Kingdom Fungi, Phyla, Classification and characteristics.
Content:Introduction Phyla of fungi Phylum Chytridiomycota Phylum Zygomycota (The Conjugated Fungi) Phylum Ascomycota (The Sac Fungi). Phylum Basidiomycota (The Club Fungi). Phylum Deuteromycota (imperfect fungi) Phylum Glomeromycota Ecological/economic importance of fungi. Ecological essence of mushroom. |
Introduction
About 250,000 species of fungi are currently extant on Earth.
All fungi are heterotrophs; they absorb nutrients through their cell walls and cell membranes.
Fungi decompose organic material, helping to recycle nutrients essential for plant growth.
Except for the unicellular yeasts, fungi consist of elongated filaments called hyphae.
Hyphae begin as cellular extensions of spores that branch, as they grow to form a network of hyphae, called mycelium.
Even the body of a mushroom consists of a mass of tightly packed hyphae attached to an underground mycelium.
Fungi are non-motile.
Reproduce employing spores produced sexually or asexually.
Many species of fungi are commercially important.
Some are used as food, such as mushrooms.
Or in the production of foods, such as bread, cheese, beer, and wine.
Other species are important in medicine, for example, in the production of the antibiotic penicillin.
Many other species of fungi are of medical and economic concern because they cause plant and animal diseases and destroy crops and stored goods.
PHYLA OF FUNGI
Fungi are classified into seven major phyla of fungi.
- Phylum Microsporidia (They are unicellular fungi that are parasitic on another unicellular life form).
- Phylum Blastocladiomycota. (Parasitic fungi, especially on fruits).
- Phylum Neocallismastigomycota. (These are fungi found in the intestine of cows, horses, and deer).
- Phylum Chytridiomycota. (These are mainly aquatic fungi)
- Phylum Glomeromycota. (Fungi that symbiotically live with plant roots (mycorrhiza).
- Phylum Basidiomycota. (They are parasitic fungi of plants that cause rust). This phylum comprises mushrooms.
- Phylum Ascomycota. (They are also parasitic fungi of plants, including truffles, yeasts, and many antibiotic-making fungi).
Blastomycetes and neocallistmastigamycetes were formerly grouped with the chytrids.
The Microsporidia are sister to all other fungi, but there is disagreement as to whether or not they are true fungi.
The phylogenetic relationships among fungi have been the cause of much debate.
Traditionally, four fungal phyla were recognized, based primarily on characteristics of the cells undergoing meiosis:
- Chytridiomycota (“chytrids”),
- Zygomycota (“zygomycetes”),
- Ascomycota (“ascomycetes”),
- Basidiomycota (“basidiomycetes”).
The chytrids and zygomycetes are not monophyletic.
Phylum Chytridiomycota
The chytrids are the simplest and most primitive Eumycota or true fungi.
- They are aquatic although some species live on land.
- They are also flagellated
- Produce haploid gametes in sexual reproduction.
- But produce diploid zoospores in asexual reproduction.
- There are about 1000 known species.
- Some species thrive as parasites on plants, insects, or amphibians, while others are saprobes
- Example is Allomyces (Allomyces produce diploid or haploid flagellated zoospores in a sporangium.)
Phylum Zygomycota (The Conjugated Fungi)
The zygomycetes are a relatively small group of fungi belonging to the Phylum Zygomycota.
Most species are saprobes, living off decaying organic material; a few are parasites, particularly of insects.
They are multicellular and filamentous.
Their body is made of a mass hypha without cross walls.
Hyphae are tubular with a lining of cytoplasm containing many nuclei.
Reproduction is by sexual and asexual process.
Examples:
Zygomycetes have a thallus of coenocytic hyphae in which the nuclei are haploid when the organism is in the vegetative stage.
The fungi usually reproduce asexually by producing sporangiospores.
The black tips of bread mold are the swollen sporangia packed with black spore
When spores land on a suitable substrate, they germinate and produce a new mycelium.
Sexual reproduction starts when conditions become unfavourable.
Two opposing mating strains (type + and type –) must be nearby for gametangia from the hyphae to be produced and fuse, leading to karyogamy.
The developing diploid zygospores have thick coats that protect them from desiccation and other hazards.
They may remain dormant until environmental conditions are favourable.
When the zygospore germinates, it undergoes meiosis and produces haploid spores, which will, in turn, grow into a new organism.
This form of sexual reproduction in fungi is called conjugation (although it differs markedly from conjugation in bacteria and protists), giving rise to the name “conjugated fungi”.
Phylum Ascomycota (The Sac Fungi).
The majority of known fungi belong to the Phylum Ascomycota, which is characterized by the formation of an ascus (plural, asci), a sac-like structure that contains haploid ascospores.
They are terrestrial organisms.
They are both unicellular species (yeast) and multicellular species.
Their body consists of hyphae with cross walls (septate).
Many ascomycetes are of commercial importance.
Some play a beneficial role, such as the yeasts used in baking, brewing, and wine fermentation, plus truffles and morels, which are held as gourmet delicacies.
Aspergillus oryzae is used in the fermentation of rice to produce sake.
Other ascomycetes parasitize plants and animals, including humans.
For example, fungal pneumonia poses a significant threat to AIDS patients who have a compromised immune system.
Ascomycetes not only infest and destroy crops directly but they also produce poisonous secondary metabolites that make crops unfit for consumption.
Filamentous ascomycetes produce hyphae divided by perforated septa, allowing the streaming of cytoplasm from one cell to the other.
Conidia and asci, which are used respectively for asexual and sexual reproductions, are usually separated from the vegetative hyphae by blocked (non-perforated) septa.
Asexual reproduction is frequent and involves the production of conidiophores that release haploid conidiospores.
Sexual reproduction starts with the development of special hyphae from either one of two types of mating strains.
The “male” strain produces an antheridium and the “female” strain develops an ascogonium.
At fertilization, the antheridium and the ascogonium combine in plasmogamy without nuclear fusion.
Special autogenous hyphae arise, in which pairs of nuclei migrate: one from the “male” strain and one from the “female” strain.
In each ascus, two or more haploid ascospores fuse their nuclei in karyogamy.
During sexual reproduction, thousands of asci fill a fruiting body called the ascocarp.
The diploid nucleus gives rise to haploid nuclei by meiosis.
The ascospores are then released, germinate, and form hyphae that are disseminated in the environment and start new mycelia.
Phylum Basidiomycota (The Club Fungi).
- They are terrestrial organisms and septate.
- Reproduction is sexual.
- The spores are borne on a club-shaped spore case, basidium (plural: basidia) which produces spores called basidiospores.
- They are either saprophytic or parasitic.
The fungi in the Phylum Basidiomycota are easily recognizable under a light microscope by their club-shaped fruiting bodies called basidia (singular, basidium), which are the swollen terminal cells of a hypha.
The basidia, which are the reproductive organs of these fungi, are often contained within the familiar mushroom, commonly seen in fields after rain, on supermarket shelves, and growing on your lawn.
These mushroom-producing basidiomycetes are sometimes referred to as “gill fungi” because of the presence of gill-like structures on the underside of the cap.
The “gills” are compacted hyphae on which the basidia are borne.
This group also includes shelf fungus, which cling to the bark of trees like small shelves.
In addition, the Basidiomycota includes smuts and rusts, which are important plant pathogens; toadstools, and shelf fungi stacked on tree trunks.
Most edible fungi belong to the Phylum Basidiomycota; however, some basidiomycetes produce deadly toxins.
For example, Cryptococcus neoformans causes severe respiratory illness.
The lifecycle of basidiomycetes includes alternation of generations.
Spores are generally produced through sexual reproduction, rather than asexual reproduction.
The club-shaped basidium carries spores called basidiospores.
In the basidium, nuclei of two different mating strains fuse (karyogamy), giving rise to a diploid zygote that then undergoes meiosis.
The haploid nuclei migrate into basidiospores, which germinate and generate monokaryotic hyphae.
The mycelium that results is called a primary mycelium.
The Mycelia of different mating strains can combine and produce a secondary mycelium that contains haploid nuclei of two different mating strains.
This is the dikaryotic stage of the basidiomycetes Lifecycle and it is the dominant stage.
Eventually, the secondary mycelium generates a basidiocarp, which is a fruiting body that protrudes from the ground this is what we think of as a mushroom.
The basidiocarp bears the developing basidia on the gills under its cap.
Examples;
- Mushroom
- Toadstool
- Bracket fungi
- Puffballs
- Rust (pathogenic)
- Smut (non-pathogenic)
Phylum Deuteromycota (imperfect fungi)
A classification group is no longer used in the present, ever-developing classification of organisms.
While Deuteromycota used to be a classification group, recent molecular analysis has shown that the members classified in this group belong to the Ascomycota or the Basidiomycota classifications.
Since they do not possess the sexual structures that are used to classify other fungi, they are less well described in comparison to other members.
Most members live on land, with a few aquatic exceptions.
They form visible mycelia with a fuzzy appearance and are commonly known as mold.
The fungi in this group have a large impact on everyday human life.
The food industry relies on them for ripening some cheeses.
The blue veins in Roquefort cheese and the white crust on Camembert are the result of fungal growth.
The antibiotic penicillin was originally discovered on an overgrown Petri plate, on which a colony of Penicillium fungi killed the bacterial growth surrounding it.
Other fungi in this group cause serious diseases, either directly as parasites (which infect both plants and humans), or as producers of potent toxic compounds, as seen in the aflatoxins released by fungi of the genus Aspergillus.
Phylum Glomeromycota
The Glomeromycota is a newly established phylum that comprises about 230 species that all live in close association with the roots of trees.
Fossil records indicate that trees and their root symbionts share a long evolutionary history.
It appears that all members of this family form arbuscular mycorrhizae:
The hyphae interact with the root cells forming a mutually beneficial association where the plants supply the carbon source and energy in the form of carbohydrates to the fungus, and the fungus supplies essential minerals from the soil to the plant.
The glomeromycetes do not reproduce sexually and do not survive without the presence of plant roots.
Although they have coenocytic hyphae like the zygomycetes, they do not form zygospores.
DNA analysis shows that all glomeromycetes probably descended from a common ancestor, making them a monophyletic lineage.
Ecological/economic importance of fungi.
They cause disease, especially in plants.
Fungi cause food spoilage
They improve soil fertility since they are decomposers.
They recycle nutrients back into the soil.
Some serve as food, for example, mushrooms.
Some are used in the fermentation industries.
Yeast is used as a rising agent in the bakery industry.
They are also used in producing antibiotics.
Some are used in sewage treatment.
Serve as a source of income.
Ecological essence of mushroom.
They release nutrients into the soil.
Mushrooms produce manure or humus.
Also brings about the recycling of nutrients.
Serve as food for other organisms.
It brings about the decay of dead organisms.
Mycorrhizae are fungi associated with roots of plants
The roots of about 90% of all plant families have species that are involved in mutualistic symbiotic relationships with certain kinds of fungi.
It has been estimated that these fungi probably amount to 15% of the total weight of the world’s plant roots.
Associations of this kind are termed mycorrhizae, from the Greek words for fungus and root.
The fungi in mycorrhizal associations function as extensions of the root system.
The fungal hyphae dramatically increase the amount of soil contact and total surface area for absorption.
When mycorrhizae are present, they aid in the direct transfer of phosphorus, zinc, copper, and other mineral nutrients from the soil into the roots.
The plant, on the other hand, supplies organic carbon to the fungus, so the system is an example of mutualism.
There are two principal types of mycorrhizae.
Arbuscular mycorrhizae,
The fungal hyphae penetrate the outer cells of the plant root, forming coils, swellings, and minute branches;
They also extend out into the surrounding soil.
Ectomycorrhizae,
The hyphae surround but do not penetrate the cell walls of the roots.
In both kinds of mycorrhizae, the mycelium extends far out into the soil.
A single root may associate with many fungal species, dividing the root at a millimeter-by-millimeter level.
Rhizopus stolonifer – Common bread mold
Diseases and infections, integrated science
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