General Characteristics of Fungi
- Fungi are cosmopolitan in distribution and can occur in any habitat where life is possible. Some fungi occur in fresh or marine water, some are terrestrial and still others are air borne. Many species are parasitic that , infects plants, animals and human beings.
- The body of fungi typically consists of branched and filamentous hyphae, which forms a net-like structure that is known as mycelium.
- The hyphae are aseptate and coenocytic (as in Phycomycetes) or septate and uni-, bi- or multinucleate (as in Ascomycetes, Basidiomycetes and Deuteromycetes).
- The septa usually have simple pores but in Basidiomycetes septa with dolipore occur. On both the sides of the dolipore, a double membranous structure – septal pore cap or parenthosome occurs. These septal pores form a valve like structure.
- With the exception of slime inolds, the protoplast remains surrounded by a distinct cell well. The main component of the cell wall is the fungal cellulose, also known as chitin. In some lower fungi (e.g. members of Oomycetes), the cell wall is composed of cellulose and glucan.
- The hyphae contain colourless and vacuolated cytoplasm. Besides endoplasmic reticulum, mitochondria and Golgi bodies, many non-living substances, like reserve food also occur in the cytoplasm.
- Fungi are entirely devoid of chlorophyll, but carotenoids are normally present.
- The hyphae are homo-or heterokaryotic, haploid, dikaryotic or diploid (diploid phase is usually ephemeral).
- The higher fungi (Ascomycetes, Basidiomycetes and Deuteromycetes) are immotile, i.e., motile cells do not occur in their life-cycle at any stage. But in lower fungi (Phycomycetes) the reproductive cells (gametes and asexual zoospores) are uni-or biflagellate.
- The flagella are of two types (i) whiplash (acronematic) flagella are smooth with 9+2 organisation, and (ii) tinsel (pantonematic) flagella with numerous minute hair like mastigonemes on their surface, the mastigonemes originate from the axial filament of flagella.
Modification of Hyphal structure
In majority of fungi the hyphae form a filet like structure, but in some advanced fungi they may undergo certain modifications in response to functional needs. The hyphal structure is variously modified into :
When the component hyphae lie more or less parallel to one another, they unite to form a rather loosely interwoven structure where their individuality is not lost.
When the hyphae become closely intertwined, forming a tissue which consists of hollow tubes running in all directions. The hyphae loose their individuality and are not distinguishable from each other. They resemble with the parenchymatous tissue of higher plants in cross section.
It is a thick strand or root-like aggregation of somatic hyphae. The hyphae lose their individuality and the whole mass behaves as an organized unit. It is believed that rhizomorph has a higher infection capacity than individual hyphae.
It is a compact globose structure, formed by the aggregation and adhesion of hyphae. It may survive for long periods, sometimes for several years, and thus represents the resting stage of the fungus. They accumulate reserve food and help in vegetative propagation.
It is a terminal simple or lobed swollen structure of germ tubes or infecting hyphae, found in many parasitic fungi such as rusts and powdery mildews. It adheres to the surface of the host and helps in the penetration of infection hyphae.
They are intracellular absorbing structures of oblige parasitic fungi, meanst for absorbing food material from the host. They vary in shape and may be knob like, elongated, finger like or branched. They secrete some specific enzymes which hydrolyse the proteins and carbohydrates of the host cell.
Hyphal traps (Snares)-
The predacious fungi develop sticky hyphae or network of hyphal loops, known as hyphal traps. They help in capturing nematodes.
These are compact somatic structures much like mattresses. Fructifications are usually formed on/in them.
Modes of Nutrition
They are heterotrophic, i.e., they do not manufacture their own food material, They are usually grouped into the following three categories on the basis of their modes of nutrition :
Those fungi which obtain their nutrition from other living plants or animals are known as parasites. Some of there fungi can live only on living protoplasm and they are known as obligate parasites (e.g. Ravenelia, Puccinia), whereas others (e.g. Ustilago, Sphacelotheca, Tolyposporium) can also grow on the dead organic material in the absence of living host and then they are known as facultative saprophytes.
These fungi obtain their nutrition from the dead decaying organic matter. Some saprophytes, such as. Mucor mucedo, can obtain their nutrition only from the dead organic material and they do not have the capacity to infect living plants or animals. They are known as obligate saprophytes. On the other hand, those saprophytic fungi (e.g. Fusarium, Pythium) which have the capacity to infect living organisms, are known as facultative parasites.
Some fungi grow on other living organisms and both are mutually benefited. Such an association is known symbiosis. Lichens and mycorrhiza are common examples of symbionts.
In most of the fungi only a part of the vegetative mycelium forms reproductive unit and the rest remains vegetative. Such fungi are known as eucarpic. However, in unicellular fungi whole vegetative cell is transformed into a reproductive unit, and as such they are known as holocarpic.
They reproduce by vegetative, asexual and sexual means.
- The vegetative reproduction may take place by fragmentation (e.g. Rhizopus, Mucor, Aspergillus, Alternaria), fission (e.g. yeast and budding (e.g. yeast, Ustilago).
- About 20% fungi propagate only by asexual means. Asexual reproduction takes place during favourable conditions by the formation of a variety of conidia or spores. The spores may be unicellular (e.g. Aspergillus, Penici-llium) or multicellular (e.g. Alternaria, Cercospora). They may be endogenous, when developed in pycnidia or sporangia (e.g. Mucor, Rhizopus), or exogenous, when developed on sporophores or conidiophores (e.g. Aspergillus, Penicillium).
- Some common asexual spores in fungi are zoospores (e.g. Achlya, Pythium, Phytophthora), Cchalamydospores (e.g. Ustilago), oidia (e.g. Collybia, Coprinus) and aplanospores (e.g. Mucor, Rhizopus, Pilobolus.
- With the exception of the class Deuteromycetes, sexual reproduction occurs in all the groups of fungi. The process of sexual reproduction is completed in the following three distinct phases:
In the first step fusion of the protoplast of two compatible gametes or sex cells takes place, and the two compatible nuclei come close to each other.
In the second step, fusion of the two nuclei from the two fusing chlamydospores gametes takes place and diploid zygotic nucleus is formed. In Phycomycetes Karyogamy occurs just after plasmogamy, but in Ascomycetes and Basidiomycetes karyogamy is much delayed. In the latter groups, the nuclei of the opposite strains get themselves arranged in pairs (dikaryon). This phase in the life cycle is known as dikaryophase and the process by which this stage is accomplished is called dikaryotization. The two nuclei of a dikaryon fuse to form a diploid nucleus.
After karyogamy, reduction division takes place in the diploid nucleus and haploid stage is re established.
The compatible nuclei are brought together by the following sexual processes:
This involves fusion of two naked motile gametes (planogametes). The planogametic copulation is of three types, depending on the nature and structure of the fusing gametes.
- Isogamy- The fusing gametes are morphologically similar but physiologically different, and are formed on different hypha (e.g. Synchytrium, Catenaria).
- Anisogamy- The fusing gametes are both morphologically and physiologically different; the male gamete is smaller and more active than the female gamete (e.g. , Allomyces).
- Oogamy- The female gamete (ovum) is non-motile and the male gamete (antherozoid) is motile; they are formed in specialized gametangia, known as oogonium and antheridium respectively.
Here gametes are neve: released from gametangia, instead the male and female gametangia come in close contact with the help of a fertilization tube. Then one or more male nuclei migrate to the female gametangium. The gametangia never fuse or lose their identity during the sexual act. The male and female gametangia are known as antheridium and oogonium (ascogonium in Ascomycetes) respectively. Albugo, Aspergillus, Pythium and Phytophthora are some common examples of gametangial contact.
This process involves fusion of the entire contents of two compatible gametangia. The gametangia come in ciose contact, their walls at the point of contact dissolve and their contents mix. The two gametangia ultimately fuse resulting in karyogamy (e.g. Mucor, Rhizopus).
In some advanced genera (e.g., Puccinia), sex organs are completely absent and the sexual process is accomplished by minute spore like spermatia (male ga netes) and specialized receptive hyphae (female gametes). The spermatia are carried by air, water or insects to the receptive hyphae, and the contents of the spermatium enter the receptive hyphae through a pore.
Here sex organs are not at all formed, but two vegetative cells.
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