In this article we will discuss about the life cycle of aspergillus with the help of suitable diagrams.
Mycelium of Aspergillus:
It is well developed and made up of a loosely interwoven mass of hyaline, bright or pale coloured, extensively branched, septate hyphae. Some of the hyphae ramify superficially upon the substratum. Others penetrate deeply into the substratum.
The latter absorb food for the entire mycelium. The hyphae are freely branched and form dense mats on the substratum. The slender, delicate hyphae have thin smooth walls.
Campbell (1970) found no evidence for multilayered nature of the hyphal wall but observed the presence of only diffuse flocculent material on the outer wall surface of A. fumigatus. The hyphae are divided into segments by cross walls.
The cross walls have each a simple, central pore which permits the streaming of cytoplasm and migration of nuclei from one cell into the next. Each segment or cell contains one to many nuclei. The nuclear membrane has typical nuclear pores.
The nuclei are embedded in a mass of granular cytoplasm which also contains the mitochondria, ribosomes lying free in the cytoplasm, endoplasmic reticulum and vesicles. Lipid globules constitute the principal reserve food.
Under certain conditions, the mycelium develops into a sclerotium. Agnihotri (1968) reported that the sclerotia of A. niger are fairly large ranging from 0.8 to 1.2 mm in dia.
Normally they are globose to subglobose bodies. Each sclerotium is surrounded by a cream to buff-coloured, thick- walled interwoven hyphae forming a pseudoparenchyma layer.
Reproduction in Aspergillus:
Asexual Reproduction (Fig. 10.2):
In addition to vegetative propagation by fragmentation, the fungus reproduces asexually by means of asexual spores known as conidia which are produced exogenously in chians at the tips of certain vertically growing aerial hyphae called the conidiophores (Fig. 10.3).
(a) Conidiophores (Fig. 10.2):
Certain cells in the older parts of the young, vigorously growing prostrate mycelium become thick-walled. These thick-walled, T-shaped cells are characteristic of the genus Aspergillus and are known as the foot cells.
Each foot cell produces a special erect branch as an outgrowth (A). These upright hyphal branches are the young conidiophores. Each conidiophore grows to a length of about 2.5 mm and swells at its tip (B) to form usually a globose or elliptical bulbous head.
This swollen end of the conidiophore is called the vesicle (C). The vesicle may, in some species, be hemispherical or clavate. The lumen of the vesicle is continuous with the upper part of the conidiophore and is multinucleate.
A. Hanlin (1976) reported that the vesicle in A. clavatus is surrounded by a thick wall. Besides the numerous nuclei, it contains mitochondria and other cell organelles.
The conidiophores, at maturity, are long, stout, unbranched and commonly unseptate, very rarely septate structures. They arise singly from the foot cells of the mycelium and are not organised into any kind of asexual fruit bodies.
From the surface of the multinucleate vesicle arise numerous, radially arranged, tubular outgrowths (E). These specialized conidiogenous outgrowths or cells are the sterigmata (Sing, sterigma) or phialides.
The sterigmata are arranged compactly side by side and thus completely cover the entire surface of the vesicle. The species with conidia bearing phialides arising directly from the vesicle are termed uniseriate species (A.flavus, Fig. 10.3).
In some other species, the phialides are borne on intermediate cells, the metullae (primary sterigmata) which are attached to the vesicle. These are termed biseriate species (Fig. 10.4 A). In biseriate species such as A. fonsecaceus the conidia bearing sterigmata are called secondary sterigmata.
Generally species tend to be either uniseriate or biseriate. Raper and Fenell (1965), however, reported cases in which both types of arrangements occur in the same species or even in the same conidial head.
The whole structure consisting of the foot cell, the upright hypha, the vesicle, the metullae and the phialides constitutes the conidiophore (Fig. 10.3).
(b) Development of phialides:
It was studied by Hanlin (1976) in A. clavatus. Phialide formation starts with the appearance of numerous thin areas in the otherwise thick vesical wall. The thin areas are formed by the dissolution of wall material.
The cytoplasm adjacent to these areas pushes out synchronously to form broadly oval, peg-like outgrowths, each with an attenuated base. These are the phialides. As the immature phialides increase in size they come in contact with one another.
Into each phialide migrates a single nucleus, mitochondria and other cell organelles from the vesicle. At maturity each phialide is cut off from the vesicle by a basal septum.
It is usually flask-shaped with a tapering apex. The opening between the vesicle and the young phialide is relatively narrow. The nucleus migrating from the vesicle into the phialide becomes elongate as it passes through this narrow opening.
(c) Abstriction of conidia (Fig. 10.4 B-C):
The entire broadly oval phialide tip pushes out at maturity. The mature phialide thus has a narrow, tapering, tubular apex (B1). The conidia are formed inside the tip of the phialide.
The phialides or sterigmata are uninucleate. At the time of the formation of first conidium, the single nucleus in the phialide divides by mitosis into two daughter nuclei. One of these migrates into the narrow, tubular phialide tip which enlarges to form the first conidium (B2).
Towards maturity the first conidium is delimited by a basal septum formed just inside the mouth of the phialide apex (B3). The basal septum has a central pore. It is too narrow to permit the migration of organelles.
The protoplast of the first conidium thus delimited by a septum rounds off and secretes a wall of its own to become a conidium which soon after assumes its characteristic shape and size (C4).
The conidial wall may fuse partially or completely with the parent cell wall or remain free from it. Meanwhile the tip of the sterigma immediately below the first conidium elongates to form a tube (C5) which is again cut off as a uninucleate cell (C6).
The latter develops into a second conidium in the same manner as the first and pushes the latter outward without disjunction (C6). With the delimitation of the second conidium, the septum separating it from the first conidium thickens greatly on both sides forming a connective between the two conidia.
This series of events is repeated. The phialide thus continues to grow and cuts off conidia one below the other. Consequently a chain of conidia is formed at the tip of each sterigma.
All the conidia in the chain are held by similar connectives. A fine channel remains open through the thickened septal pore. The conidia are formed in basipetal chains with no increase in the length of the phialide.
The youngest conidium is at the base of the chain next to the tip of the conidiophore. The oldest is at the top.
This basipetalous arrangement of conidia in the chain has the following advantages:
1. Permits ready dispersal of mature conidia by air currents.
2. Proper nourishment of the younger conidia.
As the conidial chain increases in length the connectives begin to break down separating the conidia. The conidia are thus continually shed and new ones formed below.
The conidia are uninucleate at first and remain so in the majority of species but become multinucleate in some others by successive nuclear divisions. In A. repens the conidia have 12 nuclei and in A. herbariorum 4 each.
The conidia of A. niger are binucleate and of A. nidulans uninucleate. They are black, green, brown, blue or yellow in colour, according to the species and the medium on which the fungus is growing.
The mature conidia are globose and unicellular. They have a surface which is wetted only with difficulty. The conidial wall is thick and usually differentiated into two layers, the outer epispore and the inner endospore.
The epispore in a mature conidium is finely spiny. The young conidia have a smooth epispore layer. The study of fine structure of the mature conidium revealed the spore wall to be composed of three layers.
There is an electron dense inner layer, a darker middle layer of equal thickness and a slightly thinner somewhat wavy, dark outer layer. The conidium often contains electro-transparent areas.
The conidia are very small, light and dry and thus are dispersed by wind. They are always present in the air or in dust.
On falling on a suitable substratum, each conidium germinates. It, at first, produces a germ tube which grows into a new mycelium. During germination a third wall layer forms within the two original spore wall layers in A. niger.
The germ tube wall is continuous with this third spore wall layer only. In A. oryzae the germ tube wall is an extension of the inner layer of the two-layered spore wall.
Under the microscope the conidiophore in a fresh, undisturbed preparation is seen as a long hypha tipped by an indistinct black body. The vesicle is not seen because the conidia are tightly packed and are opaque en masse.
They hide from view the vesicle which can be seen only by removing the conidia mechanically. The conidial stage is very prominent and commonly seen in nature and cultures.
Sexual reproduction (Fig. 10.5 A-C):
The sexual or perfect stage is rare. Possibly the species lacking it have lost it during the course of evolution. This view is supported by the fact that even in species that form asci there is evidence of sexual degeneration.
Different species of this genus show variation in their sexual behaviour. The antheridium is absent in some species or if present it is nonfunctional. The asci, however, develop from the female sex organ (ascogonium).
In others it is well developed and functional. Aspergillus is homothallic. A. heterothallicus is the only species that has been reported to be truly heterothallic.
A second heterothallic species, Aspergillus fishri has been reported by Chung and Sang (1974). The male and female sex organs, in the homothallic species, are developed close together on the same hypha or on separate nearby hyphae of the same mycelium (A). Both are elongate, multinucleate and generally coil around each other.
A small, loosely coiled septate hyphal branch, the archicarp arises from a vegetative hypha (A). It is differentiated into three parts. The terminal segment is generally the longest and single celled.
It contains up to 20 nuclei and is called the trichogyne. It functions as the receptive part of the female sex organ. The segment below the trichogyne functions as the female gametangium and is called the ascogonium.
The protoplast of the ascogonium is also multinucleate. It does not round off to form the egg. Below the ascogonium is the stalk consisting of a few cells. All the parts of the archicarp thus are multinucleate.
The archicarp, at first, is a loosely coiled hyphal branch (A) but later the coiling becomes tight and close (B).
(b) Male Sex Organ:
Shortly before or after septation of the archicarp, a male branch, the pollinodium grows up beside the archicarp from the same hypha or from another adjacent one (A). The pollinodium winds spirally round the tightly coiled archicarp once or several times.
Finally it arches over its apex and then cuts off a unicellular antheridium at its tip (C). The antheridiuiti is a multinucleate slightly swollen structure. The lower septate portion of the pollinodium below the antheridium constitutes the stalk. All the segments or cells are multinucleate.
Fusion may take place between the antheridium and the trichogyne. The tip of the antheridium arches over the apex of the trichogyne and fuses with it. The intervening walls dissolve.
The contents of antheridium then pass through the opening into the trichogyne. An open communication between the antheridium and the trichogyne has, however, been denied by many.
The antheridium is considered by them to be rudimentary or non-functional. In some species the male nuclei have been reported to degenerate before the antheridium has reached maturity. In still other species the antheridium may entirely be absent.
Light That Forms like Aspergillus Throw on the Degeneration of Sex Organs in the Ascomycetes:
The different species of Aspergillus illustrate the way in which elimination of male sex organ (antheridium) in the Ascomycetes has taken place. These species can be arranged in a series.
The series indicates the stages or steps in which the progressive degeneration of the male sex organ (antheridium) has been accomplished.
These steps are:
1. In A. herbariorum the pollinodium cuts off a terminal antheridium. The tip of the antheridium, however, do not migrate and fuse with the contents of the ascogonium.
2. In some species the antheridium may develop late. By that time the ascogenous hyphae have already been developed from the ascogonium.
3. In a few species the male nuclei in the antheridium are reported to degenerate before the antheridium has reached maturity.
4. The antheridium in some species such as A. flavus and A. fished and A. fumigatus is not developed at all.
(d) Development of Ascocarp (Fig. 10.5, D-E):
Whether the antheridium is functional or not the ascogonium in all cases develops into a fruit body or the fructification which is called the ascocarp. The haploid male and female nuclei in the ascogonium come to lie in pairs.
In species in which plasmogamy does not take place, the female nuclei themselves approach each other and come to lie in pairs called the dikaryons. The two nuclei in the pair may lie in the same membrane or in separate membranes.
After pairing of the nuclei, the ascogonium may become septate. The segments are binucleate. From the dikaryotic segments arise the ascogenous hyphae (D). At the same time sterile hyphae grow up from the base of the archicarp and invest the sexual apparatus and the ascogenous hyphae.
They intertwine to form a pseudoparenchymatous sheath shaped like a hollow ball of nearly the size of a pin head around the sexual apparatus. The ascogenous hyphae branch frequently. The branches are of different lengths.
The small, thin-walled asci which develop at their tips, lie at different levels presenting a scattered arrangement. The whole structure enclosed by the sheath is the fruit body or fructification. It is called an ascocarp (D).
The ascocarp in Aspergillus is a small, rounded, yellow hollow ball about 150-200 H in diameter with smooth walls. Even at maturity it remains closed. An ascocarp with a hollow, closed form is called the cieistothecium or cleistocarp.
The sheath is differentiated into a single layered yellow peridium and one or more ill-defined inner layers constituting the tapetum (E). The tapetum is nutritive in function.
The outer layer provides protection to the developing asci which are formed at different levels and thus lie in a scattered arrangement within the cieistothecium. It secretes a yellow substance which makes the cleistocarp readily recognisable. It is yellow and soft.
Development of asci and differentiation of ascospores:
The swollen terminal cells of the ascogenous hyphae or their branches give rise to asci. They are binucleate and directly function as ascus mother cells. No croziers are formed. The two nuclei in the ascus mother cell fuse.
The young ascus with the fusion nucleus enlarges. It represents the transitory diplo-phase in the life cycle. Its diploid nucleus or the synkaryon under-goes three successive divisions. The first and the second divisions constitute meiosis. The third is mitotic.
This results in the formation of eight haploid daughter nuclei. Each haploid nucleus becomes surrounded by a certain amount of cytoplasm. A wall is then secreted around each uninucleate protoplast to organise it into a meiospore which is botanically called an ascospore.
In this way eight haploid ascospores are formed from the contents of each ascus by a process known as free cell formation. The cytoplasm left over in the ascus is known as epiplasm. The mature asci may be globose, ovoid or pear-shaped (F).
They do not survive long and are not explosive. When the ascospores are formed the ascus wall, the ascogenous hypha and the tapetum all undergo degeneration to form a nutritive fluid.
The ascospores lie free in the hollow cleistothecial cavity. The nutritive fluid provides nutrition to the developing ascospores. When the ascospores are mature the peridium also undergoes decay. The ascospores are thus liberated.
Structure of Ascospores:
The liberated ascospore is a haploid, uninucleate structure (G). It is somewhat, lens-shaped with a small groove round the edge and thus shaped like a pulley wheel. The spore wall is differentiated into two layers, the outer epispore and the inner endospore.
The epispore is usually thick and sculptured. In side view the ascospore is seen like a pulley wheel but in surface view it may appear round or star-shaped (H). On falling on a suitable substratum each ascospore germinates to form a germ tube which develops into the characteristic haploid mycelium.
Alternation of Generations in Aspergillus:
From the above account, it is evident that in a single life cycle of Aspergillus there occur two phases or generations. Of these, one is more conspicuous. It is independent so far as its nutrition is concerned. This independent phase is the gametophyte.
It is characterised by the haploid number of chromosomes in the nuclei. It bears the sex organs and is concerned with sexual reproduction. In the life cycle, it starts with the formation of ascospores and ends with the formation of dikaryons in the ascogonium.
It consists of the ascospores, parent mycelium, antheridia when present and young ascogonia. There are accessory means of multiplication of this phase in the life cycle. It is done by the formation of conidia. Conidia play no role in the alternation of generations.
The second phase in the life history of Aspergillus is the sporophyte. It is not conspicuous and is dependent for its nutrition on the parent gametophyte (haplomyceliwn). The sporophytic phase starts with the formation of dikaryons in the ascogonium.
It ends with meiosis of the synkaryons in the asci.
The sporophytic phase thus consists of:
(i) Dikaryophase consisting of the ascogonium with dikaryons, the ascogenous hyphae, and
(ii) The transitory diplophase represented by the young ascus before meiosis.
These two phases sporophyte and gametophyte normally alternate in a single life cycle.
Hence Aspergillus is said to exhibit alternation of generation in the life cycle.