The Fungi - Background

Lecture 03 [Notes]
Fungi represent a separate kingdom derived from single-celled eukaryotes. They lack chlorophyll and thus must grow on some organic matter to derive their carbon source. This organic matter may be living or non­living. If the food source is living the fungus acts as a PARASITE. If the food source is non­living, the fungus acts as a SAPROBE.

Dependent upon food source, the fungi may be beneficial or detrimental to human welfare. Fungi are important DECOMPOSERS within the biosphere and are responsible for some of the initial and essential recycling of elements. Unfortunately some of the materials attacked and decomposed by fungi may be building materials, cloth, leather, waxes, jet fuel, insulation, photographic film and lenses of cameras, to say nothing of food products. Some fungal products released from the action of decomposition may be toxic and carcinogenic ­ specifically the AFLATOXINS.

Various antibiotics, organic acids, alcohols, and carbon dioxide are among the beneficial products of fungi. One of the most recent "wonder drugs" isolated from a fungus is CYCLOSPORINE an immune system repressor which has made organ transplantation a wide spread reality without the usual undesirable side effects.

Fungi play an important role in plant nutrition with the formation of MYCORRHIZAE and in symbiotic relationships with algae and bacteria in the formation of LICHENS.

Fungi are the most important single cause of garden plant and crop destruction, with over 5000 species involved. Between 50­100 species are responsible for human and animal diseases. Over 100,000 species of fungi have been identified with an estimated 200,000 still to be discovered.

Biology of the Fungi
The vast majority of fungi are terrestrial, and vary from single cells, to filaments, to multicellular structures. The fungal structure is composed of white or grayish cobwebby strands known as HYPHAE. These hyphae vary in length and are often multinucleate. The total mass of hyphae is called a MYCELIUM. Growth of hyphae occurs at the tips but proteins are synthesized throughout the hyphae and transported to the tips by CYCLOSIS.

The cell walls of fungi are composed of the polysaccharide CHITIN, the same material found in the exoskeletons of arthropods and is particularly resistant to microbial decomposition.

All parts of the fungus are metabolically active and the enzymes and other materials produced have an immediate impact on their environment. Hyphae may penetrate the soil or other substrate and obtain nutrition by direct absorption through the thin cell wall. Food is broken down within the hyphae and often stored as GLYCOGEN, or in some fungi as LIPIDS. Specialized hyphae known as RHIZOIDS serve in anchorage and function like roots. Parasitic fungi have HAUSTORIA to absorb materials directly from the host. Most fungi produce spores dispersed by the wind, and all are non­motile throughout their life cycle.

Fungi are either COENOCYTIC, that is having many nuclei within a bounded area, or SEPTATE, having septa or cross walls dividing the cytoplasm. These septa are often perforated or incomplete thus allowing for a flowing of cytoplasm between the cells. The oldest fungi are at least 500 million years old and probably were aquatic.

Fungal Reproduction
Fungi are identified on the basis of their reproductive structures. These reproductive structures are AERIAL and separated from the hyphae by complete septa. They are called GAMETANGIA if they are directly involved in the production of gametes or sex cells, and SPORANGIA or CONIDIOGENOUS (CONIDIA) if involved in the production of ASEXUAL SPORES. The gametes are equal in size or ISOGAMOUS. Meiosis immediately follows the formation of a zygote. Karyokinesis (MITOSIS and MEIOSIS) is unique in the fungi as they do not lose their nuclear envelope during the process. The nuclear envelope constricts near its midpoint and spindle fibers develop within the nuclear envelope. Fungi may have more than one type of genetic information within the various nuclei within the hyphae, a phenomenum known as HETEROKARYOSIS. This increases genetic variability within the fungus and provides for greater diversity and increased survival potential during natural selection.

Division Zygomycota
primarily saprobes on plant and animal matter although some act as parasites; approximately 600 described species; name refers to chief characteristic of division ­ production of sexual resting structures called ZYGOSPORANGIA; production of this structure follows fusion of two multinucleate gametangia; inside the zygosporangium the gametes fuse forming a zygote

asexual reproduction is accomplished by means of spores produced in specialized aerial hyphae called SPORANGIA; one of the most common of this division is Rhizopus stolonifer a black mold found on carbohydrate­rich substrate such as bread; sporangia produce thousands of spores, each capable of eventually forming a new mycelium; the genus Pilobolus exhibits a phototaxic response which forcibly ejects the spores great distances

Division Ascomycota
consist of approximately 30,000 described species; including the bread mold Neurospora used by Beadle and Tatum when they elucidated their "one gene, one enzyme" hypothesis which was an important breakthrough in the development of modern genetics; also includes the POWDERY MILDEWS, which attack leaves; Chestnut blight caused by Endothia parasitica and Dutch elm disease caused by Cerotcystis ulmi; yeasts, morels and truffles are also found within this group

asexual reproduction is accomplished by formation of specialized spores known as CONIDIA, borne on modified hyphae called CONIDIOPHORES
sexual reproduction involves formation of an ASCUS, a saclike structure containing haploid ASCOSPORES; often a total of 8 ascospores are formed after mitosis occurs in the ascus

Unicellular Ascomycetes: The Yeasts
yeasts are usually unicellular and reproduce by FISSION or BUDDING rather than spore formation; sexual reproduction occurs when two cells unite to form a zygote; yeasts are important to humans because of their ability to carry on FERMENTATION, breaking down glucose to form ethyl alcohol and carbon dioxide; most of the fermentative yeast are strains of a single species ­ Saccharomyces cerevisiae;
most yeasts are members of the Ascomycota, however a few belong to the Division Basidiomycota ­ two examples which cause a variety of human diseases are Candida albicans and Cryptococcus neoformans; there are at least 350 known species of yeasts

Division Deuteromycetes (Fungi Imperfecti)
consist of about 25,000 described species in which the sexual phase is unknown (thus it is our knowledge that is imperfect ­ not the fungi); some within this group are ascomycetes which reproduce by conidia; other are basidiomycetes based on their structural arrangement; Fungi Imperfecti is an artificial group made of diverse organisms ­ it is maintained for ease in classification and as a means of confusing students; prominent within this Division is a diverse group known as the DERMATOPHYTES responsible for ringworm or TINEA disorders

recently members of the genus Penicillium, and the genus Aspergillus, were moved to the Ascomycota

The Lichens
Lichens are symbiotic associations between ascomycetes and certain genera of green algae or cyanobacteria. Originally thought to be a mutualistic relationship whereby the photosynthetic partner provides the nutrients and the fungal partner provides protection through enzymatic action and a means of preventing dessication, the nature of the relationship is presently under review. Lichens are ubiquitous, hardy pioneers able to colonize hostile environments and establish the groundwork for future plant succession. A cross sectional view of a lichen resembles a leaf which may explain their ability to survive adverse conditions. There are three types ­ FOLIOSE or leaf­like; FRUTICOSE ­ large with upright stalks such as reindeer moss and British Soldier lichen; CRUSTOSE ­ bright colored flat mats often found on rock walls. They concentrate nutrients but have no way to release them and thus become environmental indicators. Gaseous pollutants may kill the algal partner, again indicating environmental conditions. They have a variety of uses ranging from dyes, to medicines, to perfumes, to architectural landscaping.

Division Basidiomycota
includes about 25,000 described species; consists of mushrooms, toadstools, stinkhorns, puffballs, and shelf or bracket fungi; also two groups of plant pathogens: RUSTS and SMUTS; produce BASIDIOSPORES on club­shaped structures called BASIDIA
divided into three Classes:
contain edible and poisonous mushrooms, coral fungi, and shelf or bracket fungi; Agaricus campestris is the common field mushroom; Amanita phalloides is known as the "destroying angel" and is fatal if eaten, causes hemolysis of red blood cells; some members of this group produce hallucinogenic chemicals ­ include psilocybin, mescaline, LSD

stinkhorns, puffballs, bird's nest fungi

consists of the rusts and smuts; black stem rust of wheat is caused by Puccinia graminis, uses barberry as an intermediate host

Division Oomycota (often placed in the Kingdom Protista)
consist of approximately 475 species known as water molds; examples include Saprolegnia which is parasitic on fish, fish eggs and frog eggs; also includes Plasmopara viticola which is the cause of downy mildew which destroys grapes; Phytophthara infestans was the cause of potato blight and the subsequent famine in 1846­7 which caused substantial immigration of Irish into this country; Entomophthara is an insect larval parasite and an effective means of biological control; Rhizophydium is a parasite on diatoms (algae)

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