Although these organisms are stationary like plants and thus traditionally studied in botany courses, the heterotropic fungi are fundamentally different from plants, which are autotrophic organisms.
Although some unicellular forms of fungi exist, most fungi are characterized by a mycelial growth form; that is, they generally are made up of a mass of hyphae (tubular filaments). All fungi live in their food and have an absorptive mode of nutrition by which they secrete digestive enzymes and absorb the breakdown products. They are therefore heterotrophs.
Fungi are also characterized by possession of cell walls made of chitin, synthesis of the amino acid lysine via the amino adipic acid (AAA) pathway, and possession of a ribosomal DNA sequence that classifies them more closely with other fungi than with any other group of organisms.
Fungi produce spores by either asexual or sexual means. The way they produce their spores constitutes one of the main taxonomic criteria for classifying them within the kingdom Fungi.
Ecology and Habitats
|Ecology and Habitats|
Because the saprophytic fungi grow in a radial fashion from a point of origin, the mushroom types of fungi sometimes form so-called fairy rings: At the advancing edge of the mycelia, mushroom fruiting bodies appear in the shape of an irregular ring. One mass of mycelia, for example, became so large that it occupied 37 acres (15 hectares) in the Upper Peninsula of Michigan and became known as the Humongous
Fungi are major parasites, living on live plants, animals, and other fungi. Fungal plant pathogens have evolved a variety of mechanisms to enable the fungus to penetrate the host plant and overcome the host’s defenses.
Next, the pathogen absorbs food from the host by establishing haustoria, which form a highly specialized absorbing system. These structures do not actually penetrate the host plasma membrane, but they reside within pockets of the host cell, where they secrete extrahyphal enzymes and absorb the soluble result.
Consequently, the host plant develops a series of symptoms characteristic of the infection. Farmers can suffer severe loss of crops if an infestation is left unchecked. Plant breeders attempt to breed disease resistance into crop plants as a means of combating fungal diseases.
In contrast to plant fungal pathogens, some fungi grow within plants and do not cause disease symptoms. These fungi are called endophytes. They appear to protect host plants from herbivores and from certain pathogenic microbes.
Endotrophic mycorrhizae penetrate cortical root cells with specialized hyphae that are finely branched. Other fungi grow around the root and between the cortical cells but never penetrate the cells. More than 80 percent of fungal genera that form mychorrizae are in the phylum Basidiomycota.
Lichens are an association that appears to be a controlled parasitism of an alga partner by a fungal partner. Ninety percent of lichens have one of three genera of algae as the photosynthetic partner: Trebouxia, Trentepohlia, and Nostoc. The first two are Chlorophyta (green algae), and the latter is a cyanobacterium (blue-green bacteria, formerly thought to be blue-green algae).
Of all fungal partners, 98 percent are members of the phylum Ascomycota. In this controlled parasitism, the fungus obtains minerals and water and develops a physical structure to house the algal partner. The algal partner provides photosynthetic products to the fungal partner.
These organisms can live in extreme environments such as deserts or Arctic regions. Because they have no way to eliminate toxic materials, they are extremely sensitive to air pollution. Indeed, the condition of lichens is sometimes used as an indicator of air pollution.
Some fungi have trapping mechanisms that allow them to prey upon invertebrate organisms such as nematodes, rotifers, and copepods. These trapping mechanisms involve networks of adhesive hyphae, adhesive branches, adhesive nets, adhesive knobs, nonconstricting rings, and constricting rings.
However, the chytrids are better defined by their asexually produced, motile, uniflagellated zoospores; in fact, the chytrids are the only phylum of fungi that produce such spores. There are about 790 species in this phylum.
Zygomycota reproduce sexually by producing zygospores within a zygosporangium. These fungi have a mycelial growth form that forms crosswalls only at reproductive structures. The zygomycetes produce spores asexually within a sporangium. Sexual reproduction leads to the production of a zygospore within a zygosporangium.
Mycelial cultures of a single strain may mate (in homothallic species) or may require a different mycelial strain (in heterothallic species) in order to mate. In both cases, copulation of two multinucleated gametangia occurs.
Two lateral branches grow toward each other, sometimes attracted by the hormone trisporic acid. When these branches touch, a wall is laid down near the tip of each branch, creating the multinucleated gametangia.
The wall separating the two gametangia at the point of contact breaks down, the protoplasmic contents mix, and the nuclei pair up and fuse. As the nuclei pair, a wall surrounding the fused gametangia develops into the zygosporangium. This thick-walled structure is resistant to environmental abuse.
At or just prior to germination, the nuclei within the zygospore undergo meiosis. A stalk grows out of the zygospore bearing a sporangium. Included in this phylum of about 1,060 species are Rhizopus, the black bread mold; Pilobolus, the “cap” throwing fungus; and Entomophthora, the fly fungus.
Asexually, the Ascomycota reproduce by budding, fission, or production of chlamydospores or conidiospores. The unicellular yeasts reproduce using fission and budding methods. The majority of ascomycetes reproduce asexually, using conidiospores. A cell that is going to generate a conidium grows directly from hyphae or at the end of a stalk.
The conidiogenous cell produces a swelling at the tip, into which protoplasm and a nucleus migrate. The nucleus is produced by mitotic means and is haploid. After swelling is complete and migration finishes, a wall is formed, separating the conidiospore from the conidiogenous cell.
Sexual reproduction occurs by gametangial contact between the female structure (the ascogonium) and the male structure (the antheridium). Usually, opposite mating types are required for sexual reproduction to occur.
When contact between the gametangia occurs, nuclei from the antheridium migrate into the ascogonium and pair up with nuclei in the ascogonium. Alternatively, conidia or microconidia may land on the ascogonium and fuse with it.
In both cases, fusion of paired nuclei is delayed until development of the ascus. Instead, special hyphae grow out from the ascogonium and will eventually generate the ascus. These ascogenous hyphae possess two nuclei, one of each mating type, in each cell.
Frequently the development of the ascogenous hyphae and ascus is associated with the development of a fruiting body. When the fruiting body is at the appropriate developmental stage, a hooklike structure develops at the end of the ascogenous hyphae.
At this time the nuclei fuse, producing the only diploid cell in the life cycle of these organisms. As the saclike ascus elongates, the diploid nucleus undergoes meiosis, producing four haploid nuclei.
Usually one mitotic division then occurs, resulting in eight nuclei in the elongated sac. Eventually walls are formed around each nucleus, creating the ascospore. As noted above, many Ascomycota—but not the yeasts—produce a fruiting body, called the ascoma.
The ascoma may take a variety of shapes, froma closed, ball-like structure (cleistothecium) to a pear-shaped structure with an opening at the top (perithecium) to a cup-shaped open structure (apothecium) or a stromatic structure containing cavities (ascostroma).
In all cases, the asciwith the ascospores are contained within these structures. The type of ascoma, the number of walls in the ascus, and the presence or absence of a fertile layer from which the asci arise are criteria used to distinguish among the about 32,300 species of Ascomycota.
However, the structure of these cross walls is different in that there is a swelling surrounding the pore where the wall is incomplete. There is also a curved membrane on each side of the hole, together looking much like parentheses. This septal structure is called the dolipore septum and is a secondary characteristic of the Basidiomycota.
The mycelia of most Basidiomycota pass through three stages. The primary myceliumhas cells with a single nucleus, all derived from the germinated basidiospore; it is said to be homokaryotic.
Later in development, fusion of hyphae of opposite mating types occurs, establishing the secondary mycelium, in which each cell has two nuclei; the secondary mycelium is therefore dikaryotic.
Clamp connections occur on the secondary mycelium to facilitate division of the two nuclei in limited space. Clamp connections are another secondary characteristic of the Basidiomycota. Tertiary mycelium develops in the specialized organized tissues of the fruiting bodies, the basidioma.
Asexual reproduction takes place by means of budding, fragmentation of mycelium, production of chlamydospores, or conidia. Chlamydospores are fragmented sections of mycelia that have rounded up and formed thick walls. Sexual reproduction in the Basidiomycota occurs primarily by fusion of genetically compatible hyphae, thus establishing the secondary mycelium. Fusion of these nuclei is delayed until the production of the basidium.
Nuclei of the dikaryotic cell that is to become the basidium fuse to produce one diploid nucleus that immediately undergoes meiosis, resulting in four haploid nuclei. Depending on the type of basidiomycete fungus, the haploid nucleus and associated cytoplasm migrate into a swelling that develops at the tip of the basidium.
When full sized, a wall separates the basidiospore from the basidium. Eventually the basidiospore falls off or is shot off the basidium. Classification of the approximately 22,250 species of Basidiomycota depends on the presence or absence of a fruiting body, septation, and the number of cells in the basidium.
About fifteen thousand form-species are grouped into larger form-genera and form-classes, based on the morphological characteristics of their asexual reproductive structures. Because this is an artificial classification (not based on evolutionary relationships), no basis for conclusions about relatedness within groups can be implied or inferred.
Because of a lack of sufficient fossil evidence, phylogenetic relationships have been inferred based on morphological features associated with cell structure and sexually produced structures. With the advent of sequencing analysis of proteins and nucleic acids, observations of some relationships have been confirmed, and new relationships are being discovered.
Small subunit rDNA sequence analysis shows that the fungi are derived froma flagellated animal ancestor. These data show fungi to be a monophyletic group, with the Chytridiomycota and Zygomycota as the earliest branches within the group. The facts that all fungi utilize the AAA synthetic pathway for lysine and possess cell walls of chitin support this monophyletic view of all fungi.
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Sequence analysis, however, appears to contradict morphologically based phylogeny groupings, which use structure and number of cells in the basidium and presence or absence of a fruiting body as key features. Within the Ascomycota, phylogeny appears to be monophyletic, but the evidence is not strong.