General characteristics of fungi

General characteristics of fungi: Some of the most important characters of fungi are as follows: 1. Occurrence 2. Thallus organization 3. Different forms of mycelium 4. Cell structure 5. Nutrition 6. Heterothallism and Homothallism 7. Reproduction.

OCCURRENCE:

• Fungi are cosmopolitan and occur in air, water soil and on plants and animals.
• They prefer to grow in warm and humid places. Hence, we keep food in the refrigerator to prevent bacterial and fungal infestation.

THALLUS ORGANIZATION:

• Except some unicellular forms (e.g. yeasts, Synchytrium), the fungal body is a thallus called mycelium.
• The mycelium is an interwoven mass of thread-like hyphae (Sing, hypha). Hyphae may be septate (with cross wall) and aseptate (without cross wall). Some fungi are dimorphic that found as both unicellular and mycelial forms e.g. Candida albicans.
• The thallus may be two types: Unicellular and filamentous.

Unicellular thallus:

• In some of the lower fungi, thallus is more or less a spherical, single celled structure. At the time of reproduction it becomes a reproductive unit. Such fungi are called holocarpic. In the unicellular holocarpic forms, the mycelium is absent e.g. Synchytrium.
• Some holocarpic fungi (e.g., yeast) producing bud cells in succession and these remain attached to one another in a chain. Such a chain of bud cells is referred to as pseudomycelium.
• Filamentous thallus: In most true fungi, the thallus is filamentous composed of Hyphae.
• Loosely aggregated hyphae are collectively forms a network known as mycelium. Each hypha may vary in shapes and sizes. Branching of hyphae is dichotomous. On the basis of presence or absence of septa the hyphae of mycelical fungi are of two types:
• Nonseptate or aseptate hyphae: Mycelium contains numerous nucleilying in a common mass of cytoplasm, without cross wall in the hyphae, E.g., oomycetes and zygomycetes. Such a condition is known as coenocytic. However, septa may be laid down at the time of formation of reproductive organs to delimit them from the rest of the vegetative hyphae, therefore called Pseudosepta. E.g., Allomyces.
• Septate Hyphae: Hyphae are septate and hyphal segments may contain one, two or more nuclei. E.g., Ascomycotina, Basidiomycotina, and Deuteromycotina. There are two types of septa:

• Primary septa: Primary septa are formed in association with mitotic or meiotic nuclear division, and they separate the daughter nuclei. These types of septa are found in Ascomycotina, Basidiomycotina and their asexual states.
• Adventitious septa: Adventitious septa are formed in the absence of mitosis or meiosis and
  occur especially in association with change in the local concentration of cytoplasm. These are
  found in lower groups of fungi as mastigomycotina and zygomycotina.

DIFFERENT FORMS OF MYCELLIUM

• Different forms of mycelium refer to as the various shapes and the functions of mycelium which modified according to the circumstances.

• These includes the following forms:
• a. Plectenchyma (fungal tissue): In a fungal mycelium, hyphae organized loosely or compactly woven to form a tissue called plectenchyma. It is two types:
• Prosenchyma or Prosoplectenchyma: In these fungal tissue hyphae are loosely interwoven lying more or less parallel to each other.
• Pseudoparenchyma or paraplectenchyma: In these fungal tissue hyphae are compactly interwoven looking like a parenchyma in cross-section.
• b. Sclerotia (Gr. Skleros=haid): These are hard dormant bodies consist of compact hyphae protected by external thickened hyphae. Each Sclerotium germinates into a mycelium, on return of favourable condition, e.g., Penicillium.
• c. Rhizomorphs: They are root-like compactly interwoven hyphae with distinct growing tip. They help in absorption and perennation (to tide over the unfavourable periods), e.g., Armillaria mellea.
• d. Rhizoids: A rhizoid is a short, root like filamentous branch of the thallus, generally formed in tufts at the base of the thallus. These also function as anchoring and absorbing, e.g., Rhizophydium, Rhizopus.
• e. Appressoria (Sing. appressorium): It is a terminal simple or lobed swollen mucilaginous structure of infecting hyphae which adheres to the surface of the host or other substratum and helps in the penetration of the infection hyphae. These are formed by some parasitic fungi such as powdery mildews and rust.
• f. Haustoria (Sing. haustorium): A haustorium is an organ that is developed from a hypha usually performing the function of absorption. They are characteristic of obligate parasites. They vary in shape and may be knob like or button shaped, elongated, finger-like or branched. They secrete some specific enzymes which hydrolyse the protein and carbohydrates of the host plant.
• g. Hyphal traps (Snares): The predacious fungi develop sticky hyphae or network of hyphal loops known as hyphal traps or Snares. They help in capturing nematodes.
• h. Stromata: These are compact somatic structures much like mattresses. Fructifications are generally formed on or in them.

CELL STRUCTURE

• Fungi are the eukaryotic organisms.
• The cell wall determines the characteristic shape of a cell. It protects the cell from osmotic lysis and also acts as a binding site for some enzymes.
• The chemical composition of the cell wall is not the same in all fungi. Chitin is characteristically present in the cell walls of most fungi. The chitin in fungal cell wall is not strictly identical with animal chitin, and the formula (C22H54N4O21)n has been suggested for the fungal chitin: It is a polymer of N-acetylglucosamine.
• The cytoplasm at hyphal tip contains Golgi vesicles called chitosomes which filled with cell wall materials.
• Nucleus and mitochondria are found to connect with ER. Nucleus divides by intracellular mitosis called karyochoresis where nuclear envelop remain intact during nuclear division and internal spindle develop.
• Reserve food is glycogen and oil.

NUTRITION

• The fungi lack chlorophyll. Therefore, they cannot synthesiz their own food. Depending on from where and how they get nutrition, fungi are of following types:
• a. Saprotrophs: They obtain food from dead and decaying organic matter. They secrete digesting enzymes to outside which digest the substratum and then absorb nutrients, e.g., Mucor, Agarious, Rhizopus etc.
• b. Parasitic: They obtain food from living organisms. They may be facultative or obligate. Facultative parasites grow on a variety of tissues e.g., Ustilago. The obiigate- parasites growonly upon suitable host, e.g., downy mildews. The parasitic fungi that grow on surface of host cells and absorb food through haustoria are called ectoparasites or ectophytic parasites (e.g., Mucor, Erisphae). When parasitic fungi grow inside the host tissue are called endoparasites or endophytic parasites (e.g., Pythium, Puccinia).
• c. Predacious: Some soil fungi develop ring-like noses to trap annelids, nematodes etc. e.g., Arthrobotrys, Zoophagus, Dactylella etc.
• d. Symbiotic: They live in mutualistic relationship with another organism by which both are benefited. The two common examples are lichens and mycorrhiza.
• Lichens are symbiotic associations between fungi and algae. The fungal partner is a member of ascomycetes or basidiomycetes that provides water and nutrients, while the algal partner is a green alga or cyanobacteria that prepares food by photosynthesis.
• e. Mycorrhizas: or mycorrhizae are the mutualistic symbiotic associations between soil fungi and the roots of most plant species. According to the carbohydrate theory (Bjorkman, 1949), the plants that grow in soils deficient in P and N, and high intensity light develop mycorrhizas.
• The two most common types of mycorrhizas are the ectomycorrhizas (ECM) and the endomycorrhizas (also known as arbuscular mycorrhiza). The two groups are differentiated by the fact that the hyphae of ectomycorrhizal fungi do not penetrate the cell wall of the plant’s root cells, while the hyphae of arbuscular mycorrhizal fungi penetrate the cell wall.

HETEROTHALLISM AND HOMOTHALLISM

• A. F. Blakeslee (1904) discovered mating types or genetically distinct strains in Mucor.
• He called fungi with different mating types are called heterothallic and fungi without mating types are called homothallic.
• Nowadays we call some fungi and algae homothallic if both male and female gametes produce in the same individual can fertilize each other and heterothallic if the gametes can only be fertilized by gametes from another individual of the same species.
• Heterothallism introduces variations in the species.

REPRODUCTION

• In fungi reproduction may take place by three methods; vegetative, asexual and sexual.
• During asexual and sexual reproduction processes spores are the essential structures. The spores formed after meiosis are called meiospores (e.g., ascospores, basidiospores and sporangiospores) and those resulting from mitosis, called mitospores (e.g., mitospores, zoospores, aplanospores, conidia, uredospores).

• The diploid body produced as a result of sexual fusion is known as zygote which in lower fungi is termed as resting spore, oospore or zygospore.
• In higher fungi, the zygote is represented by a diploid nucleus produced in a cell (ascus or basidium). This diploid nucleus after undergoing meiosis results in the formation of haploid nuclei serving as centres for haploid sexual spores called ascospores and basidiospores.

Vegetative reproduction:

• In this type of reproduction, a part of mycelium separate and forms a new individual. The various methods of vegetative reproduction are:
• a. Fragmentation: The hyphae break into small fragments. Each piece upon getting suitable
• conditions, germinates to form a new mycelium.
• b. Fission: This method involves the splitting of cells into two daughter cells by the formation of a constriction followed by a cell wall formation.
• c. Budding: A small bud formed from the parent cell which gradually increases in size and receives a part of nucleus. A cell wall is formed which separates the daughter cell from the parent cell.
• d. Sclerotia: These are perennating bodies formed by the compact masses of interwoven hyphae. Sclerotia under suitable conditions germinate to form new individuals e.g. Claviceps, Sclerotinia.
• e. Rhizomorphs: These are root-like elongated mycelial strands. They remain dormant under unfavourable conditions and under favourable conditions develop into a new mycelium.

Asexual reproduction:

• It commonly take place through spores, either motile or non-motile and form in a specialized part of mycelium. The various types of spores are:
• a. Zoospores: These are commonly found in lower fungi e.g., Saprolegnia, Pythium etc. They are naked spores, which after swarming, encyst, secrete a cell wall and germinate by germ tube into a thallus. They are equipped with one or two flagella
• b. Sporangiospore: The sporangiospores or aplanospores are nonmotile and lack flagella and are formed inside the sporangium e.g. Mucor, Rhizopus. These spores may by uninucleate or multinucleate and possess two-layered cell wall.
• c. Conidia: They are produced externally on branched or unbranched hyphal tips termed as conidiophores. The conidia may be formed singly or in chains. The conidial chains may be basipetal or acropetal in succession. Conidia may be uninucleate or multinucleate. The latter type is more common in the members of the form class Deuteromycetes.
• d. Oidia: They are produced by fragmentation of hyphae from apex to base. Each cell thus formed rounds off and separates as a spore which under favourable circumstances germinates and forms the mycelium.
• e. Chlamydospores: They are formed by rounding off and enlargement of terminal or intercalary cells of a hypha. These can be single or formed in chains. They do not separate from the hyphae but remain viable and germinate under favourable conditions.
• f. Ascospores: An ascospore are produced inside an ascus during the optimal condition. This kind of spore is specific to fungi classified as ascomycetes.
• g. Uredospores: A thin-walled, red, summer spore of a rust fungus, produced usually on the leaves or stems of grasses.
• h. Basidiospores: These spore produced by Basidiomycete fungi. They typically each contain one haploid nucleus that is the product of meiosis, and they are produced by specialized fungal cells called basidia.

Sexual reproduction:

• It involves the formation and fusion of gametes. Sexual reproduction found in all groups of fungi except deuteromycetes or fungi imperfecti. Sexual reproduction has three distinct phases i.e. plasmogamy (protoplasmic fusion), karyogamy (fusion of nuclei) and meiosis (reduction division of zygote). The various methods of sexual reproduction in fungi are as follows:
• a. Planogametic copulation: This is simplest type of sexual reproduction. In this process fusion of two gametes of opposite sex or strains takes place where one or both of the fusing gametes are motile (flagellated). It results in the formation of a diploid zygote. This process is usually of these types:
• Isogamy: In this process fusing gametes are morphologically similar and motile but physiologically dissimilar. These gametes are produced by different parents, e.g. Synchytrium.
• Heterogamy: When the fusing gametes are morphologically as well as physiologically different, the process is known as heterogamy. Heterogamous reproduction is of two types: anisogamy and oogamy. Anisogamy consists of the fusion of two motile gametes where the male gamete is small and more active than the female gamete, e.g., Allomyces. In oogamy the motile male gamete (antherozooid) fuses with the large, non-motile female gamete (egg or ovum) e.g., Synchytrium etc.
• b. Gametangial contact: In this process two gametangia of opposite sex come in contact with one another. The male gametangium (antheridium) transfer male nucleus or gamete into the female gametangium (oogonium) either through a pore at the point of contact or through a fertilization tube, e.g., Phytophthora, Albugo, Pythium etc.
• c. Gametangial copulation: In involves the fusion of entire contents of two gametangia to form a common cell called zygote or zygospore, e.g., Mucor, Rhizopus.
• d. Spermatization: Some fungi produce many minute, spore-like, single-celled structures called spermatia (nonmotile gametes). These structures are transferred through agencies like water, wind and insects to either special receptive hyphae or trichogyne of ascogonium. The contents migrate into receptive structure. Thus dikaryotic condition is established, e.g. Puccinia.
• e. Somatogamy: This takes place in fungi where formation of gametes is absent. In such fungi, anastomoses takes place between hyphae and their somatic cells fuse to produce dikaryotic cells, e.g, Agaricus, Peniophora etc.