ASPERGILLUS - CLASSIFICATION, VEGETATIVE STRUCTURE, REPRODUCTION, ECONOMIC IMPORTANCE

            A. CLASSIFICATION:

Division – Mycota

     Sub-division – Eumycotina

          Class – Ascomycetes

                Sub-class – Euascomycetidae

                       Series – Plectomycetes

                             Order – Aspergillales

                                    Family – Aspergillaceae

                                          Genus – Aspergillus

            The fungus is commonly called as Eurotium. The fungus comprises about 200 species which are chiefly sporophytic and widely distributed. It grows on decaying vegetables, on fatty media such as butter and ghee, on starchy media such as bread and rice, on preserved fruits as jams and jellies. Several species of Aspergillus are also found parasitically on animals as well as human beins which cause the disease known as aspergillosis.

           B. STRUCTURE OF THE VEGETATIVE BODY:- The vegetative body is the mycelium. It is composed of well developed, profusely branched, hyaline and pale coloured, septate hyphae. Some of the hyphae ramify superficially on the substratum while others penetrate deeply into the substratum. The hyphae are freely branched and form dense mats on the substratum.

        Each hyphal cell is surrounded by a distinct thin cell wall and contains many nuclei embedded in granular cytoplasm. Mitocondria, ER, ribosomes are also present in the cytoplasm. Resreve food material is the oil globule. In each septum there is a central pore through which nuclei and cytoplasmic materials flow between the adjacent cells.

                C. REPRODUCTION:- Aspergillus reproduces by vegetative, asexual and sexual methods of reproduction.

            1. Vegetative Reproduction:- Vegetative reproduction takes place by the most common method of fragmentation. In this case the hyphae break up into small segments or fragments. Each fragment grows by repeated division into a full-fledged mycelium.

            2. Asexual Reproduction:- Asexual reproduction is accomplished by the conidia. In this case, the ordinary hyphae produce a special type of aerial hyphae known as conidiophores.

            Each conidiophore is simple, erect, unbranched, hyphae. The tip of each conidiophore swells at the tip to form vesicle. The vesicle is multinucleate and from its entire surface sterigmata or phialides are borne. Sometimes two layers of phialides are developed one upon the other. From the tip of each sterigmata, a chain of conidia are produced.

            Conidia are small, spherical or oval, unicellular with externally roughened walls and greenish, yellowish or blackish in colour. At maturity the conidia are deciduous and are carried away by air. Each conidium germinates by producing germ tube to give rise to new mycelium.

            3. Sexual Reproduction:- The sexual or perfect stage is rare. Aspergillus is homothallic. Sexual reproduction takes place by the production of gametangia. The female gametangia is known as ascogonia while the male gametangia are known as antheridia.

            Ascogonium – Ascogonium develops on the mycelium in the form of specialized septate and coiled hyphal branch known as archicarp. The young archicarp is differentiated into a lower multicellular stalk, the middle oogonium and the terminal unicellular trichogyne. All the cells of ascogonium are multinucleate.

            Antheridium – It develops in the form of a male i.e., antheridial branch either from the same hyphae from which ascogonium develops or from other adjacent hypha. The male branch is also known as pollinodium. The male branch come close to the trichogyne and cut at its tip an unicellular antheridium. The lower part of the male branch is called stalk.

            Plasmogamy – The tip of the antheridium comes in contact with the trichogyne. At the point of contact, the common wall dissolves and a pore is developed between the two cells. Through the common pore, the protoplast comes to the ascogonium.

            Formation of Ascogonenous hypha, Asci and Karyogamy – Plasmogamy or autogamy is followed by formation of the ascocarp. The haploid male and female nuclei lie in pairs forming the dikaryon. After pairing of the nuclei, the ascogonium may become septate. From the dikaryotic segments arises the ascogenous hyphae.  Meanwhile, sterile hyphae grow up around the sexual apparatus and form loosely interwoven hyphae called peridium, which afford protection to the structures developing within. The whole structure is called ascocarp. Such an ascocarp with a hollow closed form is called cleistothecium.

            As a result of plasmogamy or autogamy, one or more lateral outgrowths arise from some binucleate segments of the ascogonium. Each outgrowth is known as ascogenous initial, each of which develops into a branched ascogenous hypa composed of bi-nucleate cells. From the bi-nucleate cells of each ascogenous hypha, asci (singular: ascus) are developed terminally in short chains. The two nuclei eventually fuse inside the ascus. This is called karyogamy.

            Formation of Ascospores – Soon after karyogamy, the young ascus begins to enlarge. Its diploid nucleus undergoes three successive divisions. The first and second division is meiotic and the third is mitotic, thus resulting in 8 haploid nuclei. A small amount of cytoplasm gathers around each daughter nucleus which metamorphosed into ascospores.

            Discharge and Germination of Ascospores – At maturity, the wall of the ascus dissolves and ascospores are released by the decay of the other wall of the peridium.

            The liberated ascospores remain dormant when temperature and moisture conditions are low and air supply is restricted. On falling on a suitable substratum and under suitable condition each ascospore germinate to form new mycelium.

             D. ECONOMIC IMPORTANCE:- Aspergillus are of tremendous positive and negative economic importance –

            1. Various species of Aspergillus are used in various industries, especially in the production of citric and glutamic acid. Some species of Aspergillus are also used in the manufacture of enzyme preparations and also enzymes.

            2. Some species of A. oryzae are also used in the manufacture of enzyme preparations and are sold in the market in the name of ‘Polyzime’ and ‘Takadiastase’.

            3. Many species of Aspergillus grows on cloth fabrics, leather, etc., in slight humid condition, as a result their commercial value is reduced.

            4. A. flavus produce a high toxic substance aflatoxin. This toxin has carcinogenic effects and may cause liver cancer of both humans and animals.

            5. A. niger, A. fumagatus, A. flavus, etc. cause aspergillosis disease in humans and animals.

            6. Many plant diseases e.g., crown rot of groundnut and boll rot of cotton are caused by different species of Aspergillus.

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