Nutrition, Classification and Reproduction of Fungi
Nutrition.
Unlike green plants, which use carbon dioxide and light as sources of carbon and
energy, respectively, fungi meet these two requirements by assimilating preformed
organic matter; carbohydrates are the preferred nutrient source. Fungi can readily
absorb and metabolize a variety of soluble carbohydrates, such as glucose, xylose,
sucrose, and fructose, but are also characteristically well equipped to use insoluble
carbohydrates like starches, cellulose, hemicelluloses, and lignin. To do so, they must
first digest these polymers extracellularly. Saprobic fungi obtain their food from dead
organic material; parasitic fungi do so by feeding on living organisms (usually plants),
thus causing disease.
Mycology is the study of fungi.
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Mycology is the study of fungi.
Mycology is the study of fungi.
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Mycology is the study of fungi.
Mode of Nutrition: Absorption
The mode of nutrition or the matter in which fungi "eat" is called absorption.
Among eukaryotes, absorption is unique to the fungi. Fungi obtain their food
by transporting it through their cell walls. In order to eat, the spores that give
rise to fungi must be dispersed to a location where there is food and after the
spore germinates, the mycelium of the fungus must grow into its food. Another
word, usually fungi must live in their food if they are to eat. If the food is
composed of simple molecules such as glucose or sucrose, soluble food can be
immediately transported through their cell walls. However, most food that a
fungus might consume is composed of complex, organic compounds, e.g.,
cellulose, lignin, pectin, starch, etc., which is insoluble. In order for this food
to be utilized by the fungus, it must be broken down into simpler molecules
that can be transported through their cell walls. The fungus breaks down the
complex material by secreting digestive enzymes through their cell wall that
will digest the complex organic compounds and convert them into simple
molecules that can readily be transported through their cell walls. For example,
If a fungus is growing in wood, digestive enzymes would be secreted from the
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fungus, into the wood, and break down the complex compounds of wood, e.g.
cellulose and lignin into simpler materials, such as simple sugars, which then
can be transported into the mycelium. Food must enter the hyphae in solution,
and, since most fungi have no special absorbing organs, the entire mycelial
surface is capable of taking in materials dissolved in water. Some fungi,
however, produce special rootlike hyphae, called rhizoids, which anchor the
thallus to the growth surface and probably also absorb food. Many parasitic
fungi are even more specialized in this respect, producing special absorptive
organs called haustoria.
Although this process may seem very different than our own means of
obtaining food. It is not that different. The essential difference between fungi
and animal digestive systems is that fungi digest their food first and then "eat"
it, while animals eat their food before digesting it. It is important to understand
here that different kinds of fungi will secrete only a specific number of
different enzymes. This means that they can only "eat" certain materials. A
summary of absorption is illustrated in Fig. 26, below:
Figure 26: Illustration of the process of absorption, the mechanism by
which fungi consume their food.
Although yeasts are quite different in their appearance than mycelial fungi,
their means of obtaining food is identical.
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Naming and Classification of Fungi
Taxonomy: A subject which deals with classification and naming of organisms
is called taxonomy. The species name is called a binomial, which consists of
two parts. The first part is the genus (plural: genera) and the second is the
specific epithet. For example, the species name for the human animal is Homo
sapien. When written, the species name is emphasized by italicizing or
underlining the binomial. Each known species, once it has been described by
science, is given a unique species name. Another words, there is only one valid
scientific name that is unique to a species regardless of where it may be found.
Classification of Organisms
All organisms are classified into following three domains:
a. Eukarya – eukaryotic organisms, the Eukaryotes
b. Bacteria- prokaryotic organisms, the true Bacteria
c. Archaea- prokaryotic, Archaebacteria
The Domain is a relatively new level in the classification system. The Domain
eukarya is divided into five Kingdoms namely Plants, Animals, Fungi,
Stramenopila and Protista. Fungi are very distinct from the other kingdoms.
The Kingdom Protista is a “dumping ground” for organisms that don’t fit into
the first three kingdoms. The kingdom Stramenopila includes organisms
such as the former fungi Oomycota as well as brown algae and several
other types of algae.
The classification of fungi is generally based on the following criteria:
i. Morphology of reproductive structures
ii. Types of spores formed
iii. Characteristics of the life cycle
iv. Morphology of the thallus
v. Habit
vi. Habitat
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The Fungi are classified as a kingdom that is separated from plants, animals
and bacteria. One major difference is that fungal cells have cell walls that
contain chitin, unlike the cell walls of plants, which contain cellulose. These
and other differences show that the fungi form a single group of related
organisms, named the Eumycota (true fungi), that share a common ancestor (a
monophyletic group). This fungal group is distinct from the structurally similar
slime molds (myxomycetes) and water molds (oomycetes).
According to Ainsworth fungi were classified into two kingdoms:
a) Kingdom: Myxomycota (Slime molds)
b) Kingdom: Eumycota (true fungi)
Eumycota were grouped into five sub-division, viz.
i.
Mastigomycotina (Lower fungi)
ii.
Zygomycotina (Lower fungi)
iii.
Ascomycotina (Higher fungi)
iv.
Basidiomycotina (Higher fungi) and
v.
Deuteromycotina (Higher fungi)
Sub- Division: Mastigomycotina was classified into four classes:
i.
Chytridiomycetes
ii.
Hyphochytridiomycetes
iii.
iv.
Oomycetes
Plasmodiophoromycetes
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Fungi and fungus like
organisms
Kingdom
Eumycota/Mycota
Phylum/ Division
Chytridiomycota
Zygomycota
Kingdom
Stramenopila
Slime molds
Phylum/
Division
Phylum/
Division
Oomycota
Hyphochytridio
mycota
Myxomycota
Plasmodiophorom
ycota
Dictyosteliomycota
Ascomycota
Labyrinthulomycota
Acrasiomycota
Basidiomycota
Deuteromycota
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Reproduction of fungi
Fungi exhibit three major modes of reproduction - vegetative, asexual and
sexual.
Vegetative Reproduction- It is the type of reproduction which involves the
somatic portion of the fungal thallus. It occurs by the following methods.
Fragmentation: In this process, the mycelium breaks into two or more similar
fragments either accidentally or due to some external force. Each fragment
grows into a new mycelium.
Budding: The parent cell produces one or more projections called buds, which
later develop necessary structures and detach to grow into new individuals.
Budding is common in unicellular forms like yeast.
Fission: In this process, the parent cell splits into two equal halves, each of
which develop into a new individual. Fission is also common in yeast.
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Sclerotia: In some cases, the hyphae become interwoven to form a compact
mass and get surrounded by a hard covering or rind. Such structures are called
sclerotia, remain dormant under unfavourable conditions and germinate into
new mycelia on the return of favourable conditions.
Rhizomorphs: In some higher fungi, several hyphae may become interwoven
to form rope-like structures called rhizomorphs. Under favourable conditions,
they resume growth to give rise to new mycelia.
Asexual Reproduction: The asexual (imperfect, mitotic) state of fungi is
termed the anamorph. It is the types of reproduction in which special
reproductive structures called spores or propagates are formed. The fungal
spores always result from mitosis and hence are described as mitospores.
Following are the types of spores produced in different groups of fungi:
a. Zoospores: They are flagellated, motile spores produced inside
structures called zoosporangia. These spores do not have a cell wall.
Such spores are produced in lower fungi.
b. Sporagiospores: These are non-motile spores produced inside structures
called sporangia in fungi such as Zygomycota.
c. Chlamydospores: These are thick walled resting spores which arise directly
from hyphal cells. They store reserve food.
d. Oidia: These are spore like structures formed by the breaking up of hypha
cells. They do not store reserve food and hence cannot survive under
unfavourable conditions. Such spores are produced in Erysiphe spp.
e. Conidia: These are non-motile spores produced singly or in chains at the tip
of the hypha branches that are called conidiophores. Such spores are
produced in fungi like Aspergillus and Penicillium.
Sexual Reproduction
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Sexual reproduction is known to occur in all groups of fungi except the Fungi
imperfecti or Dueteromycetes. It may involve fusion of gametes, gametangia or
hyphae. The process may involve only fusion of cytoplasm (plasmogamy) or
fusion of nuclei (karyogamy) or production of meiotic spores (meiospores) In
most of the lower fungi plasmogamy is immediately followed by karyogamy
and meiosis. In higher fungi karyogamy is often delayed so that the hyphae
remain dikaryotic. This phase of fungal life cycle is called dikaryophase. Such
fungi complete their life cycle in three phases a haplophase, a dikaryophase and
a diplophase.
Haplophase
Dplophase
Dikaryophase
Types of Plasmogamy: Plasmogamy in fungi is of different types, as shown
below:
Planogametic Copulation: Here
motile
gametes
called
planogametes undergo fusion.
When both the gametes are motile
and morphologically similar, the
fusion process is called isogamy.
Planogametic copulation
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Gametangial Contact: Here, gamete
bearing
structures
called
gametangia come closer to each
other and develop a fertilization
tube through which the male
gamete migrates into the female
gametangium.
Gametangial contact
Gametangial Copulation: Here, the
gametangia fuse with each other,
lose their identity and develop into
a zygospore
Gametangial copulation
Spermatization and Somatogamy
Spermatisation and Somato gamy:
In some fungi like Puccinia, tiny
unicellular spore like structures
called spermatia are formed. They
get
transferred
to
female
gametangia
through
various
agencies. In case of somatogamy,
two cells of different hyphae of
opposite mating type exchange
their nuclei through a fertilization
tube.
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Figure: (a) Aspergillus niger with conidiophores
Figure: (b) Conidiophores of Penicillium sp.
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