. What Fungi Are Like
A. Fungi Are Multicellular Eukaryotes
1. Fungi are mostly multicellular eukaryotes that share a common mode of nutrition.
2. Similar to animals, they are heterotrophic and consume preformed organic matter.
3. Their cells send out digestive enzymes into the immediate environment; following breakdown of molecules, the nutrients are absorbed.
4. Most fungi are saprotrophic decomposers, breaking down waste products or remains of plants and animals.
5. Some are parasitic, living off the tissues of living plants and animals.
a. Fungi can enter leaves through stomates; plants are especially subject to fungal diseases.
b. Fungal diseases account for millions of dollars in crop losses each year.
c. Fungi also cause human diseases including ringworm, athlete's foot, and yeast infections.
6. Several types of fungi are adapted to mutualistic relationships with other organisms.
a. Some fungi are symbionts of roots; they acquire inorganic nutrients for plants and receive organic nutrients.
b. Others form an association with a green alga or cyanobacterium to form a lichen.
B. Most Fungi Are Filamentous
1. Some fungi are unicellular (e.g., yeasts).
2. Most fungi are multicellular in structure.
a. The thallus (body) of most fungi is a mycelium.
b. A mycelium is a network of hyphae comprising the vegetative body of a fungus
c. Hyphae are filaments that provide a large surface area and aid absorption of nutrients.
d. When a fungus reproduces, a portion of the mycelium becomes reproductive structures.
3. Fungal cells not only lack chloroplasts, but also have cell wall made of chitin, not cellulose.
a. Chitin, like cellulose, is a polymer of glucose molecules organized into microfibrils.
b. Chitin, unlike cellulose, each glucose has an attached nitrogen containing amino group.
4. The energy reserve is glycogen, not starch.
5. Fungi are nonmotile; their cells lack basal bodies and do not have flagella at any stage in their life cycle.
6. They move toward a food source by growing toward it; hyphae can grow up to a kilometer a day!
7. Nonseptate hyphae lack septa or cross walls; the hyphae are therefore multinucleate.
8. Septate fungi have cross walls in their hyphae; however, pores allow cytoplasm and organelles to pass freely.
9. The septa that separate reproductive cells, however, are complete in all fungal groups.
C. Most Fungi Produce Spores
1. In general, fungal sexual reproduction involves (find it in the AP barrons book)
2. During sexual reproduction, haploid hyphae from two different mating types make contact and fuse.
3. If the nuclei do not fuse immediately, resulting hypha is dikaryotic or contains paired haploid nuclei, n + n.
a. In some species, the nuclei pair but do not fuse for days, months, or even years.
b. The nuclei continue to divide in such a way that every cell has at least one of each type.
4. When the nuclei fuse, the resulting zygote undergoes meiotic cell division leading to spore formation.
5. Fungal spores germinate directly into haploid hyphae without embryological development.
6. Fungal Spore Formation
a. Spores are an adaptation to life on land and ensure that the species will be dispersed to new locations.
b. A spore is a reproductive cell that can grow directly into a new organism.
c. Fungi produce spores both during sexual and asexual reproduction.
d. Although nonmotile, the spores are readily dispersed by wind.
7. Asexual reproduction can occur by three mechanisms:
a. Production of spores by a single mycelium is the most common mechanism.
b. Fragmentation is when a portion of a mycelium becomes separated and begins a life of its own.
c. Budding is typical of yeasts; a small cell forms and gets pinched off as it grows to full size.
. How Fungi Are Classified
A. Evolution of Fungi
1. The evolutionary history of fungi is not known but they evolved by about 570 million years ago.
2. Fungi may not share a common ancestor but may have evolved separately from several protist ancestors.
3. Some biologists propose that fungi evolved from red algae; both lack flagella in all stages of the life cycle.
4. R. H. Whittaker argued for their own kingdom based on their multicellular nature and mode of nutrition.
5. Fungi share similarities and have differences with other groups of protists.
6. Not knowing phylogeny, fungal groups are classified according to differences in life cycles and sporangia.
B. Zygospore Fungi Form Zygospores
1. The division Zygomycota contains about 600 species of zygospore fungi.
2. Most are saprotrophs living off plant and animal remains in the soil or bakery goods in a pantry.
3. Some are parasites of small soil protists, worms, or insects.
4. The black bread mold, Rhizopus stolonifer, is a common example from this phylum.
a. With little cellular differentiation among fungi, hyphae may be specialized for various functions.
b. Stolons are horizontal hyphae that exist on the surface of the bread.
c. Rhizoids are hyphae that grow into the bread, anchor the mycelium, and carry out digestion.
d. Sporangiophores are stalks that bear sporangia.
e. A sporangium is a capsule that produces spores called sporangiospores.
f. During asexual reproduction, all structures involved are haploid.
5. The phylum name refers to the zygospore seen during sexual reproduction.
a. Hyphae of different mating types (plus and minus) are chemically attracted and grow toward each other.
b. Ends of hyphae swell as nuclei enter, then cross walls develop behind each end, forming gametangia.
c. Gametangia merge, resulting in a large multinucleate cell in which nuclei of two mating types pair and fuse.
d. A thick wall develops around the cell, thereby forming a zygospore.
e. The zygospore undergoes a period of dormancy before meiosis, and germination takes place.
f. Germination involves the development of one or more sporangiophores, with sporangia at their tips.
g. The spores are dispersed by air currents and give rise to new haploid mycelia.
C. Sac Fungi Form Ascospores
1. The division Ascomycota contains about 30,000 species of sac fungi.
2. Most sac fungi play an essential role in digesting resistant cellulose, lignin, collagen and other materials.
3. Red bread molds, cup fungi, morels, and truffles are also sac fungi.
4. Many ascomycetes are plant parasites and include powdery mildews that grow on leaves, leaf curl fungi, chestnut blight, and Dutch elm disease.
5. Ergot is a parasitic fungus that infects rye and produces a toxin that can cause hysteria and death.
6. Yeasts are unicellular, but most ascomycetes are composed of septate hyphae.
7. Sexual reproduction involves production of eight ascospores within ascus contained within saclike ascocarp.
a. Ascus-producing hyphae remain dikaryotic except in walled-off portion that becomes ascus.
b. Asci are the fingerlike sacs in which nuclear fusion, meiotic cell division, and ascospore formation occur during sexual reproduction of sac fungi; usually surrounded by an ascocarp.
c. In most ascomycetes, the asci become swollen and burst, expelling the ascospores.
d. If released into the air, the spores are windblown.
8. Asexual reproduction, which is the norm, involves the production of conidiospores in multicellular species.
a. There are no sporangia in ascomycetes.
b. Conidiospores (conidia) develop directly on tips of conidiophores, modified aerial hyphae.
D. Yeasts Are Single Cells
1. Yeasts are unicellular organisms that reproduce asexually by mitosis or budding.
2. Saccharomyces cerevisiae is brewer's yeast; a small cell forms and is pinched off as it grows.
3. Sexual reproduction in yeasts occurs when food runs out, and involves the production of asci and ascospores.
a. Ascospores from two different mating types fuse, resulting in a diploid cell.
b. A diploid cell reproduces asexually, then undergoes meiotic cell division forming ascospores.
c. The haploid ascospores function directly as new yeast cells.
4. Yeasts produce ATP through fermentation, for which ethanol and CO2 are waste products.
5. Because of its ability to produce alcohol, yeast fermentation is important in the production of wine; special strains of yeast are added to essentiallsterile grape juice and the CO2 is kept for sparkling wines.
6. Yeasts are added to prepared grains to make beer.
7. Because of its ability to produce CO2, yeast fermentation is important in the production of bread.
8. Yeasts are also used in genetic engineering experiments requiring a eukaryote.
E. Club Fungi Have Basidiospores
1. Club fungi are in the division Basidiomycota and include about 16,000 species.
2. They have septate hyphae and include mushrooms, bracket fungi, puffballs, bird's nest fungi, and stinkhorns.
3. Sexual reproduction is the norm and usually involves production of basidiospores within clublike basidia contained within a basidiocarp; asexual reproduction is rare and involves the production of conidiospores.
a. Sexual reproduction of club fungi begins when monokaryotic hyphae of two different mating types meet and fuse to form a dikaryotic (n + n) mycelium.
b. The dikaryotic hypha mycelium, which continues its existence for years (perhaps even hundreds of years), and occasionally produces one or more basidiocarps.
c. Basidiocarps are fruiting bodies (e.g., mushrooms and puffballs) composed of tightly packed dikaryotic hyphae whose walled-off ends become the club-shaped basidia.
d. Basidia are club-shaped structures, formed within a basidiocarp from the walled-off ends of dikaryotic hyphae, where nuclear fusion, meiotic cell division, and basidiospore production occur; contains four projections into which cytoplasm and a haploid nucleus enter to form four basidiospores.
e. When released, basidiospores are dispersed by air currents; when they germinate, a new mycelium forms.
4. In a puffball, spores are produced inside parchmentlike membranes and released when it breaks down.
5. In bird's nest fungi, falling raindrops provide force to splatter "basidiospore-containing eggs" through the air.
6. Stinkhorns have a slimy cap and attract flies by their bad odor; flies pick up the spores and distribute them.
7. Smuts and Rusts Are Parasites
a. Rusts and smuts are club fungi that parasitize cereal crops (e.g., corn, wheat, oats, and rye) and are of great economic importance because of the crop losses they cause every year.
b. Rusts and smuts do not form basidiocarps, and their spores are small and numerous, resembling soot.
c. Some smuts enter seeds and exist inside the plant, becoming visible only at maturity.
d. Corn smut mycelium grows between the corn kernels and secretes substances that cause tumors on ears.
e. They have a life cycle that may be complex and may involve more than one host; thus, control measures have centered on eradicating the alternate hosts.
F. Imperfect Fungi Reproduce Asexually Only
1. About 25,000 species are imperfect fungi in the division Deuteromycota.
2. They are "imperfect" because they have not been observed to reproduce sexually and cannot be classified.
3. They reproduce asexually by forming conidiospores.
4. Cellular morphology and biochemistry indicate some are sac fungi that lost their ability to reproduce sexually.
5. Several species of imperfect fungi are of great economic importance; for example,
a. Some species of the mold Penicillium are sources of the antibiotic penicillin, while others give the characteristic aroma and flavor to certain cheeses (e.g., Roquefort and Camembert).
b. Cyclosporine, which suppresses the immune system, is from an imperfect fungus found in soil.
c. Aspergillus is used in the production of soy sauce, citric acid, and gallic acids.
6. Unfortunately, some imperfect fungi cause human diseases; for example,
a. Respiratory infection is caused by dust-borne spores of an imperfect fungus.
b. Athlete's foot and ringworm are skin infections caused by direct contact.
c. Candida albicans is a yeastlike fungus that causes infections of the vagina, diaper rash, and thrush.
Fungi Form Symbiotic Relationships
A. Lichens Improve Soil Conditions
1. Lichens are a symbiotic association between a fungus and a cyanobacterium or green alga.
2. The body of a lichen is composed of three layers: a thin, tough upper layer and a loosely packed lower layer that shield the photosynthetic cells in the middle layer.
3. Specialized fungal hyphae, which penetrate or envelope the photosynthetic cells, transfer nutrients directly to the rest of the fungus.
4. Lichens can reproduce asexually by releasing fragments that contain hyphae and an algal cell.
5. This association was considered mutualistic, but experimentation suggests a controlled parasitism by fungus of the alga.
a. It has been shown that the algae grow faster when they are alone rather than when they are part of a lichen.
b. On the other hand, it is difficult to cultivate the fungus, which does not grow naturally alone.
c. The different lichen species are therefore identified based on the fungal partner.
6. Three types of lichens are recognized.
a. Compact crustose lichens, which are often seen on bare rocks or tree bark.
b. Leaflike foliose lichens.
c. Shrublike fruticose lichens.
7. Lichens may live in extreme environments and on bare rocks; they are soil formers.
8. Lichens are efficient at acquiring nutrients; they survive areas of low moisture, temperature and poor soil.
9. However, lichens also take up pollutants and cannot survive where the air is polluted.
B. Mycorrhizae Are "Fungus Roots"
1. Mycorrhizae are mutualistic relationships between soil fungi and the roots of most plants.
2. The fungus may enter the cortex of roots but does not enter the cytoplasm of plant cells.
3. It helps the roots absorb more minerals; in turn, the plant passes on carbohydrates to the fungus.
4. Ectomycorrhizae form a mantle that is exterior to the root, growing between cell walls.
5. The truffle lives in association with oak and beech tree roots; it can be inoculated with the fungus.
6. The fossil record indicates that the earliest plants had mycorrhizae associated with them; it would seem, then, that mycorrhizae helped plants adapt to and flourish on land.