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KINGDOM FUNGI
Although fungi were previously thought to be plants they share virtually no
characteristics with them other than multicellularity and being Eukaryotes.
There are more than 70,000 species and they occupy a variety of habitats
over most of the earth. Some estimate there may be 1.5 million species. The
oldest fossils resembling fungi date back about 544 million years.
I. Characteristics of Fungi
A. Terrestrial (a few aquatics)
B. Multicellular (except yeasts)
C. No motile cells (except Chytrids)
D. Filamentous
1. Hyphae (hypha) = individual filaments
2. Mycelium (mycelia) = masses of filaments,
e.g. mushrooms is a mycelium
3. Fungi have the ability to grow very rapidly, within 24 hours
they can produce up to 1 kilometer of new mycelium
4. In some fungi the nuclei are contained in a common
cytoplasm, i.e. the hyphae are aseptate. This condition is called coenocytic
("contained in a common vessel")
5. In other fungi the hyphae are septate but the septa (cross
walls) are perforated
E. Chitin cell walls - chitin is a
polymer of polysaccharide like molecules that contain nitrogen
F. Heterotrophic via absorption. The food is ingested by
absorption after partial digestion by enzymes secreted outside the cell
1. saprobes - live on dead organic matter.
Sometimes have specialized hyphae for anchoring called rhizoids
2. parasites - feed on living matter.
Parasitic fungi have specialized hyphae called haustoria (haustorium)
which absorb food directly from the host cells
G. Glycogen primary storage polysaccharide (same as in
animals)
H. Spores are principle means of reproduction. They
are quite small and easily carried by wind. This is why they are seemingly
everywhere.
I. Reproductive structures are separated from hyphae by complete septations
1. Called gametangia if they are involved in gamete production
2. Called sporangia or conidiogenous cells if they are
involved in the production of spores
J. Unique Mitosis and meiosis - this supports their treatment as a
separate kingdom
1. During mitosis and meiosis the nuclear envelope persists. It
remains and at the end of division it constricts near its middle to separate
the chromosomes.
2. Fungi also lack centrioles (organize spindle). Replaced by spindle
pole bodies at spindle poles.
K. Fungi are haploid, at least their nuclei usually are
1. Some fungi are heterokaryotic, i.e. the nuclei in a common
cytoplasm are genetically different, either because of mutation or the fusion
of genetically distinct hyphae. Due to interaction between the 2 haploid
nuclei, the fungus can function somewhat like a diploid organism, i.e.
dominant & recessive alleles
2. In heterokaryotes, the two genetically different nuclei may sometimes fuse
to produce a diploid nucleus in which crossing over and genetic recombination
can take place. This is called the parasexual cycle. This allows
haploid organisms to behave much like diploid organisms and gives greater
evolutionary potential
II. Importance of Fungi
A. Along with bacteria, fungi are important in the world's ecosystem as decomposers,
releasing carbon dioxide to the atmosphere and nitrogen and other compounds to
the soil where they are available for other organisms. The fact that fungi are
decomposers makes them serious pests to humans
B. Fungi can grow on just about anything, cloth, paper, leather, waxes, jet
fuel, insulation, cable wires, photographic film and even optical glass.
During World War II up to half of the supplies sent to tropical areas were
unusable due to fungal destruction. They are the single most important cause
of plant diseases, particularly on our most valuable crops, such as wheat
C. Human fungal infections are quite common in tropical areas. A variety of
human respiratory infections are caused by fungi, e.g. 80% of AIDS deaths are
due to pneumonia caused by Pneumocystis which appears to be a fungus,
not protozoan.
D. Beneficial fungi include:
1. Yeasts are important in baking and brewing
2. Penicillin, the first antibiotic comes from a fungus
3. Cyclosporin, a drug which suppresses immune reactions in organ
transplants
4. Nearly 80% of all vascular plants form associations with fungi in
their roots (mycorrhizae), which aids in the plants nutrition
E. Fungi also form symbiotic relationships with algae and blue-green algae
to form lichens.
III. Classification of Fungi. Our text recognizes four phyla, we'll
cover the three largest.
A. Zygomycota - Zygomycetes - black bread molds, about 1,060 species
1. Live on decaying plant and animal matter in the soil. Some are parasites
of plants, insects and small soil organisms. A few cause serious human or
domestic animal diseases
2. Have aseptate (coenocytic) hyphae
3. Produce a zygosporangium following the fusion of two
multinucleate gametangia.
a. The gametes are simply haploid nuclei which are contained in the
zygosporangium, a black warty structure. The zygosporangium may remain in a
resting stage for a long time. Eventually the zygotes inside undergo meiosis
to form spores in a sporangium. These spores are released and they germinate
into new haploid hyphae
4. Asexual reproduction may also take place via spores produced in
sporangia
5. Rhizopus stolonifer, the black bread mold is a
typical representative. See your text for its life cycle
B. Ascomycota - Ascomycetes - the sac fungi (ascus means sac), about
32,300 species
1. Includes most of the colored molds on foods, antibiotic producers such
as Penicillium, yeasts, morels, and truffles. Also some of the worst
plant pathogens such as chestnut blight, Dutch Elm disease and powdery
mildews. Also includes the Ergot fungus on rye, the original source of LSD. Neurospora,
the red bread mold used in genetic research, is also a member of this
division.
2. Characterized by perforate septate hyphae and
production of an ascus (sac) following sexual reproduction.
The asci (pl. of ascus) occur in a mycelium called an ascocarp
and they contain ascospores
3. See your text for a typical life cycle
4. Asexual reproduction also occurs via conidia (fine dust) which
are multinucleate spores produced by conidiophores (conidia bearers)
5. Lichens are symbiotic associations between Ascomycetes (98%) and
certain green algae or cyanobacteria. There are about 13,250
"species". The photosynthetic algae provide food while the
fungus provides shelter
C. Basidiomycota - Basidiomycetes - the club fungi, about 22,300 species
1. Includes mushrooms, toadstools, puffballs, shelf fungi, rusts, bird's
nest fungi and smuts
2. Characterized by perforate septate hyphae and the
production of a basidium (club) following sexual
reproduction. The basidia (pl. of basidium) occur in a mycelium called a basidiocarp
and they produce external basidiospores
3. See your text for a typical life cycle
D. Other fungi
1. Yeasts - unicellular fungi which may belong to
the Zygomycota, Ascomycota or Basidiomycota. Most species belong to the
Ascomycota. A few filamentous forms have been discovered. About 600 species.
Economically important since they can ferment carbohydrates (anaerobic
respiration) into ethyl alcohol and carbon dioxide. They are used for
both baking and brewing
2. Deuteromycetes (Fungi Imperfecti) - artificial
assemblage of about 15,000 species of fungi which lack (or it is unknown)
sexual reproduction. They rely totally on asexual reproduction via conidia.
The majority of them are Ascomycota. Penicillium and Aspergillus
are the two best known genera. Many anthracnose diseases of plants and many
human skin diseases are deuteromycetes
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