KINGDOM FUNGI
The fungi are as distinct from the mosses and higher vascular plants
as they are from the animals and are a distinct kingdom. There are about
100,000 species of fungi that have been described, and it is estimated that
as many as 200,000 more may await discovery. There may actually be as
many species of fungi as there are species of plants, although far fewer
have been described thus far. The fungi have no direct evolutionary
connection with the plants and apparently were derived independently from a
different group of single-celled eukaryotes. The oldest fossils that
resemble fungi occur in the strata about 900 million years ago, but the
oldest that have been identified with certainty as fungi are from the
Ordovician period, 450 to 500 million years ago. Representatives of Division
Zygomycota were associated with the underground portions of the earliest
vascular plants in the Silurian period, some 400 million years ago. Fungi may
be among the oldest eukaryotes; the four major groups were in existence by
the close of the Carboniferous period, some 300 million years ago.
The fungi, together with the heterotrophic bacteria and a few other
groups of organisms are the decomposers of the biosphere. Their activities
are as essential to the continued functioning of the biosphere as are those
of the primary producers. Decomposition releases carbon dioxide into the
atmosphere and returns nitrogenous compounds and other minerals to the
soil where they can be used again by the green plants and eventually the
animals. It is estimated that, on the average, the top 20 centimeters of
fertile soil may contain nearly 5 metric tons of fungi and bacteria per
hectare (2.47 acres). As decomposers, fungi often come into direct conflict
with human interests. A fungus that feeds on wood makes no distinction
between a fallen tree in the forest or a railway tie. Equipped with a
powerful arsenal of enzymes that break down organic products, fungi are
often nuisances and are sometimes highly destructive. This is especially
true in the tropics, where the warmth and dampness promote fungal growth.
Fungi attack cloth, paint, leather, jet fuel, insulation on cables and wires,
photographic film, and even the coating of the lenses of optical equipment in fact almost any conceivable substance. Even in temperate regions, they
are the scourge of food producers and sellers alike. They can grow on
bread, fresh fruits, vegetables, meats, and other products. Fungi reduce
the nutritional value, as well as the palatability, of such foodstuffs. Some
also produce extremely poisonous toxins, some of which - the aflotoxins -
are extremely carcinogenic and show their effects at concentrations as low
as a few parts per billion. Many fungi are pathogenic, and attack living
organisms, rather than dead ones. They are the most important single cause
of plant diseases. Well over 5000 species of fungi attack economically
valuable crop and garden plants, as well as many wild plants. Other fungi are
the cause of serious diseases in humans and other animals.
Many fungi are commercially valuable. For example the yeasts, are
useful because of their abilities to produce substances such as ethanol and
carbon dioxide (which plays a central roll in baking). Others are of interest
as sources of antibiotics, including penicillin, the first antibiotic to be widely
used, and as potential sources of proteins.
Associations between fungi and other organisms are extremely
diverse. For example, about four-fifths of all land plants form associations
between their roots and fungi called mycorrhizae. Mycorrhizae increase the
absorptive surfaces of the plant roots and aid in mineral exchange between
the soil and the plant. These associations play a critical role in plant
nutrition and distribution.
Basic Structure and Nutrition
Some fungi exist as single cells and are known as yeasts. However,
most species are multicellular. The basic structure of a fungus is the hypha
(pl., hyphae)—a slender filament of cytoplasm and nuclei enclosed by a cell
wall (Figure 9). A mass of these hyphae make up an individual organism, and
is collectively called a mycelium. A mycelium can permeate soil, water, or
living tissue; fungi certainly seem to grow everywhere. In all cases the
hyphae of a fungus secrete enzymes for extracellular digestion of the
organic substrate. Then the mycelium and its hyphae absorb the digested
nutrients. For this reason, fungi are called absorptive heterotrophs.
Heterotrophs obtain their energy from organic molecules made by other
organisms.
Fungi feed on many types of substrates. Most fungi obtain food
from dead organic matter and are called saprophytes. Other fungi feed on
living organisms and are parasites. Many of the parasitic fungi have
modified hyphae called haustoria (Figure 9), which are thin extensions of the
hyphae that penetrate living cells and absorb nutrients.
Hyphae of some species of fungi have crosswalls called septa that
separate cytoplasm and nuclei into cells. Hyphae of other species have
incomplete or no septa (i.e., are aseptate) and therefore are coenocytic
(multinucleate). The cell walls of fungi are made of chitin, the same
polysaccharide that comprises the exoskeleton of insects and crustaceans.
Figure 9: Basic structure of fungus.
Reproduction in Kingdom Fungi
Reproduction in fungi can be sexual or asexual. Many, but not all,
fungi reproduce both sexually and asexually. Some reproduce only sexually,
others only asexually. All divisions, however, share similar patterns of
reproduction and morphology.
a) Asexual Reproduction
Fungi commonly reproduce asexually by mitotic production of haploid
vegetative cells called spores in sporangia, and conidia on conidiophores.
Spores are microscopic and surrounded by a covering well suited for the
rigors of distribution into the environment.
Budding and fragmentation are two other methods of asexual
reproduction. Budding is mitosis with an uneven distribution of cytoplasm
and is common in yeasts. After budding, the cell with the lesser amount of
cytoplasm eventually detaches and matures into a new organism.
Fragmentation is the breaking of an organism into one or more pieces, each
of which can develop into a new individual.
b) Sexual Reproduction
The sexual cycle of fungi includes the familiar events of vegetative
growth, genetic recombination, meiosis, and fertilization. However, the
timing of these events is unique in fungi. Fungi reproduce sexually when
hyphae of two genetically different individuals of the same species
encounter each other. Following are the four important features of the
sexual cycle of fungi:






Nuclei of a fungal mycelium are haploid during most of the life cycle.
Gametes are produced by mitosis and differentiation of haploid cells
rather than directly from meiosis of diploid cells.
Meiosis quickly follows formation of the zygote, the only diploid stage.
Haploid cells produced by meiosis are not gametes; rather they are
spores that grow into a mature haploid organism. Recall that asexual
reproduction produces spores by mitosis. In both cases, haploid
spores grow into mature mycelia.
The union of the cytoplasm of two parent mycelia is known as
plasmogamy.
The union of two haploid nuclei contributed by two parents is known as
karyogamy.
Consider the following diagram, which illustrates the generalized life cycle
of fungi.
Classification of Fungi
The taxonomy of fungi is currently the subject of much research.
Although there is considerable discrepancy in the classification of fungi, we
will treat the kingdom as having 3 divisions (phyla): the Zygomycota,
Ascomycota, and Basidiomycota. Note, that the terms phylum and division
can be used interchangeably. Where the Ascomycota and Basidiomycota are
considered the "higher fungi", the other division is known as the "lower
fungi", because they have retained many primitive characteristics. The
separation of the 3 divisions in our system is based on reproductive
structures. The Zygomycota (or zygote fungi) produce "zygospores" (heavy
walled zygotes not associated with an oogonium), as their sexual
reproductive spores. The Ascomycota (or sac fungi) produce "ascospores",
within a special structure termed an ascus. The Basidiomycota (or club
fungi) produce "basidiospores" in a basidium. As you examine members of
these major groups, carefully note variations on the fundamental structure
of vegetative mycelia and specialized structures associated with sexual and
asexual reproduction.
Division (Phylum): : Zygomycota
Most of the Zygomycota live on decaying plant or animal matter in the soil
but some are parasites on plants, insects or small soil animals. There are
approximately 750 described species of Zygomycota. The term
"Zygomycota" refers to the chief characteristic of the division; the
production of sexual resting spores called “zygospores”. Most zygomycetes
are saprophytic and their vegetative hyphae lack septa (i.e., they are
aseptate).
One representative of this division, Rhizopus stolonifera, a common
black bread mold
Rhizopus stolonifera (black bread mold)
This species is one of the most common members of this division.
This organism causes the black bread mold that forms cottony masses on
the surface of moist bread exposed to the air.
Asexual reproduction in Rhizopus occurs by spores (Figure 12).
The mycelium of R. stolonifera is composed of three different types of
haploid hyphae (another word for a filament). The bulk of the mycelium
consists of rapidly growing submerged hyphae that are coenocytic
(multinucleate) and aseptate (not divided by cross walls into cells or
compartments). From the submerged hyphae, aerial hyphae called, stolons,
are formed. The stolons form rhizoids wherever their tips come in contact
with the substrate. Sporangia form on the tips of sporangiophores, which
are erect branches formed directly above the rhizoids (Figure 10, 12). The
sporangiophores support the asexual reproductive structures: the
sporangia. Within a sporangium, haploid nuclei divide by mitosis and produce
haploid spores. The cell wall that forms around each spore is black, giving
the mold its characteristic color. The spores are released to the
environment when the sporangium matures and breaks open. Each spore can
germinate to produce a new mycelium.
Diagram illustrating the vegetative and asexual reproductive structures of
Rhizopus sp.
Rhizopus sp. growing on peaches. (Asexual sporangiouphores)
Sexual reproduction in Rhizopus occurs only between different
mating strains, which have been traditionally labeled as + and – types (or
Strain 1 and Strain 2, as seen in Figure 12). Although the mating strains are
morphologically indistinguishable, they are often shown in life cycle diagrams
as different colors). When the two strains are in close proximity, hormones
are produced that cause their hyphal tips to come together and develop into
gametangia (Figure 11, 12), which become separated from the rest of the
fungal body by the formation of septa. The walls between the two touching
gametangia dissolve, and the two multinucleate protoplasts come together.
The + and - nuclei fuse in pairs to form a young zygospore with several
diploid nuclei. The zygospore then develops a thick, rough black coat and
becomes dormant, often for several months. Meiosis occurs at the time of
germination. The zygospore cracks open and produces a sporangium that is
similar to the asexually produced sporangium, and the life cycle begins again.
Rhizopus stolonifera reproduces by sexual reproduction (gametangia,
sporangia, & zygospores).
Rhizopus stolonifera, sporangium.
Rhizopus stolonifera, gametangia.
Rhizopus stolonifera, young zygospore.
Rhizopus stolonifera, a
thick-walled zygospore.
Pictures illustrating the stages of sexual reproduction.
The life cycle of Rhizopus stolonifera (black bread mold).
B) Division (Phylum): Ascomycota
The Ascomycota comprise about 60,000 described species, including
a number of familiar and economically important fungi. Most of the bluegreen, red, and brown molds that cause food spoilage are Ascomycota. This
includes the salmon-colored bread mold Neurospora sp., which has played an
important role in the development of modern genetics. Many Ascomycota
are the cause of serious plant diseases, including powdery mildews that
attack fruits, chestnut blight, and Dutch elm disease (caused by Ceratocytis
ulmi, a fungus native to certain European countries). Yeasts, the edible
morels and truffles are also Ascomycota. This group of fungi, as a whole, is
relatively poorly known, and thousands of additional species await scientific
description.
Ascomycota, with the exception of the unicellular yeasts, are hyphal.
The hyphae are septate, or divided by cross walls. The hyphal cells of the
vegetative mycelium may be either uninucleate or multinucleate. Some
species of Ascomycota can self-fertilize and produce sexual structures from
a single genetic strain; others require a combination of + and - strains.
1) Asexual reproduction in the majority of the Ascomycota occurs by the
formation of specialized spores, known as "conidia" (a Greek word for "fine
dust"), which are cut off from tips of modified hyphae called
"conidiophores"("conidia bearers"). The conidiophores partition conidia in
longitudinal chains. Each conidium contains one or more nuclei. Conidia form
on the surface of conidiophores in contrast to spores that form within
sporangia in Rhizopus. When mature, conidia are released in large numbers
and germinate to produce new organisms. Pencillium sp., which you will
examine in this laboratory, is a common example of a fungus that forms
conidia.
2) Sexual reproduction in Ascomycota always involves the formation of an
"ascus" (pl. asci), a sac-like structure that is characteristic of this division
and distinguishes the Ascomycota from all other fungi. Ascus formation is
usually within a complex structure composed of tightly interwoven hyphae the "ascocarp". Many ascocarps are macroscopic, and are the only part of
these fungi that most people ever see. An ascocarp may be open and more
or less cup-shaped (called an "apothecium"), closed and spherical in shape
(called a "cleistothecium"), or flask shaped, with a small pore through which
the ascospores escape (called a "perithecium"). The layer of asci is called
the hymenium", or hymeneal layer, which lines the ascocarp.
Figure 13: Some typical "ascocarps" found in the division Ascomycota.
a) Sexual Life Cycle of Ascomycota
Figure 14 illustrates a typical life cycle of Ascomycota. When
examining the slides and preserved materials in this lab, try to place it into
this generalized life cycle. The mycelium is initiated with the germination of
an ascospore, or conidium. Many "crops" of conidia are produced during the
growing season, and it is the conidia that are responsible for propagation of
the fungus.
Asci formation occurs on the same mycelia that produce conidia.
They are preceded by the formation of multinucleate gametangia called
"antheridia" and "ascogonia". The male nuclei of the antheridium pass into
the ascogonium via a tubular outgrowth of the ascogonium known as the
trichogyne. "Plasmogamy", or the fusion of the two cytoplasms, has now
taken place. The male nuclei then pair with the genetically different female
nuclei within the common cytoplasm but do not fuse. Hyphae now begin to
grow out of the ascogonium. As the hyphae develop, pairs of nuclei migrate
into them and simultaneous mitotic divisions occur in the hyphae and
ascogonium. Cell division in the developing hyphae occurs in such ways that
the resulting cells are "dikaryotic" (i.e. containing two haploid nuclei, one
from each strain). The dikaryotic hyphae grow together to form a
reproductive structure called an "ascocarp".
The ascus first forms at the tip of the developing dikaryotic hypha.
The two nuclei in the terminal cell (ascus) of the dikaryotic hyphae then fuse
into a single diploid nucleus ("karyogamy"). This is the only zygote. The
ascus then elongates and the diploid nucleus divides by meiosis, forming 4
haploid nuclei. Each haploid nucleus usually divides again by mitosis, resulting
in a total of 8 haploid nuclei. These haploid nuclei are then cut off in
segments of the cytoplasm to form "ascospores". In most Ascomycota, the
ascus becomes turgid at maturity and finally bursts, sending its ascospores
explosively into the air.
Ascomycota.
3) Examples of Ascomycota
Figure 14: Life Cycle of a Typical
a) Yeast
Yeast are somewhat atypical of most Ascomycota. They are
predominantly unicellular and reproduce asexually by fission or by budding
(pinching-off of small buds), rather than by spore or conidia formation.
Sexual reproduction in yeasts occurs when either two cells or two
ascospores unite and from a diploid zygote. The zygote may produce asexual
buds, or may undergo meiosis to produce four haploid nuclei. In some
species there may also be a subsequent mitotic division producing eight
haploid nuclei. The single cell in these unicellular yeasts is acting as an
ascus, and therefore the whole organism becomes the reproductive
structure. Within the ascus/zygote wall, walls are laid down around the
nuclei so that eight ascospores are formed. These are liberated when the
ascus wall breaks down. The ascospores either bud asexually or fuse with
another cell to repeat the sexual process.
Two genera of yeasts, Saccharomyces sp. and Schizosaccharomyces
sp. are commonly used in the baking and brewing industry. In both these
genera the asci are formed by the fusion of two haploid cells.
Note the presence of 4 ascospores in each ascus. Also note that in
these unicellular yeasts, there is no "ascocarp". Figure 15 shows yeast cells
in various stages of asexual and sexual reproduction.
Figure 15: Yeast cells budding and sporulating.
b) Penicillium sp. (blue - green molds)
Penicillium sp. has become celebrated in connection with antibiotics.
Penicillin, a by-product of Penicillium notatum when liberated into the culture
medium, inhibits the growth of gram-positive bacteria. Penicillin was
discovered by Sir Alexander Fleming in 1929, but was not exploited until
World War II. The great importance of this substance is that it represses
bacterial growth without being toxic to animal tissues. Interest in penicillin
led to an intensive search for other antibiotics, and molds suddenly became
of considerable economic interest. However, Penicillium sp. is of economic
importance in other respects. For example certain species give some types
of cheese the flavor, odor, and character so highly prized by gourmets. One
such mold, P. roquefortii, was first found in caves near the French village of
Roquefort. Legend has it that a peasant boy left his lunch, a fresh piece of
mild cheese, in one of these caves and on returning several weeks later
found it marbled, tart, and fragrant. Only cheeses from the area around
these particular caves are permitted to bear the name of Roquefort.
Another species of this genus, P. camembertii, give Camembert cheese its
special qualities.
Penicillium sp. reproduces asexually by forming spores called conidia.
Penicillium sp. conidiophores and conidia.
c) Cup Fungi
The cup fungi are the most advanced group of Ascomycota. They
produce an ascocarp called an "apothecium", with the asci arranged in an
exposed layer. Although many apothecia are disc or cup-shaped, other
forms also exist.
Peziza sp.
The apothecia of Peziza sp. often exceed 10 cm in diameter. These
apothecia are usually bowl-shaped when young but will become flattened and
distorted with age. At maturity, the thousands of asci on the surface of the
cup develop hydrostatic pressure. If the cup is disturbed, the asci rupture
releasing thousands of ascospores in a visible "puff". Wind currents then
transport the spores to a new environment.
Figure 17: Diagram of the apothecium of Peziza sp. (top) and microscopic
view of asci (bottom).
Morchella sp. (morels)
These are some of the most highly prized edible fungi. The
apothecium of Morchella sp. has a stalk, or stipe, and a fertile portion called
the "pileus" (Figure 18). The pileus is essentially discoid, but it is folded
over the stipe apex and is highly contorted. These distortions greatly
increase the surface area of the pileus. The asci line the large pits, which
are separated by sterile ridges. Each "pit" is similar to the cup of Peziza sp.
In other words, each pit is an apothecium producing ascospores.
Morchella sp.
Diagram (left) and picture (right) of the apothecia of
Morchella sp.
Morchella sp.
C) Division (Phylum): Basidiomycota
The most familiar of all fungi are members of this large sub-division.
It includes some 25,000 described species, not only the mushrooms,
toadstools, stinkhorns, puffballs, and shelf fungi but also two important
plant pathogens: the rusts and smuts. The Basidiomycota are distinguished
from all other fungi by the production of basidiospores, which are borne
outside a club-shaped, spore-producing structure called the basidium (plural,
basidia).The basidia are produced by basidiocarps, which are the fruiting
bodies of the so-called higher fungi, such as mushrooms and puffballs.
Basidiocarps, like the ascocarps, are the large fruiting structures, which are
the most visible stage of the fungus. A typical mushroom is a familiar
example of a basidiocarp.
1) Life Cycle of Basidiomycota
The mycelium of the Basidiomycota is always septate and in most
species passes through three distinct phases -primary, secondary, and
tertiary- during the life cycle of the fungus. When it germinates, a
basidiospore produces the primary mycelium. Initially the mycelium may be
multinucleate, but septa soon form and the mycelium is divided into
monokaryotic (uninucleate) cells. This septate mycelium grows by division of
the terminal cell. Branches do occur, and the mycelial mass can become very
complex. Commonly the secondary mycelium is produced by the fusion of
primary mycelium from two different mating types (plasmogamy) [Figure 19].
The tertiary mycelium, which is also dikaryotic, arises directly from
the secondary mycelium, and forms the basidiocarp. The spore forming
basidia are produced by the terminal cell on millions of dikaryotic hyphae. In
a typical mushroom, basidia are found on gills, under the cup (Figure 20A).
Karyogamy occurs between the two haploid nuclei within a developing
basidium. Then, the diploid nucleus undergoes meiosis to produce four
haploid nuclei. These nuclei then migrate into four small extensions at the
apical end of the basidium, and are walled off to form the four
basidiospores.
Figure 19: Life Cycle of Typical Basidiomycota.
2) Examples of Basidiomycota
a) Gilled Mushrooms.
The basidiocarps of this group are large and conspicuous. They are
the familiar mushrooms and toadstools. The vegetative portion of the
fungus exists as a mycelial network, which grows saprobically beneath the
substrate, often as mycorrhizae with trees. The basidia are borne in a layer
on the surface of "gills" which, in turn, are produced on the underside of
fleshy umbrella-like basidiocarps. The basidiospores are forcibly ejected
form the basidium. The basidiocarp consists of a stout stalk (stipe) bearing a
circular cap (pileus) from which the lamellae (gills) hang down (Figure 20).
Most members of this order are saprobic but some are tree parasites. It
should be recognized that in the Agricales, the fruiting body (basidiocarp) is
an ephemeral structure usually lasting only a few days, whereas the
mycelium, living on organic matter in the soil, may last for years.
Figure 20A: Diagram of a Basidiocarp.
Figure 20B: Basidiocarp of Coprinus comatus (inky cap
mushrooms).
Another Coprinus sp. - "Shagy mane".
These mushrooms are the deadly Amanita musaria var. muscaria commonly
called Fly Agaric. It is deadly poisonous! Don't Eat.
Note the gills, layers of basidia, and basidiospores (Figure 21).
Gills x.s.
Gills x.s. close-up.
Basidium and basidiospores close-up.
Figure 21: Microscopic view of cross-section through the gills of the
basidiocarp.
b) Bracket Fungi
Members of this diverse group of fungi produce basidiocarps that
are woody, leathery or papery but never soft. The basidia are found
covering the surface of gills, or "teeth", or lining the inside of "pores".
Many of the common "bracket fungi" or shelf fungi, found growing on the
surface of living or dead tree trunks, are members of this group (Figure 22).
Figure 22: Some members of the bracket fungi.
c) Puffballs and Earth Stars
In the puffballs, the mature basidiocarp consists of a papery outer
covering with a small opening or "ostiole" on the top. Inside is the mass of
spores. When raindrops strike the leathery covering, "puffs" of spores are
ejected through the ostiole (Figure 23).
Note the demonstration material of puffballs (Figure 24) and earthstars
(Figure 25). Remember the structure you see is a basidiocarp similar to that
of a mushroom.
Figure 23: Young and mature basidiocarps.
Figure 24: Lycoperdon sp. (puffballs).
Figure 25: Geaster sp. (earthstars).