The life cycle of a fungus can vary considerably among species with respect to growth habit and reproduction. In general, most fungi produce some type of asexual and/or sexual spore during their life cycle that enables them to grow, be disseminated, and/or survive. Under favorable environmental conditions, spores germinate to produce a thread-like filament called a primary hypha (plural, hyphae) or germ tube. Hyphae grow by extension at the hyphal tips in a branching pattern that allows maximum absorption of nutrients from a substrate. The hyphae become organized into the vegetative body of the fungus called a mycelium (plural, mycelia). Fungi are nonphotosynthetic and rely on
absorption to obtain their nutrition. They produce an array of extracellular degradative enzymes necessary to break down complex molecules from dead and/or living cells into simpler compounds that can be readily absorbed.
As fungi colonize their substrate, spores may be produced via asexual or sexual reproduction. Spores produced via asexual reproduction develop as a result of mitosis without the fusion of gametes or nuclei. In contrast, spores produced via sexual reproduction involve the fusion of nuclei and/or gametes via meiosis. Examples of asexual spores include aleuriospore, chlamydospores, conidia, sporangiospores, urediospores, and zoospores (Figure 6.2). Asexual spores can form singly on a hyphal cell or form in groups into a specialized structure (e.g., synnema, sporodochium, Figure 6.3). Asexual spores can also be produced on hyphal cells in an enclosed or partially enclosed fruiting structure (pycnidium, acervulus, Figure 6.3).
The morphology and ontogeny of asexual spores (conidia) provide a basis for their classification in the Deuteromycota. Multiple generations of asexual spores may be produced in a given year depending on environmental conditions and host susceptibility. Asexual spores are important for rapid dissemination and propagation of identical genetic individuals of a pathogen and for subsequent epidemic development. Although sexual spores can have functions similar to those of asexual spores, they are also important for long-term survival and for serving as a mechanism for generating genetic diversity
through recombination. Fungi can also produce specialized structures such as microsclerotia or sclerotia (a compacted mass of melanized cells) to enhance their survival during periods of unfavorable temperature and moisture conditions. Examples of sexual spores include aeciospores, ascospores, basidiospores, oospores, sporidia, and zygospores. In general, ascospores are produced in a specialized fruiting structure called an ascocarp. Four different types of ascocarps have been described: apothecium, cleistothecium, perithe-cium, and pseudothecium (Figure 6.4). However, some fungi in the phylum Ascomycota do not produce ascospores in an ascocarp. Basidiospores are produced on a basidium that is or is not enclosed in a fruiting body or mushroom (basidiocorp), while oospores and zygospores are usually borne directly on or in hyphal cell (Figures 6.5 and 6.6).
For more detailed information see the websites by Tom Volk, http:// botit.botany.wisc.edu/toms_fungi/, and Kathy Hodge, http://biodiversity. uno.edu/~fungi/.
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