Kingdom Fungi:
Fungi are heterotrophs, which secrete digestive enzymes (exoenzymes) into their surroundings, and
absorb the organic molecules after the food has been digested.
In this mode, the fungi are known as SAPROPHYTES or SAPROBES (live on dead matter).
Those that aren't saprophytes are PARASITES.
Most fungi are multicellular. Some produce highly elaborate reproductive structures.
Fungi are decomposers, which help with the cycling of important nutrients. Fungi are also used
commercially in such products as cheese, antibiotics, bread and beer (alcohol).
Some fungi cause diseases; for example, Dutch elm disease is caused by an ascomycete, and
Aspergillus contaminates grain or peanuts. Aspergillus produces alfatoxin that is carcinogenic.
You will notice that when we talk about fungi, that many of their names end in MYCOTA or
MYCETES. MYKETOS is Greek for fungus; the root word is also the source for the term
MYCELIUM-- the body of many fungi. The mycelium is made up of tread-like structures known
as HYPHAE (singular: hypha). Septate hyphae are tubular cells with one nucleus per cell.
Coenocytic hyphae have no cell wall separating the nuclei, so the hyphae consist of a continuous
cytoplasmic mass.
Cell walls are composed of CHITIN. The MYCELIUM is the feeding structure that secretes the
digestive enzymes and absorbs the products. The mycelium is usually the result of germination and
growth from one spore. Growth occurs at the tips of hyphae. Parasitic fungi often have specialized
hyphae called HAUSTORIA, which have nutrient absorbing tips that penetrate the tissues of the
host. The rest of the hyphae remain outside the host cell membrane.
Many fungi develop erect spore producing organs, SPORANGIA, which makes tiny spores by the
millions. Sporangia are made on specialized hyphae called SPORANGIOPHORES. The spores
are everywhere and are a means of asexual reproduction. Spores are small and able to survive long
periods of drought and extreme temperatures. Often sporangia are raised above the mycelium by
the sporangiophores. The spores are caught up and transported by air currents. The bright colors
and powdery textures associated with many types of molds are the colors and textures of spores and
sporangia. Fungi also reproduce sexually-often in complex lifestyles. In fact, the nuclei of hyphae
and spores are almost always haploid.
In sexual reproduction, the fusion of gametes produces two events called Syngamy. Syngamy is the
sexual union of cells from two different individuals; it occurs in two stages: 1) plasmogamy: the
fusion of cytoplasm and 2) karyogamy: the fusion of the nuclei. In fungus, after plasmogamy the
nuclei don't fuse or form a dikaryon (a cell with two nuclei). These cells behave like diploids in
that two haploid nuclei can compensate for harmful mutations. The dikaryon stage is called the
heterokaryotic stage. Finally the nuclei fuse to form a diploid nucleus (zygote) but then it
immediately undergoes meiosis, forming haploid nuclei again.
Divided into five divisions: The divisions are named after the cells in which karyogamy occurs.
1
Chytridiomycota: chytrids.
Zygomycetes: Conjugating molds, e.g. bread mold.
Ascomycetes: sac fungus, e.g. neurospora and yeasts.
Basidiomycetes: club fungi, e/g. toadstools and mushrooms.
Deuteromycota: Fungi Imperfecti. Includes fungi in which sexual reproduction is unknown or lost
evolutionarily or unobserved. Also included are fungi whose sexual cycles are known to be closely
related.
Chytridiomycota:
Chytrids are the organisms are believed to be the link between protists and fungus. They are
mainly aquatic. Some are saprobes, while others parasitize protists, plants, and aquatic
invertebrates. Chytrids form uniflagellated sperm (protist characteristic) called zoospores.
Scientists analyzed their DNA and protein products and determined that chytrids are a fungus.
Their cell walls are made of chitin and they are composed of coenocytic hyphae.
Zygomycetes: Conjugating molds.
Most zygomycetes are terrestrial fungi that are saprobes.
Zygomycetes have sexual reproduction characterized by the formation of zygospores. They can
form mycorrhizae, which are mutualistic associations with the roots of plants.
Example:
Common bread mold grows on many foods. The spores land on the food and germinate. Hyphae
grow in all directions. Some of the hyphae extend vertical hyphae called rhizoids that anchor the
fungus to the substrate, secrete digestive enzymes, and absorb dissolved organic matter. When the
bread mold hyphae have grown for a long time, they develop horizontal hyphae called stolons.
When sporangia appear on vertical hyphae, these hyphae are called sporangiophores.
Sex begins with conjugation. When opposing mating types are grown in the same medium,
specialized hyphae are produced (no male or female, we will refer to the compatible types as + and
-). Hormones that diffuse in gas attract the two types to each other. When + and - hyphae fuse, they
form a gametangium (plasmogamy), which contains two haploid nuclei. The nuclei fuse,
karyogamy, to form diploid zygospores. These can stay dormant for a long time (the structure is
now called a zygosporangium). When conditions are right, meiosis occurs within the zygospore
followed by the degeneration of all but one haploid nucleus. A haploid sporangium is formed on a
sporangiophore. Many spores are formed, each spore released may produce a new mycelial mass.
Ascomycetes and Basidiomycetes are considered higher fungus.
Ascomycetes: Include the edible mushroom Morchella; powdery mildew, blue and green molds;
Neurospora, yeasts; and ergot.
There are 60,000 species of ascomycetes plus an additional 25,000 species of lichens, most of
which have an ascomycete component.
If ergot infects rye bread, it has the strange property of constricting blood vessels in the body's
extremities. People on a continuous diet of ergot infected bread suffer from "holy fire" or "St.
Anthony's fire" which causes hallucinations and burning sensations in the hands and feet. The
2
restricted blood flow in these parts can result in gangrene and subsequent loss of extremities (last
reported case in 1960's in France). Ergot is now used to control internal bleeding.
The most famous ergot produces a compound called lysergic acid diethylamide, LSD. People have
tied together moist seasons with the increase of witch accusations. The rye became moist and
infected with ergot, people ate rye bread and began hallucinating… accused people of being
witches.
Asexual Reproduction:
Ascomycetes produce conidiospores during the asexual stage. Conidiospores are produced in long
chains at the end of specialized hyphae known as conidiophores.
Sexual Reproduction:
There is great diversity in the sexual characteristics of ascomycetes. The name 'sac fungi' originates
from the production of ascospores in a sac like container called an ascus. A group of asci can be
found in a fruiting structure known as an ascocarp, which is usually spherical in shape. What goes
on inside the ascocarp is unusual. The diploid ascocarp is produced by delayed fertilization.
When the hyphae of different mating types come into contact, one hypha produces a large
multinucleated swelling called an ascogonium (a sort of ovary). The other hypha produces a
multinucleated antheridium (sperm producing organ).
The antheridium grows towards and contacts the ascogonium, releasing nuclei into it. Each
reproductive structure supplies many nuclei. The nuclei do not immediately fuse. The hyphae,
which emerge from the ascogonium, contain one nucleus of each mating type in each cell. The
fusion of the nuclei occurs after the ascocarp is formed. Meiosis immediately follows, completing
the sexual process.
The four products of the meiotic division undergo one round of mitosis to produce 8 haploid
ascospores. When mature, the ascus breaks open, and the spores are forcibly ejected.
Basidiomycetes: includes mushrooms, rusts, smuts, and shelf fungi.
The mycelial mass of the common mushroom lies deeply embedded in organic matter, breaking it
down and using the nutrients. The mushroom that we see is the spore-producing BASIDIOCARP,
which produces BASIDIOSPORES. The mature basidiocarp develops a large number of GILLS or
PORES or TEETH on its underside. These contain spore-producing cells called BASIDIA
(singular: BASIDIUM). Meiosis occurs in each basidium producing four haploid spores. The
spore nuclei migrate through minute projections from the basidium.
Each mushroom can develop millions to billions of spores.
The sexual phase of the mushroom occurs before the basidiocarp develops. The basidiocarp
contains two nuclei in each cell (like the ascocarp). The nuclei of the terminal cell fuse, making the
basidium diploid. Then the basidial cell nucleus quickly undergoes meiosis, producing four haploid
3
nuclei that will develop into basidiospores.
Deuteromycota: Fungi Imperfecti
This group encompasses a number of fungi in which sexual reproduction is unknown. Asexual
reproduction is the production conidia on specialized hyphae called conidiosphores.
There are about 25,000 species, some are parasitic: ringworm, athlete’s foot, and thrush.
Some are important in the production of Roquefort and Camembert cheeses. They can also produce
the antibiotics penicillin and the drug cyclosporin, which suppresses the human immune system and
allows successful transplants to take place.
Symbiosis and Fungi:
Lichens: There are 25,000 types of lichens. Lichens are formed by a symbiotic relationship
between an alga (or cyanobacterium) and a fungus. They are important soil builders. Lichens
erode rock surfaces and harbor bits of organic matter in their crusty bodies. Lichens can live in
harsh conditions such as the tundra without many nutrients.
Lichens may be found on rocks, trees or soil. They reproduce asexually by fragmentation or with
soredia (small clusters of hyphae with embedded algae—very small fragments). Lichens can
reproduce sexually by forming ascocarps or basidiocarps. The chloroplasts reproduce on their
own.
Mycorrhizae are the symbiotic relationships between zygomycetes and the roots of vascular plants.
In some mycorrhizal associations, ENDOMYCORRHIZAE, the fungal hyphae extend into the root
cells, forming coils, swellings or bridges. The hyphae also extend into the surrounding soil. About
80% of vascular plants have endomycorrhizal associations.
ECTOMYCORRHIZAE--The ascomycete or basidiomycete forms a sheath around the root, but it
doesn't penetrate the root. This relationship is found in some trees and shrubs, including pines,
beeches, and willows. Truffles are an example of ascomycetes forming ectomycorrhizae
relationship.
No one understands the precise relationship between the fungi and roots. Roots secrete sugar,
amino acids and other organic substances used by the fungi. The fungi can convert minerals in the
soil and decaying matter into available forms for transport into the plant. It has been shown that
mycorrhizae transport phosphorous from the soil to the roots. Fungi also facilitate water uptake.
4