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Asexual reproduction is the production of new individuals by mitotic divisions from a single parent. It does not involve the fusion of gametes, also called sex cells. There are many types of asexual reproduction, all producing individuals that are genetically identical to the parent.
Fission
Fission occurs in lower plants and animals such as the bacteria, blue-green algae and protozoa. In this process, the cell divides after the genetic material has divided. If the cell divides into two it is called binary fission. The DNA or the nucleus of a mature cell divides first and then the cell divides into two daughter cells of almost the same size. It is seen in bacteria and protozoans like amoeba and paramoecium.
If the parent cell divides into many daughter cells, it is called multiple fission. It is seen in the life cycle of the protozoa, plasmodium (the malarial parasite). The nucleus divides many times and then the cytoplasm divides and surrounds the nuclei.
The parent cell may get covered by a protective coat called the cyst which bursts to release the daughter cells during favourable condition. Multiple fission is also seen in Chlamydomonas, an alga.
Budding
It is seen in certain fungi and multicellular animals. In budding, the parent cell or body gives out a lateral outgrowth called the bud. The nucleus divides and one of the daughter nuclei passes into the daughter cell. The bud grows in size while being attached to the parent body. It then gets separated from the parent by the formation of a wall. It then falls off and germinates into a new individual. Thus budding results in the formation of daughter cells of unequal sizes that later grow to adult size. For example, yeast, a fungus and Hydra, a multicellular animal. In case of hydra, the daughter hydra even develops hypostome and tentacles develop around the hypostome before being detached from the parent body.
Spore Formation
It is generally seen in bacteria and most fungi. One of the cells enlarges and forms the sporangium (literally meaning spore sac). The nucleus divides many times and then the daughter nuclei are surrounded with protoplasm bits to form daughter cells called spores. The spores are covered with a thick wall called the cyst. On maturation, the sporangium bursts and releases the spores. The spores germinate on getting favourable conditions. For example, fungi such as Mucor and Rhizopus
Spore formation also occurs in bacteria such as Clostridium and Bacillus. The bacterial spores are also called endospores. They are thick-walled and lightweight.
Fragmentation
It takes place in some lower plants and animals such as some worms. The mature organism breaks up into two or more pieces or fragments. The fragments then grow into complete organisms.
For example: Spirogyra, an alga.
Worms such as ribbon-worms (Nemertinea) and flatworms (Turbellaria).
Fragmentation takes places in hydra, planaria and sponges. The fragments regenerate to form an entire individual.
Regeneration is one of the methods of asexual reproduction in lower or less evolved animals. When the parent body breaks into pieces, each piece can grow into a new organism.
Fascinating Ability of Regeneration in Planaria
- Planaria have a strong sense of head and tail.
- When cut, the anterior cut surface almost always regenerates a new head and the posterior cut surface regenerates tail structures.
- Planaria is a little half-inch flatworm.
- It often lives under rocks.
- It can be cut into, as many as 32 pieces and each fragment is able to rebuild a miniature flatworm complete with head, tail, eyes, mouth and internal organs!
- In contrast to higher vertebrates, free-living flatworms like planaria show a fascinating ability of regeneration.
- Cut off its head and it grows a new one.
- Divide its head laterally into two, three or more parts and a two, three or multi-headed worm will result.
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