Asexual Reproduction in Animal - Fission

Fission:

• The term “fission” means “to divide”.
• Fission is the division of the parent body into two or more identical daughter individuals.
• Five types of fission are seen in animals. They are binary fission, multiple fission, plasmotomy, strobilation, and sporulation.

A) Binary fission:

• In binary fission, the parent organism divides into two halves and each half forms a daughter individual.
• The nucleus divides first amitotically or mitotically (karyokinesis), followed by the division of the cytoplasm (cytokinesis).
• The resultant offsprings are genetically identical to the parent.
• Depending on the plane of fission, binary fission is of the following types,

1. Simple irregular binary fission:
   Simple irregular binary fission is seen in Amoeba like irregular shaped organisms, where the plane of division is hard to observe. The contractile vacuoles cease to function and disappear. The nucleoli disintegrate and the nucleus divides mitotically. The cell then constricts in the middle, so the cytoplasm divides and forms two daughter cells.
   

   Simple irregular binary fission in Amoeba

2. Transverse binary fission: 
   In transverse binary fission, the plane of the division runs along the transverse axis of the individual. e.g. Paramecium and Planaria. In Paramecium, the macronucleus divides by amitosis and the micronucleus divides by mitosis.
   

   Transverse binary fission in Paramecium

3. Longitudinal binary fission: 
   In longitudinal binary fission, the nucleus and the cytoplasm divides in the longitudinal axis of the organism. In flagellates, the flagellum is retained usually by one daughter cell. The basal granule is divided into two and the new basal granule forms a flagellum in the other daughter individual. e.g. Vorticella and Euglena.

   Longitudinal binary fission in Euglena

4. Oblique binary fission:
   In oblique binary fission the plane of division is oblique. It is seen in dinoflagellates. e.g. Ceratium.

   Oblique binary fission - Ceratium

B) Multiple fission:

In multiple fission, the parent body divides into many similar daughter cells simultaneously. First, the nucleus divides repeatedly without the division of the cytoplasm, later the cytoplasm divides into as many parts as that of nuclei. Each cytoplasmic part encircles one daughter nucleus. This results in the formation of many smaller individuals from a single-parent organism. If multiple fission produces four or many daughter individuals by equal cell division and the young ones do not separate until the process is complete, then this division is called repeated fission. e.g. Vorticella.

During unfavorable conditions (increase or decrease in temperature, scarcity of food) Amoeba withdraws its pseudopodia and secretes a three-layered, protective, chitinous cyst wall around it and becomes inactive. This phenomenon is called encystment. When conditions become favourable, the encysted Amoeba divides by multiple fission and produces many minute amoebae called pseudopodiospore or amoebulae. The cyst wall absorbs water and breaks off liberating the young pseudopodiospores, each with a fine pseudopodia. They feed and grow rapidly to lead an independent life.

C) Sporulation:

During unfavourable conditions, Amoeba multiplies by sporulation without encystment. Nucleus breaks into several small fragments or chromatin blocks. Each fragment develops a nuclear membrane, becomes surrounded by cytoplasm, and develops a spore-case around it. When conditions become favourable, the parent body disintegrates and the spores are liberated, each hatching into a young amoeba.

D) Strobilation:

In some metazoan animals, a special type of transverse fission called strobilation occurs. In the process of strobilation, several transverse fissions occur simultaneously giving rise to a number of individuals which often do not separate immediately from each other e.g. Aurelia.

E) Plasmotomy: 

Plasmotomy is the division of multinucleated parent into many multinucleate daughter individuals with the division of nuclei. Nuclear division occurs later to maintain normal number of nuclei. Plasmotomy occurs in Opalina and Pelomyxa (Giant Amoeba).