5.3 Fungi  (Page 3/9)

The Zygomycota (zygomycetes) are mainly saprophytes with coenocytic hyphae and haploid nuclei. They use sporangiospores for asexual reproduction. The group name comes from the zygospores that they use for sexual reproduction ( [link] ), which have hard walls formed from the fusion of reproductive cells from two individuals. Zygomycetes are important for food science and as crop pathogens. One example is Rhizopus stolonifer ( [link] ), an important bread mold that also causes rice seedling blight. Mucor is a genus of fungi that can potentially cause necrotizing infections in humans, although most species are intolerant of temperatures found in mammalian bodies ( [link] ).

The Ascomycota include fungi that are used as food (edible mushrooms, morels, and truffles), others that are common causes of food spoilage (bread molds and plant pathogens), and still others that are human pathogens. Ascomycota may have septate hyphae and cup-shaped fruiting bodies called ascocarps . Some genera of Ascomycota use sexually produced ascospores as well as asexual spores called conidia , but sexual phases have not been discovered or described for others. Some produce an ascus containing ascospores within an ascocarp ( [link] ).

Examples of the Ascomycota include several bread molds and minor pathogens, as well as species capable of causing more serious mycoses. Species in the genus Aspergillus are important causes of allergy and infection, and are useful in research and in the production of certain fermented alcoholic beverages such as Japanese sake . The fungus Aspergillus flavus , a contaminant of nuts and stored grains, produces an aflatoxin that is both a toxin and the most potent known natural carcinogen. Neurospora crassa is of particular use in genetics research because the spores produced by meiosis are kept inside the ascus in a row that reflects the cell divisions that produced them, giving a direct view of segregation and assortment of genes ( [link] ). Penicillium produces the antibiotic penicillin ( [link] ).

Many species of ascomycetes are medically important. A large number of species in the genera Trichophyton , Microsporum , and Epidermophyton are dermatophytes , pathogenic fungi capable of causing skin infections such as athlete’s foot, jock itch, and ringworm . Blastomyces dermatitidis is a dimorphic fungus that can cause blastomycosis , a respiratory infection that, if left untreated, can become disseminated to other body sites, sometimes leading to death. Another important respiratory pathogen is the dimorphic fungus Histoplasma capsulatum ( [link] ) , which is associated with birds and bats in the Ohio and Mississippi river valleys. Coccidioides immitis causes the serious lung disease Valley fever . Candida albicans , the most common cause of vaginal and other yeast infections, is also an ascomycete fungus; it is a part of the normal microbiota of the skin, intestine, genital tract, and ear ( [link] ). Ascomycetes also cause plant diseases, including ergot infections, Dutch elm disease, and powdery mildews.

Saccharomyces yeasts, including the baker’s yeast S. cerevisiae , are unicellular ascomycetes with haploid and diploid stages ( [link] ). This and other Saccharomyces species are used for brewing beer.