perforated so that cytoplasm and nuclei can move from one cell to another. During the sexual stage, the hyphae of different mating types fuse to form a new cell. Subsequent division of the new cell will produce up to eight ascospores within the ascus. In most Ascomycetes, the asci are formed in complex structures called ascocarps or fruiting bodies. In the cup fungi, the ascocarp is composed of millions of hyphae tightly packed together in the shape of a cuplike container which is exposed above the surface.5

Ascomycetes also produce asexual spores called conidia. Conidia are pinched off at the tips of certain specilized hyphae called conidiaphores (conidia bearers). Conidia are a means of rapidly propagating new mycelia when environmental conditions are favorable. Each conidium that lands on a suitable food source rapidly grows into a new mycelium, which in turn produces conidia. Many generations of conidia are produced during the growing season.1,2

Edible Ascomycetes

True morels, known as sponge mushrooms, are one of the best edible groups of mushrooms, widely hunted throughout North America and the rest of the world. They can be dried and kept in sealed plastic bags or glass jars for several years and will quickly revive in warm weather. Although most morels do not have toxins, they should be cooked before eating. False morels are also eaten, but poisonings sometimes occur.



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Cup fungi are often brightly colored and often fruit in large numbers. They are often found on logs, stumps, rotten wood, and wood debris. Several hundred known species of cup fungi are present in North America.

Aspergillus

The truffles are well known by almost everyone but few ever see them in nature. They develop in the soil and are not visible to the casual observer often associated with the roots with the trees. At maturity they produce a specific odor that attracts rodents, deer and insects. The rodents and deer dig them up and eat them. There are more than 55 species of truffles in North America some of which are commercially grown.3

Pathogenic Ascomycetes

Certain Aspergillus species contaminate house dust ad cause allergies and respiratory disease. Aspergillus species also produce dangerous chemicals called aflatoxins. The aflatoxins from contaminated foods cause damage to nervous system. They have been investigated as agents of bioterrorism because they can be produced as bioweapons.

When Claviceps purpurae infects rye grain, and when humans consume the contaminated grain or bread baked from it, the fungus causes the nervous system disorder called ergot disease.5

It should be noted that in immunocompromised persons (HIV/AIDS individuals) any yeast, which has the ability to grow at 37°C (normal body temperature of human), can be pathogenic and cause infection. The severity of infection caused by these yeasts depends only on the type and state of immune-suppression of the host and the number of yeast organisms and the site of infection.7

Fungal diseases (mycoses) of domestic animals are most commonly caused by yeasts. The majority of these diseases are skin infections, but fatal systemic infections also occur. Currently, most common pathogenic yeasts of domestic animals are included in the genera Candida, Cryptococcus, and Malassezia. C. albicans is the most commonly isolated yeast from dogs and cats with urinary tract infections. Aspergillus and Fusarium filamentous fungi often cause fungal keratitis (ocular disease) in horses which can be triggered by an eye trauma or excessive usage of topical corticosteroids. Infection with Candida species is the principal fungal infection of digestive tract of poultry.6

References

  1. The Microbial Worl. Roger Y. Stanier, John L. Ingraham, Mark L. Wheelis, Page R. Painter
  2. Introductory Botany: Plants, People, and the Environment. Linda Berg, Linda R. Berg
  3. North American mushrooms: a field guide to edible and inedible fungi. Orson K. Miller, Hope Miller
  4. Botany: an introduction to plant biology. James D. Mauseth
  5. Microbes and Society. Benjamin S. Weeks, I. Edward Alcamo
  6. Pathogenic Yeasts. Ruth Ashbee, Elaine M. Bignell
  7. Yeast Biotechnology: Diversity and Applications. T. Satyanarayana, Gotthard Kunze

 

 

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