Introduction to Fungi

Fungi are a vast assemblage of 95,000 organisms. All of them completely lack photosynthesis. They are heterotrophs that depend upon other living or dead matter for nutrition. As parasites many are serious pathogens on other plants. As saprotrophs they, along with bacteria, degrade dead organisms and release organic chemicals and nutrient elements so they can be recycled. About 13,500 fungal species have a unique association with some algal partners resulting in symbiotic structures known as lichens. The majority of higher plants possess mycorrhizal association where some species of fungi live as symbionts inside or around the roots.

Fungi are eukaryotes. They are an ancient group. Fossil evidence shows that all major fungal groups known today had already evolved by the end of the Paleozoic era, about 280 million years ago. At a time when all living things were grouped under either th animal or the plant kingdom the fungi were thought to be plants. We now place all fungi in the kingdom, Fungi (Myceate). Members of this kingdom lack plastids. They are mostly filamentous in construction. Except in one group their walls contain chum rather than cellulose. Fungi do not store starch as plants do. The filamentous structures that make up the fungal body are known as mycelia (singular. mycelium). Although the filaments are microscopic. the extensive growth of fungal mycelium can be seen as a fuzzy mass. The reproductive bodies of some fungi such as the mushrooms are made up of well defined aggregates of mycelia. Complex tissues and organs characteristic of the plants are never found among the fungi. Fungi reproduce by spores.

In spite of the many features that seem to unite the members of the fungal kingdom the fungi are a heterogeneous group. Fungi are classified into 7 divisions (Table 1.2). Relationships among these groups are shown in Fig. 2.2. The slime molds (Myxomnycota) are not true fungi. They appear to have evolved independently from some protozoan ancestors. In their vegetative phase the slime molds lack a cell wall. The wall-less cells aggregate lo form an amoeba-like mass that moves around and engulfs bacteria and other organio matter. Two groups of slime molds are known: the plasmodial slime molds with a multinucleate true plasmodium and the cellular slime molds. The vegetative body of cellular slime molds is a pseudo plasmodium where the aggregating cells retain their cell membranes and individuality. Slime molds produce motile spores. The oomycetes or water molds differ from other fungi by the possession of cellulose in their cell walls. The fungal body is diploid rather than haploid as in other true fungi. These and other features of reproduction and metabolism suggest that the oomycetes are not related to other fungal groups. They might have evolved from some green or yellow-green algal ancestors after losing their plastids.

The chytrids are simple water molds that live as parasites or saprotrophs. Because they possess motile spores they are often classified with the oomycetes. However, the ehytrids have chitin and their filaments are haploid. They are probably distantly related lo the bread molds and other true fungi.

The zygomyeetes (bread molds). Ascomycetes (sac fungi) and basidiomycetes (club fungi) are evolutionarily related as shown in Fig. 2.2. None of them produce motile cells at any stage of their life cycle. The frugal filaments do not have septa (cross walls) in the zygomyceetes. The mycelium is septate in the other two groups.

Fungi reproduce asexually and sexually. In sexual reproduction the ascomycetes produce characteristic structures known as asci (singular. ascus). Basidia are the equivalent structures among the basidiomycetes. A fungal species can he assigned to either one of these groups only when they produce an ascus or basidium. A vast number of fungi.

about 22.000 species, reproduce only asexually, or sexual cycle has not been observed yet. Because their life cycle is imperfectly known and they cannot he assigned with confidence to either one of the groups they are known as Fungi Imperfectly. The divisional name Deuteromycota is often used for this group of imperfect fungi. When the sexual life cycle is known the species is automatically assigned to-either the ascomycetes or the basidiomycetes.

fungi

Lichens are unique organisms consisting of a fungal and an algal panner. Less than 40 Introduction to Cyanobacteria.. algal or cyanobacterial species enter into this association. Yet, there are about 13,500 species of lichens! The characteristic form of each lichen appears to be determined by the fungal component. About 2% of the species have either a basidiomycete or an imperfect fungus as [he fungal panner. The remaining 98% of lichens are composed of ascomycete species. The lichens are not considered to be a separate taxonomic category. Rather, they are treated as members of the respective fungal divisions, and the name of a lichen refers co the name of its fungal partner.

In Table 2.2 the fungi are divided into 7 formal divisions. in other classifications only two divisions are recognized, the Myxomycota (slime molds) and Eumycota (true fungi). The latter is divided into subdivisions and classes etc.

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