Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

The Setting
The Kingdom of Protists
Fungi

Lacking the capacity for Photosynthesis and living predominantly in the soil, though occasionally also in Water, Fungi are represented by a vast number of species—around 10⁵, which is only half the number of vascular plant species. Fungi are characterized by two distinct features: they lack chlorophyll and they grow as a mass of numerous branching tubes (typically 6–8 µm in diameter); these tubes are called hyphae, and the mass they form is known as the mycelium. In most cases, fungi are coenocytic organisms, meaning their hyphae consist not of discrete Cells, but of a mass of protoplasm containing numerous nuclei, with the tube being divided by septa only on rare occasions. Nutritionally, fungi are predominantly Saprophytes, though some are parasitic, causing serious and intractable diseases in humans. A major medical challenge is the lack of Antibiotics capable of effectively treating fungal infections (mycoses). On the other hand, fungi are responsible for synthesizing many vital antibiotics, such as penicillin. Furthermore, fungi produce some of the most potent toxins known to science.

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FIG. 1-8. Fungi commonly used in laboratory research: Neurospora crassa and the Yeast Saccharomyces cerevisiae. (Webster I., Introduction to Fungi, Cambridge Univ. Press, London, New York, 1970.)

Lower fungi (Phycomycetae) are simple freshwater slimes and Molds. Higher fungi are subdivided into ascomycetes (Ascomycetae) and Basidiomycetes (Basidiomycetae) based on the method of sexual spore formation. In ascomycetes, spores are formed within small sac-like structures called asci (Fig. 1-8). An ascus contains four or eight linearly arranged spores, where each group of four represents the result of a single pair of meiotic divisions (with subsequent mitosis yielding eight spores). Due to this unique mode of sporulation, one particular ascomycete, Neurospora crassa (Fig. 1-8), has become a favorite model Organism for genetic research [36]. Ascospores can be isolated from the ascus in a precise ordered sequence and cultured individually to map the outcomes of Crossing-over events that occurred during Meiosis.

Neurospora can also reproduce via haploid spores known as conidia. Haploid mycelia occur in two mating types, and conidia or mycelia of one mating type are capable of fertilizing Cells of the opposite type (located within a specialized Structure called the protoperithecium) to form zygotes. The zygotes immediately undergo meiosis and mitosis to form eight ascospores.

Ascomycetes include highly prized edible truffles and morels. However, the majority of edible mushrooms are the fruiting bodies of basidiomycetes. Basidiomycetes also encompass rust fungi, which inflict catastrophic damage on harvests of wheat and other cereal crops.

Yeasts are fungi adapted to environments with a high sugar concentration; they typically remain unicellular and reproduce by budding (Fig. 1-8). Periodically, their haploid cells fuse in pairs to form diploid cells and sexual spores. Some yeasts are ascomycetes, while others belong to basidiomycetes. Saccharomyces cerevisiae, the active agent in both baker's and brewer's yeasts, is an ascomycete capable of robust growth in both the diploid and haploid phases, with diploid cells being somewhat larger than haploid ones [37].