The fungi are a group of eukaryotic, non-vascular organism. Which are of diverse forms, sizes, physiology and reproduces both by sexual (meiotic) and asexual (mitotic) spores. Examples of fungi :-Mushrooms, yeasts, molds, Penicillium- the first of the wonder drugs, penicillin, was isolated from this fungus and crop parasites.Fungi are most often associated with the roots of some plant species, and this type of symbiotic associations is known as mycorrhizae.
The study of fungi is known as mycology (Mykes = mushroom + logos = discourse) or mycetology and who study about fungus is known as mycologist.
General characteristics of Fungi
- Most fungi are eukaryotic,multinucleate, multicellular organisms, except yeasts which are unicellular.
- Nutrition Fungi are heterotrophic because these lack chlorophyll (green pigment) and thus cannot create their own food through photosynthesis.Fungi acquire their nutrients from dead organic matter by secretion of extracellular enzymes followed by absorption. These are either :-
- Saprophytes or Saprobes :- Most fungi are saprophytes ,obtaining nutrients by absorbing dead organic matter.
- Parasitic :-Some fungi are parasites, living in or on another organism (called a host) from which they obtain their nutrients. This relationship usually harms the host. Such parasitic fungi usually have specialized tissues called haustoria that penetrate the host's body for food absorption.
- Symbionts :- Some fungi live in a mutually beneficial symbiotic relationship with another organism.
- Lichen (association of fungi + algae)Some fungi are associated with either cynobacteria or green algae and this type of symbiotic associations is known as lichen.
- Mycorrhizae (fungi + plants) :- Some fungi are most often associated with the roots of some plant species, and this type of symbiotic associations is known as mycorrhizae.Mutualistic association of plant roots and fungi increase the absorptive surface area of plant roots. (Fungus gets organic nutrients from plant; Plant gets minerals from the soil via the fungus).
- Thallus organization (structure):-
- Most of the fungi, are filamentous and the vegetative (=assimilative) stage is a tubular branching thread-like filament called a hypha (plural = hyphae).
- The hypha extends by tip growth, and multiplies by branching, creating a fine network called a mycelium.
- Two of the phyla Zygomycotina and the Chytridiomycotina (also known as lower fungi) vegetative mycelium is without cross -walls known as non-septate (aseptate) or coenocytes. Complete septa are only found in reproductive structures. In aseptate mycelium numerous nuclei lie in a common mass of protoplasm.This condition is called coenocytes.
- The other two phyla Ascomycotina and the Basidiomycotina( higher fungi)the hyphae have cross walls that break them up into cells. A cross wall in fungi is called a septum ( plural septa).The septum has a central pore which enable exchange of nuclei and most organelles within the cytoplasm between adjacent compartments.
- Septum formation is a simple process. Wall ingrowth towards the centre of the compartment results in a complete or incomplete blockage of the hypha. Inward growth may be followed by modification of the outer wall. The septum also increases rigidity of the hypha as it can function as a structural support to the turgor pressure within the compartment.
- Ultrastructure of fungal thallus:-
- Rigid cell walls are strengthened with chitin, a polymer of N-acetylglucosamine (except oomycetes where cellulose present).
- Food reserves are in the form of glycogen
- Hyphae contain nuclei, mitochondria, ribosomes, golgi and membrane-bound vesicles within a plasma-membrane bound cytoplasm. The sub-cellular structures are supported and organized by micro-tubules and endoplasmic reticulum. The cytoplasm and most organelles and inclusions of fungal cytoplasm are typical of eukaryotic organisms. However, chloroplasts or plastids are absent.
- Reproduction :-
- At the time of reproduction when entire cell asin case of unicellular fungus may be converted into reproductive structure. Such a condition is known as holocarpic. But if only a portion of thallus is converted into reproductive structure, it is called eucarpic. Some fungi are unicellular and are termed as yeasts. These grow by binary fission or budding, creating new individuals from the parent cell.
- Members of the Ascomycotina produce asexual conidiospores and sexual ascospores in sac-shaped cells called asci.
- Fungi from the Basidiomycotina rarely produce asexual spores, and produce their sexual spores from club-shaped basidia in complex fruit bodies
Classification of Fungi
Over 60,000 species of fungi are known. Fungi are classified primarily by their method of reproduction (both sexual and asexual) and fruiting bodies (asexual or sexual spores surrounded by highly organised protective structures). In the earliest classification, there were only two recognized kingdoms:- Plants and Animals.This two kingdom system was used until Whitaker (1969) proposed that organisms be classified into five kingdoms :- Monera (=Bacteria), Protista (=Mostly Algae and Protozoans), Plantae (=Plants), Mycetae (=Fungi) and Animalia (=Animals).
Fungi belong to Domain-Eukarya and Kingdom -Mycetae (=Fungi) The classification of fungi, as proposed by Ainsworth (1973), is commonly followed :-
The more recent systems of classification that is based, in part, on molecular research are :-