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31 Fungi: Recyclers, Pathogens, and Plant Partners
31 General Biology of the Fungi Diversity in the Kingdom Fungi Fungal Associations
31 General Biology of the Fungi The fungi live by absorptive nutrition, secreting digestive enzymes that break down large food molecules and absorbing the breakdown products. Some are saprobes (feeding on dead matter); others are parasites. A few have mutually beneficial (symbiotic) relationships with other organisms.
Figure 31.1 Parasitic Fungi Attack Other Living Organisms
31 General Biology of the Fungi The production of chitin is a shared derived trait for fungi, choanoflagellates, and animals. The presence of chitin in fungi is evidence that all fungi are more closely related to animals than to plants. The kingdom Fungi consists of four phyla: Chytridiomycota, Zygomycota, Ascomycota, and Basidiomycota. The four phyla are primarily distinguished by their methods and structures of reproduction.
Table 31.1 Classification of Fungi
31 General Biology of the Fungi Unicellular forms are found in all of the fungal phyla. Those of the Zygomycota, Ascomycota, and Basidiomycota are called yeasts. Yeasts may reproduce by budding, fission, or sexual means.
Figure 31.2 Yeasts Are Unicellular Fungi
31 General Biology of the Fungi The vegetative body of a multicellular fungus is called the mycelium (plural mycelia). The mycelium is composed of threadlike hyphae (singular hypha). Within the hyphae of two clades, cell-like compartments are formed by incomplete cross- walls called septa (singular septum). Pores in septa allow free movement of organelles, sometimes even nuclei, and other materials. Some hyphae are coenocytic—they have no internal separations into distinct cells.
Figure 31.3 Most Hyphae Are Incompletely Divided into Separate Cells
31 General Biology of the Fungi The hyphae may be widely dispersed to forage for nutrients or they may be clumped together in a cottony mass to exploit a rich nutrient source. Sometimes the mycelium becomes reorganized into a fruiting (reproductive) structure, such as a mushroom. Rhizoids are modified hyphae, which anchor Chytridiomycota to a substrate. These rhizoids are not homologous to the rhizoids of plants because they are not specialized to absorb water and nutrients.
31 General Biology of the Fungi Some parasitic fungi may have modified hyphae that take up nutrients. Fungal parasites can invade wounds on plants and grow mycelium throughout the plant. Some hyphae produce haustoria, branching projections that push into the living plant cells and absorb nutrients within them.
Figure 31.4 A Fungus Attacks a Leaf
31 General Biology of the Fungi The mycelium has a very high surface area-to- volume ratio. Throughout the mycelium (except in fruiting structures), all the hyphae are very close to their environmental food source. Fungi are tolerant to highly hypertonic environments. Many can tolerate temperatures as low as 5–6 C below freezing. Some can tolerate temperatures as high as 50 C or more.
31 General Biology of the Fungi The majority of fungi are saprobes, living on dead organisms. Saprobic fungi (along with bacteria) are Earth’s primary decomposers. Fungi are the principal decomposers of cellulose and lignin. Many fungi can use ammonium (NH 4 + ) ions or nitrate (NO 3 – ) as a sole source of nitrogen. Most are unable to synthesize their own thiamin or biotin, but they can synthesize some vitamins that animals cannot.
31 General Biology of the Fungi Facultative parasites can attack living organisms but they can also be grown on defined media. Obligate parasites grow only on their specific host. Most predatory fungi secrete sticky substances from the hyphae. Trapped prey are penetrated by hyphae and eventually killed. Some species form a ring with modified hyphae that constricts around nematodes. The crawling nematode triggers these rings to swell and trap the worm. Hyphae quickly invade and digest the worm.
Figure 31.5 Some Fungi Are Predators
31 General Biology of the Fungi Lichens are symbiotic associations of a fungus with a cyanobacterium, a unicellular photosynthetic eukaryote, or both. Mycorrhizae are mutualistic associations of fungi and plant roots. The fungus obtains organic compounds, while the plant is provided with water and soil minerals.
31 General Biology of the Fungi Asexual reproduction among the fungi includes: The production of haploid spores within sporangia. The production of naked spores at the tips of hyphae (not within sporangia) called conidia. Cell division by unicellular fungi—either equal division (fission) or production of a daughter cell (budding). Simple breakage of the mycelium.
31 General Biology of the Fungi Sexual reproduction involves fusion between different mating types. Some fungi have more than two mating types. Mating types cannot be distinguished morphologically. Mating can occur only between different mating types, which prevents self-fertilization. Fungi reproduce sexually when hyphae (or motile cells in chytrids) of different mating types meet and fuse.
31 General Biology of the Fungi In many fungi, the zygote nuclei are the only diploid nuclei of the life cycle. These nuclei undergo meiosis, producing haploid nuclei. Haploid spores divide mitotically to form haploid hyphae. This type of life cycle is called a haplontic life cycle and is a characteristic of many protists.
31 General Biology of the Fungi Fungal pathogens are a major cause of death among people with compromised immune systems. Most patients with AIDS die of fungal diseases such as Pneumocystis carinii. Candida albicans and other yeasts also cause severe diseases in those with AIDS. Other less severe and common diseases include ringworm and athlete’s foot. Plant diseases include black stem rust and others.
Figure 31.6 Phylogeny of the Fungi
31 Diversity in the Kingdom Fungi The chytrids (phylum Chytridiomycota) are the earliest diverging fungal lineage. They are aquatic microorganisms, formerly classed with protists but now classed with fungi because of the chitin in their cell walls. They are the only fungi that have flagella at any stage of the life cycle. Chytrids are parasitic or saprobic, but some are found in the rumen of ruminants. Most live in fresh water or moist soil; some are marine. Some are unicellular; others have coenocytic hyphae. They reproduce both sexually and asexually.
31 Diversity in the Kingdom Fungi Allomyces displays alternation of generations. A haploid zoospore germinates to form a haploid organism, which later forms female and male gametangia. Both have flagella. The female gametes produce a pheromone that attracts male gametes. The male and female gametes fuse to produce a diploid organism, which produces numerous diploid flagellated zoospores. These disperse and produce more diploid organisms. These eventually produce resistant resting sporangia that can survive dry and freezing weather. The nuclei in sporangia eventually undergo meiosis to produce haploid zoospores.
Figure 31.7 Reproductive Structures of a Chytrid
31 Diversity in the Kingdom Fungi Zygomycetes (phylum Zygomycota) have coenocytic hyphae; they have only one diploid cell, the zygote. Most form occasional stalked reproductive structures called sporangiophores. Sporangiophores may have one or many sporangia. One group are the fungal species in the most common mycorrhizal associations. Black bread mold is Rhizopus stolonifer.
Figure 31.8 A Zygomycete
Figure 31.9 Sexual Reproduction in a Zygomycete (Part 2)
31 Diversity in the Kingdom Fungi The ascomycetes (phylum Ascomycota) are a large and diverse group with septate hyphae, and distinguished by the production of asci (singular ascus). The ascus contains the products of meiosis. There are two groups of ascomycetes: Those with an ascocarp are called euascomycetes (“true ascomycetes”). Those without are called hemiascomycetes (“half ascomycetes”).
Figure Asci and Ascospores
31 Diversity in the Kingdom Fungi Most hemiascomycetes are microscopic and some are unicellular. Baker’s or brewer’s yeast (Saccharomyces cerevisiae) is an ascomycete. Hemiascomycete yeasts reproduce asexually by budding or fission. Sexual reproduction occurs when two haploid cells of opposite mating types fuse. In some, the zygote immediately undergoes meiosis. The entire cell becomes an ascus. Four or eight ascospores are produced depending on whether the cells divide once after meiosis.
31 Diversity in the Kingdom Fungi The euascomycetes include some of the fungi known as mold. Neurospora is pink bread mold. Many euascomycetes are plant parasites such as chestnut blight and Dutch elm disease. Powdery mildews infect cereals, lilacs, roses, and other plants. Cup fungi such as morels and truffles are euascomycetes. These produce huge numbers of spores and can be several centimeters in diameter.
Figure Two Cup Fungi
31 Diversity in the Kingdom Fungi Penicillium is a genus of green molds. Some species produce the antibiotic penicillin. P. roquefortii and P. camembertii provide the flavors to the cheeses Roquefort and Camembert. Aspergillus tamarii is used to ferment soybeans to make soy sauce. A. oryzae is used in brewing the Japanese alcoholic beverage sake. Some Aspergillus species that contaminate peanuts and pecans produce powerful mutagens called aflatoxins.
Figure The Life Cycle of a Euascomycete
31 Diversity in the Kingdom Fungi About 25,000 species of basidiomycetes (phylum Basidiomycota) have been described. They produce a wide variety of fruiting structures (basidiocarps): puffballs, mushrooms, and giant bracket fungi. There are more than 3,250 species of mushrooms. Agaricus bisporus is the common edible one; some Amanita mushrooms are deadly poisonous. Bracket fungi are tree parasites. Smut fungi parasitize cereal grains. Basidiomycetes have septate hyphae.
31 Diversity in the Kingdom Fungi Fungi not yet placed in any existing phyla are grouped as imperfect fungi or deuteromycetes. Deuteromycetes currently include 25,000 species. The sexual cycle has yet to be observed in these species. DNA sequences can now be used to determine actual relationships between deuteromycetes and other fungi.
31 Fungal Associations Almost all tracheophytes have mycorrhizae, which help make water and minerals more available to the plant. Ectomycorrhizae are fungi that wrap around the root tips and acts as a sponge. Endomycorrhizae infect the interior of the root. The fungi get sugars, amino acids, and some vitamins from the plant. The fungi might supply growth hormones as well, and protect the plant against attack by microorganisms. Fungal–plant root interactions have existed for hundreds of millions of years.
Figure Mycorrhizal Associations
31 Fungal Associations Lichens are a meshwork of two different organisms. One is a fungus, and the other is a photosynthetic organism. Lichens can survive harsh environments. In spite of this hardiness, lichens are sensitive to air pollution because they cannot excrete toxic substances. Hence they are good biological indicators of air pollution. The fungi of most lichens are ascomycetes. The photosynthetic component may be either a cyanobacterium or a unicellular green alga.
31 Fungal Associations There are about 13,500 “species” of lichens. The fungal components cannot grow independently of their photosynthetic partners. The reindeer “moss” is a lichen that is very important in the diet of large mammals in the arctic, subarctic, and boreal regions. Lichen growth forms include crustose, foliose, and fruticose.
Figure Lichen Body Forms (Part 1)
Figure Lichen Body Forms (Part 2)
31 Fungal Associations The most widely held interpretation is that the lichen relationship is a mutualistic one. The algal cells in a lichen “leak” photosynthetic products at a greater rate than do similar cells growing on their own. On the other hand, the photosynthetic cells from lichens grow more rapidly on their own than when combined with a fungus. Therefore, we could consider lichen fungi as parasitic on their photosynthetic partners.
31 Fungal Associations Lichens can reproduce simply by fragmentation of the vegetative body called the thallus. They can also reproduce by means of specialized structures called soredia (singular soredium). These are composed of fungal hyphae and a few photosynthetic cells. These become detached, are dispersed by air currents, and then develop into a new lichen. If the fungal partner is an ascomycete or a basidiomycete, the fungus may undergo a sexual process, but the spores are released alone into the environment and fail to reestablish the lichen relationship.
Figure Lichen Anatomy (Part 1)
Figure Lichen Anatomy (Part 2)
31 Fungal Associations Lichens are often the first colonists on newly exposed areas of rock. They satisfy most of their nutritional needs from air, rainwater, and from the minerals absorbed from dust. A lichen usually begins to grow shortly after a rain event. Eventually, as the lichen grows, its water content drops to less than 10 percent of its dry weight, and it becomes highly tolerant of temperature extremes.