The following article will guide you about how do amoeba reproduce asexually during favourable and unfavourable environmental conditions.
Binary Fission:
This is the commonest and simplest method of reproduction in amoeba. In this, the whole body divides, like an ordinary cell, into two daughter amoebae by mitosis. Hence, it is termed binary fission. In favourable conditions, when food is plentiful, amoeba feeds well, grows rapidly and continues reproducing by binary fission.
In binary fission of amoeba, the different phases of mitosis are accompanied with changes in the shape of body. In Prophase, body becomes spherical with numerous small pseudopodia all around, contractile vacuole stops functioning, and the nucleus elongates, to form an intra-nuclear spindle. 500 to 600 (according to different species) chromidia now appear as duplicated chromosomes in the nucleus.
In metaphase, chromosomes arrange at the equator of the spindle-shaped nucleus to form a metaphase plate. In anaphase, the chromatids (= daughter chromosomes) separate and move towards opposite poles of spindle, pseudopodia becomes larger and irregular in shape and the nucleus first becomes dumb-bell-shaped by a middle constriction and finally divides into two daughter nuclei.
In telophase, pseudopodia assume normal shape- the body first elongates, then constricts in the middle, and finally divides into two daughter amoebae, each having a daughter nucleus. The contractile vacuole is retained by one of the daughter amoebae; the other one acquires a new vacuole. Under optimum conditions each binary fission is completed in about 30 mols.
Multiple Fission:
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According to some earlier workers, Amoeba divides by multiple fissions during unfavorable conditions of food and temperature. Pseudopodia are withdrawn, the animal becomes rounded, the streaming movements of endoplasm stop, large granules dissolve and protoplasm becomes granular. The distinction between ectoplasm and endoplasm is lost.
The animal rotates and secretes a three layered protective, chitinous cyst around the body and becomes inactive. The nucleus inside divides repeatedly to form several daughter nuclei which migrate towards periphery. Each daughter nucleus gets enveloped by a portion of cytoplasm to form daughter amoebae called amoebulae or pseudopodiospores.
At the approach of favourable condition, the cyst wall absorbs water and bursts, the daughter amoebae escape and each grows into an Amoeba. A portion of central protoplasm remains as residue. However, the recent authors discard this sort of encystment and multiple fission.
Sporulation:
Some species of Amoeba reproduce by sporulation, either normally and regularly, or when depressed due to repeated binary fissions, or under unfavourable environmental conditions. Sporulation begins with breakdown of nuclear membrane and release of chromatin blocks in groups of 2 or 3 into the cytoplasm. Each group becomes a small daughter nucleus by acquiring a nuclear membrane around it. Now, the cytoplasm of parent body segregates into small masses, each around a daughter nucleus.
Encystment:
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If conditions of life becomes unfavourable, as when the pond dries up or when the food supply runs low the animal forms a membranous protective shell or cyst around it to tide over the difficult period. The animal rounds up and secretes over itself a hard and impervious covering, the cyst, within which its rate of metabolism falls to a minimum.
In this state the amoeba can with stand extremes of heat and cold as well as desiccation, as their power of resistance is considerably increased. Encystment not only enables amoebae to tide over unfavourable circumstances but also furthers their distribution, as in the encysted state they may be blown by wind or transferred from one pond to another in the mud that clings to the feet of birds or other animals. Since encystment is mainly valuable as a means of surviving some of the dangers of the outside world, it is not surprising that it is universal among the parasitic amoebae.
Replaced in a suitable environment the cyst ruptures and the enclosed animal emerges and resumes its usual activities. According to some the amoeba does not emerges from encystment as its old self; instead a cluster of swarm spores comes out, produced by repeated divisions within the cysts. In some species such as Paramoeba, there are a dozen or so of these swarm spores, each with two whip-like flagella. In the course of time the flagella are lost and the swarm spore comes to look like ordinary amoeba.