Ascomycetes make up 64% of all known fungal species. They include important wood-rotting fungi, many ectomychorrizal species, and significant pathogens of both animals and plants. Ascomycetes form the fungal partner in most lichens—approximately 40% of all ascomycete species occur in lichens. Ascomycetes loom large in human history, contributing the baker’s and brewer’s yeasts used to make bread and beer, the antibiotic penicillin, and model systems employed in laboratory investigations of eukaryotic cell biology and genetics. Ascomycetes are used to produce soy sauce, sake, rice vinegar, and miso, and to transform milk into Brie, Camembert, and Roquefort cheeses. Athlete’s foot and other skin infections are caused by ascomycetes, as are many more serious fungal diseases.

Ascomycetes include both species that form multicellular fruiting bodies and ones that form unicellular yeasts. A remarkable feature of ascomycete fruiting bodies is that they contain both dikaryotic and haploid cells. The proliferation of dikaryotic cells leads to the production of many asci, while the growth of haploid cells contributes to the bulk of the fruiting body. Two ascomycete lineages consist largely of yeasts. The earliest branching members in these groups, however, produce hyphae. Thus, the unicellular nature of yeasts is a derived feature evolved through loss of hyphae and is not an ancestral condition.

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Fig. 34.19 illustrates the life cycle of a common ascomycete, the brown cup fungus. In ascomycetes, meiosis is followed by a single round of mitosis, resulting in asci that contain eight haploid spores. When mature, the spores are ejected from the top of the asci, expelled by turgor pressure. In many ascomycetes, fruiting bodies elevate the asci on one or more cup-shaped surfaces, from where the spores are easily caught and carried away by wind.

In the fruiting bodies of some ascomycetes, the asci are completely enclosed by a layer of tissue and thus must be dispersed by other organisms rather than by the wind. The truffles prized in cooking provide an example. The edible truffle is the fruiting body of an ascomycete that grows as an ectomycorrhizal fungus on tree roots. Not only do truffles encase their spores in protective tissues, but they also develop underground. How, then, can their spores be dispersed? Developing truffles release androstenol, a hormone also produced by boars before mating. The hormone attracts female pigs, which unearth and consume the fruiting body. The spores pass through the pig’s digestive tract without damage and are released into the environment in feces, thereby dispersing the spores.

Another ascomycete is thought to have played a role in the events that unfolded in Salem, Massachusetts, in 1692. That year, several girls came down with an unknown illness manifested by delirium, hallucinations, convulsions, and a crawling sensation on the skin. The girls were thought to be bewitched, and subsequent accusations led to the infamous Salem witch trials. They were executed for witchcraft, but today many scholars prefer a simpler diagnosis for the illness: poisoning by the ascomycete fungus ergot. Ergot is a common pathogen of rye and related grasses, and ingestion of contaminated plants can lead to the symptoms reported in Salem. Like many defensive compounds produced by plants, alkaloid molecules produced by ergot and their derivatives are used in medicine in low doses, such as in the treatment of migraines.

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Ascomycetes are even known for producing so-called zombie ants, a topic explored in Fig. 34.20.