Asexual reproduction in Rhizopus stolonifer
Asexual reproduction in Rhizopus stolonifer takes place by vegetative methods and sporulation.
It takes place by the following two methods :-
It is the commonest method of propagation of the mycelium stage during the growing season. Fragmentation consists in the separation of hyphae from the aerial mycelium. These are known as the fragments. Each fragment by, apical growth and branching grows into a new mycelium.
Some species of Rhizopus under unfavourable conditions (shortage of waters) produce specially modified thick-walled resting cells called the Chlamydospores. The protoplasmic contents of the hyphae accumulate at certain points and lose water. The intervening parts remain empty. Each localized protoplasmic accumulation secretes a thick wall around it to become Chlamydospore. The Chlamydospores in Rhizopus are thus intercalary. The contain sufficient reserve food and very resistant to desiccation. They are in fact resting cells. As the substratum, on which the fungus grows, dries out the mycelium perishes. The Chlamydospores survive. On the return of conditions suitable for vegetative growth, the surviving (perennating) Chlamydospores germinate, each producing a new mycelium. The Chlamydospores thus enable the fungus to tide over conditions unfavourable for growth. Chlamydospores are rarely formed in R. stolonifer but they are commonly found in other species.
Multiplication by oidia formation is absent in R. stolonifer but is common in other species of Rhizopus (R. oryzae). When the mycelium grows submerged in a nutritive medium such as sugar solution the young coenocytic hyphae develop septa and divide into short multinucleate segments. These segments separate from each other and become rounded to from Oidiospores or oidia. The latter increase in number by budding. This is known as the torula stage of the fungus. The oidiospores germinate on solid organic substrata and grow into new mycelia.
During the growing season the fungus reproduces repeatedly by the formation of small, non-motile wind-disseminated spores. They are produced in large numbers within round black structures, the sporangia. They are differentiated by mitosis from the protoplast of sporangium endogenously and thus are termed the sporangiophores. The sporangiophores serve to multiply the mycelium stage in the life cycle. They belong to the category of mitospores and play no role in the phenomenon of alternation of generations.
Development of sporangia-
As the sporangiophores elongates by linear growth of the region behind the tip, the latter swells into a knob-like vesicle . It is more or less spherical in form. The vesicle grows in size due to flow into it of cytoplasm carrying nuclei and food from the sporangiophore. It is the young sporangium.
Differentiation of Spores-
The young sporangium contains a certain amount of cytoplasm and many nuclei it also accumulates a considerable amount of reserve food. These protoplasmic contents soon become differentiated into two zones. Most of the multinucleate protoplasm concentrates thick dense layer lining the wall Thus results in the appearance of a highly vacuolated dome-like central zone in the sporangium. The central zone contains dense, vacuolar protoplasm with fewer nuclei. A layer of vacuoles then appears between the two portions. The vacuoles flatten and finally fuse laterally to form a cleft between the two zones. A wall finally appears in this cleft The sporangium is now divided into the denser, peripheral, soporiferous zone and the central dome-shaped zone, the columella. The protoplast of the columella is continuous with that of the sporangiophores. The soporiferous zone undergoes cleavage to from a number of multinucleate segments. These segments round off and secrete walls around them to become the sporangiophores.
Structure of sporangium-
The young sporangium is a white spherical structure situated at the free end of the sporangiophores. At maturity it turns black. The black colour is due to the presence of sooty spores within. In the centre of the sporangium is a large, dome-shaped structure called the columella. Surrounding the columell are the numerous tiny sporangiophores which are covered and protected by a non-cellular layer constituting the sporangial wall. The sporangia of Rhizopus are haploid. They are protected on the haploid mycelium and contain haploid spores differentiated by mitosis.
Dehiscence of the sporangium-
As the spores mature, there is an increase in the quantity of fluid in the sporangiophores and the columella. It exerts a considerable pressure and makes the columella bulge more strongly into the sporangium. Consequently the fragile sporangium wall cracks into small fragments in dry air or with slightest disturbance the spores which form a dry powdery mass exposed. A part of the wall remains as a collar-like torn fringe at the base of the distended columella.
Dispersal of Spores-
The spores in the exposed mass are minute and light. According to Ingold, immediately after the dehiscence of sporangium the mature columella collapses and takes on the form of an upside down bowl with the spores spread over its bulging surface. Form there the spores are blown away by the lightest drought to a great distance through the air. In this way, they attain such a wide distribution that they come to be present almost everywhere. The jerking and twisting of the thick-walled sporangiophores also assists in the liberation of spores.
The liberated spore is non-motile globose to oval or somewhat angular in form. It is microscopic and consists of a tiny mass of multinucleate protoplast surrounded by a resistant, single layered spore wall which is thick, rigid and electron opaque. It bears numerous prominences representing the ridges. Within the spore wall is the plasma membrane closely investing the cytoplasm which contains several nuclei, mitochondria lipid droplets and ribosome. Endoplasmic reticulum strands are sparse. Dense membrane bound inclusios termed cytosomes are abundant. Small vesicles are also present. The spores are coloured bluish or brown but in a mass appear black
Under dry or other unsuitable conditions, the spores remain viable for long periods. The spores of R. stolonifer, when kept dry, are known to germinate after twelve years. On falling on a suitable substratum and under suitable conditions they germinate immediately. Adequate moisture, favourable temperature (between 15° to 20°C) and starchy matter in the substratum are the three conditions essential for their germination. Finding these, the Rhizopus spore germinates immediately. It absorbs moisture. The rigid spore wall softens and the protoplast within swells rupturing the spore wall. The spore protoplast emerges through the split in the form of one or more short protuberances called the germ tubes. There is a suggestion for enzymatic breakdown of the single-layered spore wall at the point of emergence of germ tube rather than a mechanical rupture. The spore wall is apparently weakened by enzyme action. Prior to emergence of the germ tube, a second wall layer known as the vegetative or germination layer appears only in the germinating spores inside the spore wall. It surrounds the emerging germ tube (Hawker and Abbot, 1963; Buckley, Soneuer and Matsumato, 1968). There is a mass flow of protoplasm towards the germ tube apex in early stages of extension. During spore germination, the nuclei divide, vacuoles increase in size and mitochondria increase in size and number.
Development of Mycelium
The germ tube elongates by apical growth with amazing speed and branches extensively to form the internal feeding mycelium. In the young mycelium all the hyphae are alike, multinucleate and asepate. After a few days growth within the substrate the internal mycelium produces some hyphae which come to the surface of the substrate to form the aerial mycelium which is still connected with the internal feeding mycelium.
Propagation of the vegetative (somatic) phase by vegetative methods of fragmentation and sporulation is quite common. It may continue for several generations under favourable conditions.
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