Sunday, July 26, 2020

Biological Classification - Notes | Class 11 | Part 4: Kingdom Fungi

2. BIOLOGICAL CLASSIFICATION

3. KINGDOM FUNGI

-    It is a unique kingdom of heterotrophic organisms.

-    Fungi are cosmopolitan.

-    They grow in warm and humid places.

-    E.g. mould on bread & rotten fruits, mushroom, toadstools.

-    White spots on mustard leaves are due to a parasitic fungus.

-    Some fungi are the source of antibiotics, e.g., Penicillium.

-    Some unicellular fungi (e.g. yeast) are used to make bread and beer.

-    Other fungi cause diseases in plants and animals. E.g. wheat rust-causing Puccinia.

-    Except yeasts, fungi are filamentous. Their bodies consist of thread-like structures called hyphae.

-    The network of hyphae is known as mycelium.


-    Hyphae are 2 types:


o  Coenocytic hyphae: They are continuous tubes filled with multinucleated cytoplasm.


o  Septate hyphae: They have septae or cross walls.


-    Fungal cell wall is made of chitin & polysaccharides.

-    Most fungi are saprophytes (absorb soluble organic matter from dead substrates). Some are parasites.

-    Some live as symbionts. E.g. Lichens (fungi+ algae), mycorrhiza (fungi + roots of higher plants).


Reproduction:


·     Vegetative propagation: By fragmentation, fission & budding.

·     Asexual reproduction: By spores such as conidia, sporangiospores and zoospores.

·    Sexual reproduction: By oospores, ascospores and basidiospores. They are produced in distinct structures called fruiting bodies.


-    The sexual cycle involves 3 steps:


a.    Plasmogamy: Fusion of protoplasm between two motile or non-motile gametes.

b.    Karyogamy: Fusion of two nuclei.

c.    Meiosis in zygote to give haploid spores.


-    When a fungus reproduces sexually, two haploid hyphae of compatible mating types come together and fuse.

-    In some fungi, the fusion of two haploid cells immediately results in diploid cells (2n).

-    In ascomycetes and basidiomycetes, a dikaryotic stage or dikaryophase (n + n i.e. two nuclei per cell) occurs. Such a condition is called a dikaryon. Later, parental nuclei fuse and the cells become diploid.

-    The fungi form fruiting bodies in which reduction division occurs, leading to formation of haploid spores.


Based on morphology of mycelium, mode of spore formation & fruiting bodies, Fungi are classified into different classes:

1. Phycomycetes

2. Ascomycetes

3. Basidiomycetes

4. Deuteromycetes

I. Phycomycetes (Lower Fungi)


-    They occur in aquatic habitats and on decaying wood in moist and damp places or as obligate parasites on plants.

-    The mycelium is aseptate and coenocytic.

-    Asexual reproduction: By motile zoospores or by non-motile aplanospores. These are produced in sporangium.

-    Sexual reproduction: Zygospores are formed by fusion of two gametes. These gametes are isogamous (similar in morphology) or anisogamous or oogamous (dissimilar).

-    E.g. Mucor, Rhizopus (bread mould) and Albugo (parasitic fungi on mustard).


II. Ascomycetes (sac-fungi)


-    They are unicellular (e.g., yeast, Sacharomyces) or multicellular (e.g., Penicillium).

-    Mycelium is branched and septate.

-    They are saprophytic, decomposers, parasitic or coprophilous (growing on dung).

-   Asexual reproduction: By conidia produced exogenously on the special mycelium called conidiophores. Conidia germinate to produce mycelium.

-    Sexual reproduction: By ascospores produced endogenously in sac like asci (sing. ascus). The asci are arranged to form fruiting bodies called ascocarps.

-    E.g. Aspergillus, Claviceps and Neurospora.

-    Neurospora is used in biochemical and genetic work.

-    Morels & truffles are edible.


III. Basidiomycetes


-    Includes mushrooms, bracket fungi or puffballs.

-    They grow in soil, on logs and tree stumps and in living plant bodies as parasites (e.g. rusts and smuts).

-    The mycelium is branched and septate.

-    The asexual spores are generally not found, but vegetative reproduction by fragmentation is common.

-    The sex organs are absent, but plasmogamy occurs by fusion of two vegetative or somatic cells of different strains or genotypes. The resultant structure is dikaryotic which gives rise to basidium. Karyogamy and meiosis take place in basidium producing four basidiospores exogenously. Basidia are arranged in fruiting bodies (basidiocarps).

-    E.g. Agaricus (mushroom), Ustilago (smut) and Puccinia (rust fungus).


IV. Deuteromycetes


-    Commonly known as imperfect fungi because only their asexual or vegetative phases are known.

-    When perfect (sexual) stages were discovered, they were often moved to ascomycetes or basidiomycetes.

-    It is also possible that asexual and vegetative stage have been given one name placing under deuteromycetes and the sexual stage another name placing under another class. When the linkages were established, the fungi were correctly identified and moved out of deuteromycetes.

-    They reproduce only by asexual spores (conidia).

-    The mycelium is septate and branched.

-    Some are saprophytes or parasites. Majority are decomposers of litter and help in mineral cycling.

-    E.g. Alternaria, Colletotrichum and Trichoderma.

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