KINGDOM OF FUNGI
Kingdom Fungi includes some of the largest, oldest, and most ecologically and economically
important species in the world. Fungi are very diverse and nearly ubiquitous in nature, although
this may not be obvious to the untrained or unequipped observer. The presentation below will
reveal this secretive aspect to fungi as well as the importance of these neither plant nor animal
species.
Recent data towards a phylogeny of Kingdom Fungi and their relation to other kingdoms reveals
Kingdom Fungi is more closely related to Kingdom Animalia then it is to Kingdom Plantae. It is
currently hypothesized that fungi and animals had a common protozoan ancestor.
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Only the most ancient fungi, ‘chytrids’, have flagellated cells. Many ‘Protists
kingdoms’, plants and animals have flagellated cells.
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Fungi, like animals only, store carbohydrate as glycogen.
Fungus more than likely facilitated the transition of ancient plants to land habitats. Fossils dating
back to the Silurian reveal that the ‘first land plants’ had fungal associations on their roots. Such
fungus will be described below as part of the ecological significance of fungi.
WHAT ARE THE MAJOR EVOLUTIONARY GROUPS OF FUNGI?
There are about 100,000 currently described species but there are undoubtedly many undiscovered
and unnamed species. One estimate of the world’s fungus diversity is 1.5 million species. That is a
great deal of work for mycologists. Mycologists are biologists that study fungi.
Fungi are classified into four divisions: Zygomycota, Basidiomycota, Ascomycota, and
Chytridiomycota. These divisions are largely distinguished and defined by their life cycle and
sexual reproductive structures. They are evolutionarily valid clades with several synapomorphies.
Fungi species that are asexual, without known sexual phases or structures, they are classified
in a ‘catch-all group’, Deuteromycota (imperfect fungi). (SEE BELOW)
GENERAL STRUCTURE AND LIFE CYCLE OF A FUNGUS
Fungi are absorptive heterotrophs. They secrete digestive enzymes onto the substrate on which
they live; digestion occurs outside the fungal body and feeding occurs as material is ‘transported’
into the fungus. Fungi are usually multicellular, sometimes multinucleate, and have a cell wall
composed of chitin. Fungus (excluding ‘chytrids’ which are flagellated) are stationary organisms at
the mercy of wind, water, and animals for dispersal. In this regard they resemble plants rather than
animals.
The vegetative structure is the nutritionally active (feeding part) fungus body and is hidden from
view and spread throughout the food source. The basic unit of the vegetative structure is called
hyphae. Hyphae are thread-like tubes interwoven to form the mycelium. The mycelium is the
extensive ‘feeding net’ of the fungus. It is underground or within the substrate (food such as wood
or leaf litter) that the fungus is attached to.
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Because of the structure of mycelium and hyphae, fungi have a tremendous amount
of surface area in contact with their food. This maximizes the amount of food that can be
digested and absorbed. When fungi grow, they add length to hyphae and exploit new
food sources by reaching out to them.
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The tubular walls of the hyphae are composed of chitin and the wall encloses a plasma membrane
and cytoplasm with the typical compliment of eukaryote structures. One exception however is
that many fungi have dikaryotic cells (two nuclei) or are huge ‘multinucleate’ cells.
Above is a micrograph of sepatate hyphae.
septum.
Above is a micrograph of a nucleus passing the
The hyphae may be subdivided by septa. Septa are not walls; they allow ‘flow’ of cytoplasm, even
nuclei, throught the body of the fungus. Fungi with undivided hyphae (the hyphae looks like one
big cell) are aseptate. Fungus with aseptate cells are sometimes called coenocytic fungi. In these
fungi, hyphae are a continuous, cytoplasmic mass with many nuclei. This gives the appearance of a
massive, multinucleate cell. The hyphae may have specialized structures. Parasitic fungi have
specialized hyphae structures called haustoria that penetrate the tissue of the host.
FEEDING FORMS OF FUNGI
Fungi may be categorized by their nutritional / absorptive mode (How and what they eat). All
fungi are heterotrophic. Some fungi are:
saprobic heterotrophs - feed off dead or decaying organic matter
parasitic heterotrophs - feeding off of a living host at cost to the host
mutualistic heterotrophs - feeding off of or with a living host without detriment to the host,
both host and fungus benefit
REPRODUCTION IN THE FUNGI
Reproduction in fungi may be sexual, asexual, or both. Many fungi reproduce sexually when
conditions change. This is adaptive because sexual reproduction generates genetic diversity.
Asexual reproduction is used to maximize production when conditions are stable and genetic
diversification is less urgent. This is a trade-off between generating variation and simply generating
offspring. This sexual / asexual trade-off is typical for species with both sexual and asexual
reproduction.
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Spores are formed during sexual and asexual phases of the life cycle by specialized
structures. Specific sexual structures will be presented later to distinguish fungi of each
division.
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Nuclei of hyphae and spores are haploid except during sexual stages.
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Hyphae may become diploid-like by fusion of hyphae with different nuclei.
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Mating is between fungi of different mating types. There are not males and females,
as we know sexes.
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Nuclear material may intermingle and even recombine or nuclei may remain
segregated producing a mosaic organism.
During sexual reproduction, genetic heterogeneity is introduced by two events that occur in
different stages at different times. Syngamy is the union of cells of two different individuals of two
different mating types. First their is cytoplasmic fusion, plasmogamy. Second is nuclear fusion,
karyogamy. A dikaryon is produced by plasmogamy. The two nuclei may coexist for years until
the nuclei fuse. Meiosis immediately follows karyogamy and sexual spores are produced.
MORPHOLOGICAL AND ECOLOGICAL GROUPINGS OF FUNGI
There are major ecological and morphological specializations that evolved in all divisions of fungi.
Such species may be grouped by their ecological role and general morphology. Recognize of
course that these are not evolutionarily legitimate groupings.
The group for which sexual stages are not known, Dueteromycota, ‘Fungi Imperfecti’
Because fungus divisions are determined largely on the sexual aspects of the fungus,
deuteromycetes are grouped together. It is likely that the lack of a sexual phase or form has
evolved many times in fungi. Said another way, it is likely that some deuteromycetes evolved
from ancestors of the three major divisions (Zygomycota, Basidiomycota, and Ascomycota) of
fungus.
The above circumstance leads to evolutionarily artificial nature of this ‘taxon’. In some cases,
deuteromycetes have been found to be ‘protists’. Others have been classified, using molecular
phylogeny techniques, into the appropriate one of the three fungal divisions.
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Molds are imperfect fungi that are rapidly growing, asexually reproducing, and
usually saprobic or parasitic. The spore of a mold is called a conidia and it is presented on
a conidophore.
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Yeasts are unicellular, perhaps only asexual, and they inhabit moist or liquid habitats
including moist plant and animal tissues. Some yeasts have sexual reproduction and form
either basidia or asci. To what divisions would such yeasts belong?
Mycorrhizae are mutualistic associations involving plant roots and fungi. The plant benefits by an
increased surface area as the hyphae of its symbiotic fungi are intimately integrated with its roots.
The fungus benefits by feeding from the tissues of the plant such as its sap. In many cases
mycorrhizae are absolutely essential for seed germination and greatly increase the growth potential
of certain plant species. Many oak, birch, and pine trees have mycorrhizae. The vast majority of
mycorrhizae are basidiomycete fungi but all three main divisions have mycorrhizae species. One
estimate is that 95% or more vascular plant species have mycorrhizae. Now that is what I mean
by nearly ubiquitous in nature.
Lichens are symbiotic associations involving fungi and a small photoautotrophic organisms, mostly
algae but in some cases a cyanobacterium. By far most lichens involve ascomycete fungi but there
are also many known basidiomycete lichens. There are about 25, 000 described species of lichens.
Lichens are very significant ecologically for their role in primary succession; making the abiotic
world inhabitable to the biotic world by forming soil from rocks. Lichens also are sensitive
environmental indicators as they do not tolerate air pollution well.
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Because lichens are not single organisms, classification is complicated. Fungi, lichens for sure, defy
human perception of the individual. Of course, if you don’t know the individual a species
designation is impossible. Lichenologists classify lichens according to their fungus and characterize
the ‘individuals’ based upon various chemical tests and microscopic study. As enigmatic as fungi
are, lichens are down right bizarre.
Below is a rock with several morphotypes of lichens. Notice they can be brightly colored due to
photosynthetic pigments of their algal symbionts. The fungus of a lichen can be very obvious; the
one in the middle is an ascomycete lichen. What about the one on the right? Of course, another
ascomycete lichen!
PHYLOGENY OF THE FUNGI
Molecular evidence substantially supports that the fungi and the animals had a common ‘protist’
ancestor. Evolution within fungi, like plant evolution, was and is driven by selection for a
terrestrial existence and reproduction on land. The most ancient fungus has flagella and the more
derived fungi have distinctive spore presenting structures and no flagella. This is the same trend as
found in plants as they weaned from water. Most funguses present very little of themselves to the
atmosphere except to reproduce. Fungus still tend to inhabit very moist ecological niches and
therefore maintain a significant reliance on water. Fungi spores literally exist by the millions almost
everywhere a person would look. These spores are exceptional at escaping unfavorable
environments (dryness included) and a fabulous means of dispersal. Fungal spores are at least as
impressive as pollen, maybe more. Also, like pollen, fungal spores are a significant allergen for
some people.
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CHITRIDIOMYCOTA
chytrids
The ‘ballons’ sticking out of Spirogyra are ‘parasitic’
Above is a ‘vegetative phase’ of a chytrid
Below is a flagellated zoospore of a chytrid
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‘Chytrids’ are the basal, most ancestral division of fungus.
Most species from this division are microscopic.
One can distinguish them from the other fungi by their flagellated cells. Both the
spores and gametes move by flagella.
This division has the only obligate, aquatic fungi.
Similar to the three other divisions, ‘chytrids’ have cell walls with chitin and an
absorptive mode of nutrition with many of the same enzymes and metabolic pathways.
Chytrid cells are unusual among fungus cells because they have centrioles like animal
cells do. The other fungus divisions have a spindle pole body rather then centrioles.
'Chytrids' are sometimes parasitic on plants and flies.
Some research even points to chytrid related disease conditions in amphibians. In
this regard, this fungus is becoming a fairly common, supportable explanation for some
amphibian declines.
ZYGOMYCETES, ASCOMYCETES, AND BASIDIOMYCETES
In these divisions spores are not motile, that is, they lack flagella or cilia. The vegetative body, the
hyphae, is multicellular and haploid. Also, these divisions have morphologically similar but
genetically different (different haploid nuclei) mating strains.
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ZYGOMYCOTA
On the left is a sporangium; zygomycetes do not have mushrooms.
On the right is a zygosporangium; zygomycetes form a ‘zygote’.
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There are about 600 described species of ‘zygote fungi’. Black bread mold
(Rhizopus) and dung fungus (Pilobolus) are well known examples. Some zygomycetes are
mycorrhizae.
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Hyphae of zygomycetes are aseptate. The cells are haploid, multinucleated.
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Spores are commonly dispersed by air currents.
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No dikaryotic hyphae during sexual phase
Life cycle of Zygomycota: These fungi produce dikaryotic zygosporangia following plasmogamy.
This is a metabolically inactive stage that is resistant to harsh environmental conditions and
therefore may lie dormant for long periods until favorable conditions return. Upon return of
'good' conditions, karyogamy and meiosis occur. A sporangium erupts from the zygosporangium;
then haploid, sexual spores are dispersed.
ASCOMYCOTA
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This is a morel, mmmm good!!
This is scarlet cup, a brightly colored ascomycete.
There are over 60,000 species of ‘sac fungi’. Included are the luscious mycorrhizal fungi, truffles
and morels, as well as some of the most devastating plant pathogens. Some plant pathogens are
economically relevant as they cause huge financial losses in agriculture.
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Almost all lichens have ascomycete fungi.
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Most ‘classified’ imperfect fungi have turned out to be ascomycetes.
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Some ascomycetes provide defenses to plants by secreting toxins into the leaves and
stems in which they live. This associaition may render the plant tissues unpalatable or
poisonous. These symbiotic fungi are called endophytic fungi. In exchange the fungus has a
place to live and ‘nutrients’ to draw upon.
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Some yeasts are ascomycetes. Baking and brewing yeast are beneficial single-celled
ascomycetes. Candida albicans is problematic. Human yeast infections are caused by this
fungus and include symptoms such as diaper rash, thrush, and vaginitis.
Life cycle of Ascomycota
1) These fungi produce ascogonia following plasmogamy. An ascogonium produces dikaryotic
hyphae that produces the ascocarp. The ascocarp is the macroscopic fruiting body of the fungus,
the mushroom. At the tips of the dikaryotic hyphae in the ascocarp are the asci (sacs) that house
the sexual spores.
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The ‘golden eggs’ are ascospores; the columns are asci.
above.
Notice the 8 spores in the ascus
Karyogamy occurs in the asci forming a diploid cell that divides by meiosis to produce for
haploid cells each of which divides by mitosis producing eight ascospores. Each ascus
forcibly ejects its spores when the sac collapses. The collapse of neighboring sacs triggers
those sacs in a chain reaction. If you happen to see ascomycetes eject their spores it looks
like a cloud of ‘smoke’.
2) A limited dikaryotic hyphae is formed.
3) Ascomycetes may reproduce asexually by haploid spores known as conidiaspores that are
produced by conidia. Once again, such ascomycete fungi are called yeasts.
BASIDIOMYCOTA
Amanita; very deadly!!
Agaricus; supermarket mushrooms
There are about 25,000 species of ‘club fungi’ known. Typical looking mushrooms and shelf
fungus are familiar basidiomycetes. Many of our edible mushrooms are basidiomycetes but so are
the most dangerous and poisonous mushrooms. Among the most toxic are species from the genus
Amanitas, known by the common names ‘death cap’ and ‘death angel’. Only experts should
harvest wild mushrooms.
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Rusts and smuts are very problematic plant parasites, they are basidiomycetes.
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Saprobes from this division are the best wood decomposers. These fungi are the
only organisms that can fully degrade lignin. They also ‘digest’ cellulose. Now that is
impressive!
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About half of the ‘basidiomycete mushrooms’ are saprobes and the other half are
mycorrhizae.
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Most mycorrhizae are basidiomycetes. Half of them are mycorrhizae of pine, birch,
and oak species.
Life cycle of Basidiomycota
1) The dikaryotic stage is extensive and may exist for many hundreds of years over thousands of
square meters. Not kidding! So they are the oldest and biggest organisms on earth. Mushrooms,
the above ground structures, are produced annually. ‘Fairy rings’ are formed by mycelium and
basidiocarps. What is the individual you ask!? In this case the individual is the field of mushrooms
above ground and the ‘unseen’ mycelieum below ground. Mushrooms challenge the imagination
and stretch the limits of basic, biological definitions like individual.
2) Plasmogamy produces a dikaryotic mycelium and basidiocarps, the mushroom. Gills lined with
basidia (pedestals) cover the lower surface of the mushroom cap. Karyogamy occurs in the basidia
that are at the terminus of the dikaryon at the gill surface on the underside of the cap of the
mushroom. Sexual basidiospores are electrostaticly released from the gill region under the cap.
Spores drop from the mushroom and are dispersed by the wind. Note that the spores are
produced and presented 4 at a time. How does this differ compared to ascomycetes?
Answer: Meiosis reduces the diploid nucleus to haploid and generates four cell products.
Mitosis doesn’t occur as it does in ascomycete spore production.
3) Asexual reproduction is uncommon. When it occurs, it is similar to ascomycetes; that is it is
yeast like.
REVIEW OF THE ECOLOGICAL IMPACT OF FUNGI
Saprobic fungi are decomposers of plant material such as leaves, wood and fruits, and animal waste
and remains. Decomposition by fungus returns carbon, nitrogen, and other elements to the cycle
of the ecosystem. Other fungi, mycorrhizae, greatly increase the nutrient absorption capacity of
plants or as in endophytic fungi provide plants defense against predators.
Fungi, as lichens, inhabit rather inhospitable places on earth and function to make inorganic matter,
such as rocks, suitable for life.
These two ecological contributions are vital towards ecosystem function.
ECONOMIC AND SOCIAL IMPACT OF FUNGI
THE GOOD THE BAD AND THE UGLY
Penicillin, the first antibiotic and cyclosporine, a drug given to suppress immunity following organ
transplants, are isolated from fungus. Fungi also cause health problems, allergies, more serious
respiratory illness, reproductive tract infections, and athlete’s foot.
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Some fungi have potent hallucinogenic properties. A species of Psilocybe is consumed during
religious ceremonies of Mexican Indians. While another is cultivated or ‘hunted’ for ‘recreational
drug use’. Below is a Psilocybe project.
Claviceps purpae has accidentally been milled into contaminated grain several notable times and
resulted in many people with fits of hallucinogenic hysteria (ergotism) and sometimes death. The
‘purple’ projections from the grass below are ergot fungus.
One of the most potent natural carcinogens (cancer causing things), aflotoxin, is produced by
Aspergillus and is known to occasionally contaminate peanuts.
FUNGI AS FOOD AND IN FOOD PREPARATION
Cheeses are inoculated with fungi to create ‘blue cheese’. Soy is acted upon by fungus to make
tempeh.
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Penicillium roquefortii is used to make Roquefort.
delight.
Rhizopus makes tempeh; a vegetarians’
Yeasts are the best thing since sliced bread; actually sliced bread owes itself to yeast.
Saccharomyces, are used to ferment starches and sugars in order to raise bread. This yeast also
enables us to produce ‘alcoholic beverages’ such as wine and beer.
Yeast; notice they are budding (reproducing asexually).
Aspergillus is used to prepare soy sauce from soybeans and to make saki (rice wine).
Fungi may also be serious and dangerous pests of crops such as ‘corn smut’ and various fungi that
destroy 10 – 50% of the worlds fruit harvest per year.
In France, organized ‘hunts’ are held in order to find and auction ‘truffles’ because many people
find them very delicious and will pay about 600 dollars a pound. In America there are festivals
and celebrations to hunt and eat ‘morels’.
Mushrooms are eaten worldwide and easily account for billions of dollars of sales annually.
KINGDOM OF FUNGI