In this article we will discuss about:- 1. Occurrence of Aspergillus 2. General Characteristics of Aspergillales 3. Salient Features.
Occurrence of Aspergillus:
This fungus is also called Eurotium. It is chiefly a saprophytic fungus which is widely distributed. It grows on decaying vegetables; on fatty media such as butter and ghee; on starchy media as bread and rice; on preserved fruits as jams and jellies.
It is very commonly found on rotting oranges and phyllanthus fruits. It thus grows on a wide variety of substrata by virtue of the large number of enzymes it produces. All that the fungus requires are some organic matter and little of moisture.
Aspergillus appears in the form of greenish, smoky patches along with Mucor, Rhizopus and Penicilliutn on moist bread when kept under a bell jar for a couple of days. The other common shades are yellow, black and blue.
In all Aspergillus includes about 200 species. Majority of these are known only in the conidial stage. A few produce cleistothecia (perfect stage).
At first when the connection between the two stages was not fully established the sexual or perfect stage was called Eurotium and the conidial or imperfect stage as Aspergillus.
Now both the stages are usually denoted by the common generic name Aspergillus because conidial stage is common and predominant and moreover generic name Aspergillus was introduced first.
General Characteristics of Aspergillales:
The order Aspergillales {Eurotiales) is also called Plectascales. The sex organs in this order develop before the envelope surrounding the ascogenous hyphae and the gametangia. Sterile hyphae develop only with the formation of the ascogenous hyphae.
The order includes several hundred species which are placed under 50 genera. Majority of them are saprophytes. A few species, parasitic on plants and animals, also occur.
The saprophytic species such as the blue and green molds are of great economic importance. Some of these are sources of antibiotic drugs, organic acids and commercial enzymes. A few are essential in the ripening of cheese.
The saprophytic forms are found almost on any decaying organic matter. The chief means of multiplication are the conidia. They are produced in enormous numbers. It is the conidial mass which gives hyphal mats their characteristic tint.
In fact they owe their success to two features:
(i) Production in enormous numbers of light, dry, air dispersed conidia.
(ii) High osmotic pressure of the cell sap which enables them to germinate in comparatively drier habitats and in concentrated solutions of salts and sugars in which most other fungi and bacteria are unable to grow. The conidial stage in these fungi is predominant. The ascal stage is rarely seen.
The asci are globose or broadly club-shaped. They arise at different levels and thus are scattered irregularly within a round closed ascocarp called the cleistothecium. There is thus no hymenial layer.
The ascospores are released in the cleistothecium by the dissolution of ascus wall. Subsequently they are liberated by the decay or breaking of the peridium. Bessey (1950) recognises seven families in the order.
Walker makes four (Gymnoascaceae, Aspergillaceae, Elaphomycetaceae and Onygenaceae) but Alexopoulos (1962) reduced the number to three. He raised Onygenaceae to the rank of an order Onygenales. Martin (1961), however, classified the order into two families. Of these Eurotiaceae or Aspergillaceae is considered here in detail.
Salient Features of Aspergillus:
The life history of Aspergillus consists of three phases. They are the haplophase, the dikaryophase and the transitory diplophase.
Hapiophase:
1. It is represented by the extensively branched mycelium consisting of septate, colourless hyphae. The cells are multinucleate.
2. Besides fragmentation, the haplomycelium is propagated asexually by means of non- motile, wind disseminated, exogenously produced spores called the conidia which are uninucleate at first but soon become multinucleate.
3. Each conidium is globose. It has thick, echinucleate wall and is unicellular.
4. The conidia are formed one below the other in a long chain at the top of somewhat bottle-shaped sterigma.
5. The sterigmata arise side by side completely covering the entire surface of the globose, swollen head, the vesicle developed at the top of a special long, erect unbranched, unseptate, stout hypha called the conidiophore.
6. Each conidiophore arises as an outgrowth from a special thick-walled T-shaped cell of the mycelium. It is called the foot cell.
7. Sometimes the sterigmata are produced in two layers. Those of the upper layer are called the secondary sterigmata and of the lower layer as primary. The former bear the conidia.
8. On falling on a suitable substratum each conidium germinates by a germ tube which soon develops into the haplomycelium.
9. The sex organs antheridia and ascogonia are produced close together on the hyphae of the same mycelium. The female branch is called the archicarp and the male pollinodium.
10. The sex organs are elongated, multinucleate structures. The pollinodium is coiled round the ascogonium.
11. The tip of the antheridium comes in contact with the trichogyne. The intervening double wall dissolves at the point of contact.
12. If the migration of male nuclei takes place they pair with the female nuclei, otherwise the ascogonial nuclei approach near each other to lie in pairs in the ascogonium. Each such pair is called a dikaryon. With the formation of dikaryons the haplophase ends and the dikaryophase starts.
Dikaryophase:
13. After the establishment of dikaryons, the ascogenous hyphae arise from the ascogonium. They are septate and branched. The branches lie at different levels.
14. The asci are formed from the terminal binucleate cells of the ascogenous hyphae or their branches.
15. The young binucleate ascus mother cell is the last structure of the dikaryophase which consists of the ascogonium with dikaryons and the ascogenous hyphae.
16. The dikaryophase is parasitic on the haplomycelium.
17. The whole sexual apparatus in the meantime is enclosed in a compact layer of sterile hyphae forming the sheath or peridium.
18. The fructification thus formed is a small, globose ascocarp. The wall is smooth and yellow. The ascocarp in Aspergillus is soft. It has no opening and thus is closed and called the cleistothecium.
Diplophase:
19. With the fusion of the two nuclei in the ascus mother cell the dikaryophase ends and the transitory diplophase starts.
Future Hapiophase:
20. During further development, the young ascus enlarges and the synkaryon undergoes meiosis. The resultant four haploid nuclei divide mitotically to form eight nuclei which are organised into ascospores. With the establishment of ascospores starts the new hapiophase.
21. The walls of the evanescent asci dissolve, the ascospores are released into the cavity of the cleistothecium. Eventually the wall of the cleistothecium decays to liberate the ascospores.
22. Each ascospore germinates under suitable conditions to form a new characteristic haplomycelium.