Question

How to make flow chart of asexual reproduction?......

Answer 1

Asexual Reproduction:

In this type of reproduction neither the sex cells (nor gametes) are formed nor do they unite to form the zygote. Moreover, the participation of two organisms (male and female) is not required, only one organism reproduces. During asexual reproduction the body (somatic) cells divide, their nucleus divides either by mitosis or amitosis, therefore, such type of reproduction is also known as somatogenic or blastogenic reproduction. The asexual reproduction is commonly found in lower animals such as protozoans, sponges, coelenterates, certain worms and tunicates.

Principal forms of asexual reproduction are:

1. Binary fission

2. Multiple fission

3. Gemmulation

4. Budding

5. Fragmentation

6. Regeneration.

1. Binary Fission:

This is the simplest and most common method of asexual reproduction seen in unicellular organisms. This occurs under the favourable conditions of the environment. After the organism grown to its full size, the parent divides into two daughter cells which are genetically and morphologically similar. During this process, the nucleus divides into two, followed by the division of the cytoplasm.

According to the plane of division, following types of binary fission have been recognized in the organisms:

(a) Simple Binary Fission:

This type of binary fission occurs in the irregular-shaped organisms such as Amoeba in which the plane of division is difficult to ascertain (Fig. 3(A).1).

(b) Transverse Binary Fission:

If the plane of division is at right angle to the long axis of the animal, it is known as transverse binary fission as in Paramecium and Planaria

(c) Longitudinal Binary Fission:

In this type the plane of fission is parallel to the long axis, as in Euglena, Vorticella and in some corals

During binary fission the organelles of parent body either divide equally between two daughter individuals or one daughter individual retain them and other must develop new organelles.

2. Multiple Fission:

In multiple fissions the parent nucleus undergoes repeated divisions to form a large number of daughter nuclei. This is followed by the division of the cytoplasm into as many parts as there are nuclei, each part enclosing one nucleus. As a result a number of daughter cells are formed from a single parent cell at the same time. This process usually takes place under unfavorable environmental conditions. The multiple fissions occur in most algae, fungi and some protozoans, e.g., Amoeba, Plasmodium (malaria parasite) and Monocytes etc.

3. Gemmule in Sponges or Gemmulation:

Asexual reproduction occurs in sponges in various ways; the best known method is gemmulation. In freshwater sponges and a few marine sponges buds are formed within the parent body and are called gemmules. These are also called as endogenous buds or internal buds.

Gemmulation begins when a small group of cells (mostly archaeocytes) become ladden with reserve food granules and become isolated at the internal surface of a sponge. Bach one mass is covered over by a protective covering and is called as a geinmule. The gemmules are expelled from the adult sponge and this is a normal reproductive process in some marine sponges.

Sometimes, gemmule formation is a means to tide over unfavourable conditions. After degeneration of the parent sponge due to drought or temperature extremes, the gemmules are liberated and germinate to adult sponge.

The freshwater sponges under family spongillidae undergo a slight different form of gemmulation. Here the gemmules consist of mass of archaeocytes ladden with reserve food materials and, in addition, they are surrounded by protective membranes formed by the archaeocyte cells. The protective covering is generally reinforced by spicules, the skeletal materials of sponges. The gemmules of freshwater sponges allow a species to survive in unfavourable conditions. In cold regions, gemmulation occurs in winter and the inactive gemmules hibernate.

In warm regions, gemmulation occurs in summer and the gemmules are said to estivate. In next spring or autumn, as the case may be, when favourable conditions return, the gemmules germinate. Their archaeocytes emerge through an opening called micropyle. The various cellular types differentiate and a new sponge grows.

4. Budding in Hydra:

During the process of bud formation or budding an outgrowth or bud appears on the parent body.

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Answer 2

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