It is that type of asexual reproduction in which a fully grown parent organism divides into two or more than two daughter cells.
In this case , the reproductive unit is whole parental body.
Types of Fission:
On the basis of number of daughter cells produced, fission is of two types:
(a) Binary fission
(b) Multiple fission.
(a) Binary Fission: It is division of adult parental body into two nearly equal daughter cells during favourable conditions.
It is the simplest and most common method of asexual reproduction found in protists (Amoeba, Euglena), bacteria, and green algal forms (Chlamydomonas) and planarians (flat worms).
In binary fission, the karyokinesis (division of nucleus) is followed by cytokinesis, so that nothing is left with the parent. Daughters feed, grow and repeat the process. The organisms undergoing binary fission are called immortal as after binary fission nothing is left with the parental body so there is no natural death.
On the basis of plane of cytokinesis, the binary fission is of three types:
(i) Irregular or simple binary fission
Here cytokinesis may take place along any plane but its plane is always perpendicular to that of karyokinesis e.g., Amoeba. So nothing is left with parental Amoeba which completely divides into daughter cells, so it is called immortal as it suffers no natural death.
(ii) Longitudinal binary fission:
Here cytokinesis takes place along longitudinal axis e.g., in flagellates (Euglena).
(iii) Transverse binary fission :
Here cytokinesis takes place along transverse axis e.g., in ciliate protozoans (Paramecium).
In Opalina and Pelomyxa (both protozoans), the peculiar binary fission called plasmotomy occurs in which a multinucleate adult parent undergoes cytokinesis to form two multinucleate daughter cells followed by karyokinesis in each daughter cell. In Planaria, the parent undergoes transverse binary fission as shown in fig.
(b) Multiple fission: It is that type of asexual reproduction in which the parental body divides into many daughter cells simultaneously during the unfavourable conditions to increase the chances of survival of daughter cells. In this, the reproductive unit is whole parental body.
Occurs in: Multiple fission is found in a number of organisms e.g., algae among plants, Plasmodium malarial parasite, Amoeba and Monocystis.
How it happens: During multiple fission, the nucleus of parent divides by repeated amitosis into many nuclei, each nucleus takes a bit of cytoplasm and forms a daughter cell. Some cytoplasm of the parental body remains unused and is called residual body.
For example, during erythrocytic schizogony in the life cycle of P. vivax , the cryptomerozoite enters the RBC, feeds on the haemoglobin and other cellular contents saprozoically and becomes fully grown and is called schizont. Then multiple fission occurs and 12-24 merozoites are formed.
RBC ruptures and merozoites are released which repeat the process. Similarly in an encysted oocyst called sporont present on the stomach wall of female Anopheles host, the multiple fission called sporogony occurs and many sporozoites are formed. Similarly, Amoeba undergoes multiple fission during unfavourable conditions in an imencysted form as well as in encysted form.
In the former, daughter cells get encysted (called encystation), and are called spores, so the process is called sporulation. The cysts also help in perennation and dispersal. In the later case.Amoeba withdraws its pseudopodia and secretes a three-layered chitinous cyst wall (encystation). On the approach of favourable conditions. Amoeba undergoes multiple fission and produces many small sized amoebulae or pseudopodiospores .