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Division Ascomycota

Asexual Spores

The fungi form nonmotile conidia from the tip or side of a sporogenous (conidiogenous) region or cell. Conidia are not surrounded by a second wall, as in the case of sporangiospores. The spores are usually dispersal units and a few may travel great distances. Arthrospores and some conidia may be long-lived. Viability of conidia is reduced rapidly in the presence of UV radiation, and in dry conditions. Other spores have high metabolic rates which limit their viability.

Conidia may arise from conidiogenous regions in several ways. In blastic ontogeny, the conidium originates from a narrow portion of the region which swells before being cut off by a septum. In thalloblastic ontogeny, the conidium originates from a broad region of the hypha, and as with blastic ontogeny, the cell swells before delimitation by the septum. In thallic ontogeny, the conidia arise from a length of hypha; septa are laid down before each conidium swells. Arthric conidia are specialised thallic conidia formed in chains that disarticulate readily.

Conidia mostly germinate by germ tube extension. In a few cases, conidia germinate to form yeast cells, or to form a further conidium.

Taxonomy

Ascomycota is the largest division of fungi. Classification of the division is in a state of change due to the integration of molecular information. The most recent analyses (eg Schoch et al 2009) have proposed a major reconsideration of the group. These analyses benefit from using both molecular, morphological and developmental data. The orders suggested by Schoch et al (2009) include: Sordariomycetes, Laboulbeniomycetes, Leottiomycetes, Lichinomycetes, Lecanoromycetes, Eurotiomycetes, Dothidiomycetes, Arthoniomycetes, Orbiliomycetes, Pezizomycetes, Saccharomycetes, Schizosaccharomycetes, Pneumocystidiomycetes and Noelectomycetes. This hypothesis is based on 434 taxa. As the division contains many fungi, the classification will, inevitably, be expanded and refined as further information is included.

The following indicate only the diversity within the division.

Order 1: Laboulbeniomycetes

The fungi form parasitic attachments to arthropods.

Order 2: Schizosaccharomycetes

A group lacking ascogenous hyphae and ascomata. The asci formed lack a protective covering if they form from a hymenium. The taxon contains some of the ascomycetous yeasts.

Order 3: Pezizomycetes

A huge group containing fungi which form large and obvious ascomata. The orders are separated on the structure of the ascus and the manner of ascus opening. The ascocarp is often seen as a flattened sheet, or one that has variably evolved to a series of cup shaped structures held above or at the surface of the supporting substrate.

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Cultural Conditions

Nutritionally, most are either saprobes or weak plant parasites. Others include lichen symbionts, endophytes of angiosperms, arthropod parasites and a few form ectomycorrhiza. Most of these fungi have the capacity to utilise simple sugars as a source of organic energy. Some parasites produce enzymes that enable them to digest structural carbohydrates such as cellulose, chitin and pectin.

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Ecology

The vast majority of these fungi are terrestrial, a few are aquatic. Many species are of great importance because they are pathogens of plants and animals causing serious disease. Several of the fungi produce metabolites of commercial importance. One yeast is used for production of bread and alcohol. A few filamentous species are the basis of the production of antibiotics, or immune suppressants, or used in food processing and flavouring. A few species are important producers of toxins in animals, especially human foods. Among the taxa are the most xerotolerant organisms known, those that live on dried foods. Many ascomycota form associations with algae called lichens, and others form ectomycorrhizas.

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Hyphal Characteristics and Pores

The yeast fungi are found as single cells. The cells divide either by holoblastic budding or fission. Fission fungi may form asci following fusion of cells of opposite mating types. The cells are separated by a septum which is perforate.

The hyphae of Ascomycota have bilayered walls with primary septa at regular intervals along the hypha. The cell is divided by a septum which has a simple pore. The pore may be associated with an electron dense structure known as the Woronin body. Woronin bodies are only found in the hyphal phase of Ascomycota. In true yeasts and thalli transforming to a yeast phase, Woronin bodies are not seen.

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Sexual Spores and Mating Types

Anastomosis of somatic hyphae of opposite mating type leads to the dikaryotic phase only within the ascoma. The dikaryotic phase is restricted and leads immediately to the formation of the crozier, usually within a hymenium.The reflexed tip, or crozier, enables mitotic division of the nuclei, delimiting the ascus mother cell in which karyogamy is followed immediately by meiosis. In other words, plasmogamy, karyogamy and meiosis are closely linked spatially and temporally in Ascomycota.

Haploid ascospores, usually eight, are formed within an ascus immediately following meiosis and one mitotic division. Ascospores are released with the opening or breaking of the wall of the ascus. Asci are sometimes held singlely. Compound structures have hymenia either within an inoperculate spherical structure called a cleistothecium, an ostiolate flask-shaped structure called a perithecium, or an open hymenium called an apothecium.The compound ascoma may be held within locules of a stroma, and the stroma may be epigeous or hypogeous during spore release. Spore release is usually by active ejection from apothecia and perithecia. Hypogeous ascocarps are usually consumed by animals which disperse the spores during deposition of faeces.

Macroscopic ascocarps are rare. The largest is an apothecium which may reach 15cm across (e.g. Peziza) or 15cm high (e.g. Morchella). Ascocarps rarely exceed 2 to 4 mm in any one dimension.

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Further Reading

Alexopoulos, Mimms & Blackwell 1996 Introductory Mycology (4th edition), Ch 7/15.

Schoch CL et al. 2009 The Ascomata Tree of Life: A Phylum-wide Phylogeny Clarifies the origin and evolution of Fundamental Reproductive and Ecological Traits. Systematic Biology 58: 224-239.

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