Fungi- Definition, Examples, Structure, Classification & Reproduction
What are fungi? How fungi differ from other plants?
Ø Fungi (fungus) in Latin literally means mushrooms
Ø Fungi are eukaryotic organisms with true nucleus and membrane bounded organelles
Ø They are non-vascular Cryptogams included in Thallophyta along with algae due to their undifferentiated plant body.
Ø Study of fungi is called mycology and the one who study fungi is called Mycologist
Ø The word in Latin ‘mykes’ means fungus
Ø Kingdom fungi includes about 5100 genera and 50,000 species
Ø Fungi are heterotrophs since they lack chlorophyll
Ø Fungi cannot synthesize their own carbohydrates using, carbohydrates, water and sunlight.
Ø Plant body of fungi consists of thallus make up of hyphae which together constitute the mycelium
Ø Fungi show much diversity in form, structure of plant body and method of reproduction
Ø Fungi are cosmopolitan in distribution, occurs in any habitat where life is possible
Ø Some fungi are aquatic, which may be fresh water or marine
Ø Most of the fungi are terrestrial forms
Ø Many species of fungi are parasite to plants, animals and human
Ø Majority of the diseases of cultivated plants are caused by fungi and thereby they have immense economic importance
What is mycelium and hyphae?
Ø Plant body of fungi typically consists of branched and filamentous hyphae
Ø Hyphae forms a net like structure called mycelium
Ø Hyphae may be aseptate (without septa) or septate (with septa)
Ø In aseptate forms, the hyphae will be coenocytic (multinucleate condition)
Ø In septate forms, the hyphae may be uninucleate or binucleate or rarely multinucleate
Ø Septa usually have simple pore (hole) at the centre for the cell to cell communication
What is dolipore septum?
Ø In Basidiomycetes (a division of fungi) the septa is highly advanced, here dolipore septa occurs
Ø On both sides of dolipore septum a double membrane structure called septal pore cap or parenthosome occurs
Ø Parenthosomes of dolipore septa act as valves which can regulate the passage of cell contents between cells
How fungal cell wall is different from the cell wall of other plants?
Ø Protoplast of fungi is surrounded by distinct cell wall
Ø In slime molds (lower fungi, closely related to Protistis) the cell wall is absent
Ø Main component of cell wall is chitin (a major difference of fungal cell wall from plant cell wall)
Ø Chitin is a polymer of an N-acetylglucosamine, a derivative of glucose
Ø In some lower fungi (Oomycetes) cell wall is composed of cellulose and glucan
How mitosis in fungi is different from other eukaryotes?
Ø The nucleus of fungi is very small
Ø Fungi have special type of mitosis called nuclear mitosis
Ø Mitosis in fungi is different from that in other eukaryotes
Ø In fungi, during mitotic cell division, the nuclear envelope does not break down and re-form. Instead, mitosis takes place within the nucleus
Ø Spindle apparatus is formed within the nucleus, which drag the chromosomes to two opposite poles of nucleus (not the cell as in most other eukaryotes)
What is meant by dikaryotization?
Ø Fungal hyphae may be homo-karyotic (only one strain of nuclei) or hetero karyotic (different strains of nuclei)
Ø Hyphae may be haploid, diploid or dikaryotic
Ø Dikaryotic condition is seed in higher fungi only (members of Ascomycetes and Basidiomycetes)
Ø In dikaryotic mycelium, two different strains of nuclei (+ and -) stay separately in the cell and they simultaneously divide when the cell divides
Ø The process of formation of dikaryotic mycelium is called dikaryotization
What are the hyphal modifications in fungi?
Ø In majority of fungi, hyphae are simple
Ø In some advanced fungi, hyphae may undergo certain modification in response to functional needs
Ø Important hyphal modifications in fungi are: Prosenchyma, Pseudoparenchyma, Rhizomorphs, Sclerotia, Appressorium, Haustoria, Stroma and Hyphal traps (snares)
Learn more: Hyphal Modifications in Fungi
What is the type of nutrition in fungi?
Ø Fungi are heterotrophic in nutrition (they do not manufacture food)
Ø Fungi are entirely devoid of chlorophyll, however carotenoids and other colour pigments are present
Ø On the basis of made of nutrition, fungi are classified into three groups:
1. Parasites: grow and feed on other living plants or animals (hosts)
2. Saprophytes: grown and feed on dead organic matters
3. Symbionts: mutual association between algae or roots of higher plants (lichens and mycorrhiza are examples)
Ø Fungi digest food first and then ingest (absorb) the food into cells, to accomplish this the fungi produce exoenzymes (enzymes which acts outside the cell)
Ø The food reserve of fungi is glycogen (similar to animals cells)
Learn more: Nutrition in Fungi
How fungi reproduce?
Ø Fungi reproduce by vegetative, asexual and sexual methods
Ø Sexual state of fungi is called teleomorph
Ø Asexual state of fungi is called anamorph
Ø Vegetative reproduction in fungi takes place by Fragmentation (Eg. Rhizopus, Aspergillus), Fission (Eg. Yeast) and Budding (Eg. Yeast)
Ø About 20% fungi (mostly members of Deuteromycetes) propagate only by asexual means, they completely lack sexual reproduction
Ø Asexual reproduction takes place during favorable condition by the formation of a variety of conidia or spores
Ø Fungal spores may be unicellular (Aspergillus, Penicillium) or multicellular (Alternaria, Cercospora)
Ø Spores may be endogenous when enveloped in pycnidia or sporangia (Mucor, Rhizopus) or they may be exogenous when developed on sporophores or conidiophores (Aspergillus, Penicillium)
Ø Common asexual spores of fungi are: Zoospores, Conidiospores (conidia), Clamydospores and Oidia:
Learn more: Different types of Spores in Fungi
Ø In lower fungi reproductive cells are flagellated and motile
Ø Flagella are of two types in fungi
1. Whiplash (Acronematic): smooth flagella with 9+2 organization
2. Tinsel (Pantonematic): Flagella with numerous minute hair like projections called mastigonemes, originate from the axial filament
Ø Motile reproductive structures are completely absent in higher fungi (Ascomycets, Basidiomycets and Deuteromyces)
Holocarpic vs Eucarpic fungi
Ø In some unicellular forms the whole vegetative cell is transformed to a reproductive unit and such a fungi is called holocarpic fungi
Ø In most fungi only a part of the vegetative mycelium forms the reproductive unit and the rest remain vegetative, such a fungi is called eukarpic fungi
How sexual reproduction occurs in fungi?
Ø Sexual reproduction takes place in all group of fungi except Deuteromycetes (fungi imperfectii)
Ø Fungi may be monoecious (bisexual) or dioecious (unisexual)
Ø Moneoecious species produce the two types of sex organs (male and female) in the same thallus, and hence they are called homothallic forms
Ø Dioecious forms produces male and female sex organs in separate thalli, hence called heterothallic forms
Ø The process of sexual reproduction is completed in three distinct phases
1. Plasmogamy: fusion of cytoplasm
2. Karyogamy: fusion of nucleus
3. Meiosis: reduction division
Ø In higher groups of fungi, plasmogamy do not immediately follows kaaryogamy, the cell will hold the two nuclei separately in order to establish dikaryotization
Ø In sexual reproduction the two compatible nuclei (male and female) are brought together by any of the following processes:
1. Planogametic copulation: fusion of two motile gametes by isogamy, anisogamy or oogamy
2. Gametangial contact: male and female gametangia connect each other by fertilization tube
3. Gametangial copulatin: entire content of two gametangia fuse together
4. Spermatization: fusion of minute spores called spermatia and specialized receptive hyphate acting as male and female structures respectively
5. Somatogamy: sex organs not formed, two vegetative cells fuse together, highly advanced type
Ø Fungi exhibit the phenomenon of alteration of generation
Ø Fungi have immense economic importance to man