The Fungi
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Paul Stamets started Fungi Perfecti with the goal of building the bridge between people and fungi.Since its inception in 1980, Fungi Perfecti has become synonymous with cutting-edge mycological research and solutions. Our continued mission is to explore, study, preserve, and spread knowledge about the use of fungi for helping people and planet. Read More
Premise of the study: Fungi are major decomposers in certain ecosystems and essential associates of many organisms. They provide enzymes and drugs and serve as experimental organisms. In 1991, a landmark paper estimated that there are 1.5 million fungi on the Earth. Because only 70000 fungi had been described at that time, the estimate has been the impetus to search for previously unknown fungi. Fungal habitats include soil, water, and organisms that may harbor large numbers of understudied fungi, estimated to outnumber plants by at least 6 to 1. More recent estimates based on high-throughput sequencing methods suggest that as many as 5.1 million fungal species exist.
Key results: Molecular methods have dramatically increased our knowledge of Fungi in less than 20 years, revealing a monophyletic kingdom and increased diversity among early-diverging lineages. Mycologists are making significant advances in species discovery, but many fungi remain to be discovered.
Conclusions: Fungi are essential to the survival of many groups of organisms with which they form associations. They also attract attention as predators of invertebrate animals, pathogens of potatoes and rice and humans and bats, killers of frogs and crayfish, producers of secondary metabolites to lower cholesterol, and subjects of prize-winning research. Molecular tools in use and under development can be used to discover the world's unknown fungi in less than 1000 years predicted at current new species acquisition rates.
These fungi grow as saprophytes, parasites, or both by using specific proteolytic, glycolytic, or lipolytic enzymes to extracellularly break down substrates and to absorb the products of digestion through the fungal cell envelope.
The true fungi obtain their carbon compounds from nonliving organic substrates (saprophytes) or living organic material (parasites) by absorption of nutrients through their cell wall. Small molecules (e.g., simple sugars and amino acids) accumulate in a watery film surrounding the hyphae or yeast and simply diffuse through the cell wall. Macromolecules and insoluble polymers (e.g., proteins, glycogen, starch, and cellulose), on the other hand, must undergo preliminary digestion before they can be absorbed by the fungal cell. This process involves release of specific proteolytic, glycolytic, or lipolytic enzymes from the hypha or yeast, extracellular breakdown of the substrate(s), and diffusion of the products of digestion through the fungal cell envelope (Fig. 73-2). Fungal pathogens rely on these digestive enzymes to penetrate natural host barriers.
Not all species of fungi have cell walls, but in those that do, cell wall synthesis is an important factor in determining the final morphology of fungal elements. Thus, our knowledge of fungal morphogenesis has evolved in parallel with our understanding of fungal cell wall biosynthesis. The fungal wall also protects cells against mechanical injury and blocks the ingress of toxic macromolecules. This filtering effect may be especially important in protecting fungal pathogens against certain fungicidal products of the host. The fungal cell wall is also essential to prevent osmotic lysis. Even a small lesion in the cell wall can result in extrusion of cytoplasm as a result of the internal (turgor) pressure of the protoplast. The composition of fungal cell walls is relatively simple and includes substances not typically found in animal and plant hosts (e.g., chitin). On this basis, it may be possible to identify pathogen-specific molecular targets from investigations of the biosynthesis of fungal wall components. Such targets may prove pivotal for the successful development of antifungal drugs that are not toxic to mammalian cells.
Fungi, like bacteria, are ecologically important as decomposers as well as parasites of plants and animals. Both groups of microbes often inhabit the same ecosystem and thus compete for the same food supply. Associated with this competition is the production by both the fungi and bacteria of secondary products that function as microbial growth inhibitors or toxins. These compounds constitute a rich library of antimicrobial agents, many of which have been developed as pharmacologic antibiotics (e.g., penicillin from Penicillium chrysogenum, nystatin from Streptomyces noursei, amphotericin B from S niveus).
The superficial morphologic similarities between actinomycetes (filamentous bacteria) and molds suggest that the two groups have undergone parallel evolution. Despite the production of branching filaments and mold-like spores, the actinomycetes are clearly prokaryotes, whereas fungi are eukaryotes. Moreover, the sexual reproduction of bacteria, which typically occurs by transverse binary fission, should not be confused with asexual processes of budding and fragmentation associated with mitotic nuclear division in fungi. Most of the molds that produce septate vegetative hyphae reproduce exclusively by asexual means, giving rise to airborne propagules called conidia. On the other hand, elaborate mechanisms of sexual reproduction are also demonstrated by members of the Eumycota. Four distinct kinds of meiospores (products of karyogamy-meiosis-cytokinesis) are recognized: oospores (Oomycetes), zygospores (Zygomycetes), ascospores (Ascomycetes), and basidiospores (Basidiomycetes).
Sexual reproduction in the fungi typically involves fusion of two haploid nuclei (karyogamy), followed by meiotic division of the resulting diploid nucleus (Fig. 73-5A). In some cases, sexual spores are produced only by fusion of two nuclei of different mating types, which necessitates prior conjugation of different thalli. This condition of sexual reproduction is known as heterothallism, and the nuclear fusion is referred to as heterokaryosis. Normally plasmogamy (union of two hyphal protoplasts which brings the nuclei close together in the same cell) is followed almost immediately by karyogamy. In certain members of the Basidiomycotina, however, these two processes are separated in time and space, with plasmogamy resulting in a pair of nuclei (dikaryon) contained within a single cell. Karyogamy may be delayed until considerably later in the life history of the fungus. Meanwhile, growth and cell division of the binucleate cell occur. The development of a dikaryotic mycelium results from simultaneous division of the two closely associated nuclei and separation of the sister nuclei into two daughter cells (Fig. 73-5B). An alternative mechanism of sexual reproduction in the fungi is homothallism, in which a nucleus within the same thallus can fuse with another nucleus of that thallus (i.e., homokaryosis). An understanding of these nuclear cycles is fundamental to investigations of fungal genetics.
As mentioned above some fungi are classified as strictly asexually reproducing forms. These include the large group of asexual (imperfect) yeasts (e.g., Candida species) and conidial fungi (e.g., Coccidioides immitis). Most members of this group have permanently lost their ability to produce meiospores. A few undergo rare sexual reproduction, and perhaps for some species we have yet to discover their sexual (perfect) stage. The most common methods of asexual reproduction, in addition to simple budding in yeasts, are blastic development of conidia from specialized hyphae (conidiogenous cells), fragmentation of hyphae into conidia, and conversion of hyphal elements into conidia or chlamydospores (thick-walled resting spores) (Fig. 73-6).
Despite the absence of meiosis during the life cycle of these imperfect fungi, recombination of hereditary properties and genetic variation still occur by a mechanism called parasexuality. The major events of this process (Fig. 73-7) include the production of diploid nuclei in a heterokaryotic, haploid mycelium that results from plasmogamy and karyogamy; multiplication of the diploid along with haploid nuclei in the heterokaryotic mycelium; sorting out of a diploid homokaryon; segregation and recombination by crossing over at mitosis; and haploidization of the diploid nuclei. Sexual and parasexual cycles are not mutually exclusive. Some fungi that reproduce sexually also exhibit parasexuality.
An extensive foundation of knowledge on the basic biology of fungi is at hand, including fungi that cause superficial, deep-seated, and systemic infections of humans and other animals. Much less is known, however, of the intricacies of interactions between these largely opportunistic pathogens and their hosts. Many areas of research in medical mycology are still in their infancy and offer formidable challenges and potential rewards. The current application of methods of recombinant DNA technology to problems of fungus-host interactions, especially the identification of pathogenicity genes, holds promise for significant contributions to our knowledge of medically important fungi.
There's more to fungi than just mushrooms. Fungi are the cause of scores of life-threatening diseases, they are the earth's best degraders of organic matter and they are proving to be more useful to science and manufacturing every year. They come in many forms, ranging from single-celled yeasts on the order of 10 Ì