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Asexual Reproduction in Organisms

Asexual Reproduction in Organisms Synopsis

Synopsis

 

Reproduction

  • The period from birth to the natural death of an organism is called its lifespan.
  • Reproduction is defined as a biological process in which an organism gives rise to an offspring similar to itself.

 

Events in Reproduction

 

 

Features of Reproduction

  • Replication of DNA
  • Cell division 
  • Formation of reproductive bodies or units 
  • Development of reproductive bodies into offspring

 

Significance of Reproduction

  • Reproduction ensures continuity of life on the Earth, while sexual reproduction provides great scope for variation.
  • Variation is important for survival of species.
  • Variation helps species to adapt to different environmental changes.

 

Types of Reproduction

 

 

Asexual Reproduction 

 

The process by which offspring is produced by a single parent with or without the formation of gametes is called asexual reproduction.

 

Characteristics of Asexual Reproduction

  • A single parent is involved.
  • Gametes are not formed.
  • Involves only mitotic cell division.
  • Offspring are genetically identical to the parent. They are called clones.
  • No mixing of genetic material, hence does not promote variation and evolution.

 

Sexual Reproduction 

Sexual reproduction is the development of new individuals through the formation and fusion of gametes produced either by the same individual or by different individuals of the opposite sex.

 

Characteristics of Sexual Reproduction

  • Mostly biparental
  • Gamete formation takes place
  • Involves fertilisation
  • Involves both meiosis and syngamy
  • Daughter individuals are always different from parents

 

Modes of Asexual Reproduction

 

Fission

  • Fission is the division of the parent body into two or more daughter individuals identical to the parent organism. 
  • It can take place through binary fission, multiple fission and plasmotomy.

 

1. Binary Fission

  • In binary fission, the parent organism divides mitotically into two halves, each half forming an independent daughter individual.
  • The offspring produced are genetically identical to the parent and one another. 
  • Depending on the plane of division, binary fission is of the following types:
    1. Simple Binary Fission: Division occurs through any plane, e.g., Amoeba.
    2. Longitudinal Binary Fission: The plane of division passes along the longitudinal axis of the organism, e.g., Euglena.
    3. Transverse Binary Fission: The plane of division runs along the transverse axis of the organism, e.g., Paramoecium, Planaria.
 
 
 
 
2. Multiple Fission
  • In multiple fission, the parent body divides into many similar daughter individuals.
  • It produces a number of daughter cells from a single parent cell at the same time. 
  • Multiple fission occurs in protozoa such as Amoeba (sporulation) and malarial parasite such as Plasmodium (schizogony).

 

3. Plasmotomy

  • Plasmotomy involves division of a multinucleate parent into several multinucleate daughter individuals without the division of nuclei. 
  • The division of the nuclei occurs later on to maintain the number of nuclei constant. 
  • Plasmotomy occurs in protozoans such as Opalina and Pelomyxa.

 

Budding

  • In budding, one or more small protuberances called buds develop from a single parent body.
  • Each bud then separates from the parental body and assumes an independent existence.
  • Budding is seen in sponges such as Scypha, coelenterates such as Hydra, annelids such as Syllis and tunicates such as Salpa.
  • Budding in animals is of the following types:
    1. Exogenous/External Budding: In exogenous budding, the bud grows externally on the surface of the body, e.g., Sycon, Hydra.
    2. Endogenous/Internal Budding: In endogenous budding, buds are formed within the parent’s body, e.g., Spongilla and few marine sponges.
    3. Strobilation: The process of repeated formation of similar segments by the process of budding is called strobilation, e.g., Aurelia, in the neck of Taenia

 
Fragmentation
  • In fragmentation, the parent body breaks up into several parts called fragments. 
  • Each fragment then develops into a complete organism.
  • Fragmentation is observed in sponges, coelenterates such as sea anemones, echinoderms, algae such as Spirogyra, fungi such as Rhizopus, bryophytes such as Riccia, pteridophytes such as Selaginella etc.

 

Regeneration

  • Regeneration is the ability of organisms to generate lost or damaged parts.
  • In some animals, the entire organism is regenerated, while in others only a part of the organism is regenerated.  
  • Regeneration is of the following types:
    1. Morphallaxis: In morphallaxis, the entire body grows from a small fragment, e.g., Sponges, Hydra, Planaria etc.
    2. Epimorphosis: Epimorphosis is the replacement of lost body parts. It is of the following types:
      1. Reparative Regeneration: Only some damaged tissues can be regenerated
      2. Restorative Regeneration: Several body parts can be regenerated.

 
Gemmae Formation
  • Gemmae are propagules that detach from the parent body and grow into new individuals.
  • Reproduction through gemmae is observed in Marchantia.

 
Sporulation
  • During sporulation, the parent plant or sporophyte produces hundreds of reproductive units called spores inside the sporangia. 
  • When the spore case ruptures or bursts, the spores spread into the air.
  • When the air-borne spores land on food, soil or any suitable substratum under favourable conditions, they germinate and give rise to new organisms with many vegetative hyphae.
  • Sporulation is observed in Rhizopus, Mucor, Aspergillus.

 
Vegetative Propagation
  • In vegetative reproduction, new plants are produced from the vegetative parts of plants.
  • The newly formed individuals are genetically identical to the parent plant.
  • Vegetative reproduction is a common method of reproduction in flowering plants.
  • There are two types of vegetative reproduction:
    1. Natural Vegetative Reproduction
    2. Artificial Vegetative Reproduction
 
Natural Methods of Vegetative Reproduction
  • In natural vegetative propagation, a somatic part of the plant detaches from the body of the mother plant and develops into a new independent plant under suitable environmental conditions.
  • The detachable somatic part that functions in vegetative propagation is called vegetative propagule.

 
Propagation by Roots
  • Tap roots, fleshy roots as well as adventitious roots take part in vegetative propagation.
  • Tap roots of some plants develop adventitious buds to form new plants 
  • Propagation by roots takes place in Guava, Poplar, Sweet potato, Dahlia, Asparagus.

 
Propagation by Underground Stems
  • Different types of underground stem structures can take part in vegetative propagation.
    1. Tubers: E.g., Artichoke, Potato.
    2. Bulbs: E.g., Garlic, Onion.
    3. Corms: E.g., Colocasia, Crocus.
    4. Rhizomes: E.g., Ginger, Turmeric.
    5. Suckers: E.g., Mint, Chrysanthemum.





Propagation by Subaerial/Creeping Stems
  • Creeping stems are of three types-runners, stolons and offsets.
    1. Runners: E.g., Cynodon, Oxalis.
    2. Stolons: E.g., Strawberry, Vallisneria.
    3. Offsets: E.g., Eicchornia, Pistia.

 
Propagation by Aerial Stems
  • Each segment of fleshy phylloclades can give rise to a new plant.
  • Propagation by aerial stems takes place in Opuntia, Sugarcane etc.

 
Propagation by Leaves
  • Adventitious buds present on the leaves of several plants help in vegetative propagation.
  • Propagation by leaves takes place in Begonia, Bryophyllum.

 

Propagation by Bulbils

  • Bulbils are multicellular fleshy buds that give rise to new plants.
  • Propagation by bulbils takes place in Oxalis, Agave, Pineapple etc.

 

Propagation by Turions

  • Turions are fleshy buds in aquatic plants that help in perennation.
  • They get detached from the parent plant and remain inactive through the winter and give rise to a new plant in the following spring.
  • Propagation by turions takes place in Utricularia, Potamogeton etc.

 

Artificial Methods of Vegetative Reproduction

  • In artificial method of vegetative propagation, a part of the somatic body of a plant is made to develop into new independent plants.
  • Artificial methods help to propagate plants of desired varieties.

 
 
Cuttings
  • Cuttings are cut pieces of root, stem and leaves which are planted in nurseries.
  • Cutting is removing a portion of the stem, leaf or root and fixing it into the soil to allow the growth of roots and shoots.
  • Root promoting chemicals like Indole-butyric acid (IBA) and Naphthalene acetic acid (NAA) are used for propagation through cuttings.
  • Propagation through cuttings takes place in the following ways:
    1. Root Cuttings: E.g., Lemon, Orange.
    2. Stem Cuttings: E.g., Rose, Sugarcane.
    3. Leaf cuttings: E.g., Snake plant, Saintpaulia.

 
 
Layering
  • Layering is a method in which adventitious roots are induced to develop on a soft stem while it is still attached to the plant. 
  • In layering, a branch of the plant is covered with some material and supplied with water to produce roots.

 
  • Propagation through layering takes place in the following ways:
    1. Tip Layering: E.g., Blackberry, Raspberry.
    2. Trench Layering: E.g., Walnut, Mulberry.
    3. Serpentine Layering: E.g., Clematis.
    4. Mound Layering: E.g., Apple, Pear.
    5. Air Layering: E.g., Litchi, Pomegranate.
 
Grafting
  • Grafting is a technique of joining two parts, usually a root system and a shoot system of two different plants in such a way that they unite and later develop as a composite plant.

 
  • Propagation through grafting occurs in Mango, Apple, Pear, Peach, Pine etc.
  • Grafting can be carried out in the following ways:
    1. Tongue Grafting
    2. Crown Grafting
    3. Wedge Grafting
    4. Side Grafting
    5. Approach Grafting
    6. Bud Grafting
 
Micropropagation
  • Micropropagation is the technique of production of new plants from cells or tiny pieces of plant tissues that are removed from the growing tips of a plant and put into a suitable growth medium called the culture solution to produce callus, which gets differentiated into a plantlet.
  • Micropropagation is used for the commercial production of Orchids, Carnation, Gladiolus, Chrysanthemum and other ornamental plants.

Advantages and Disadvantages of Vegetative Propagation
 
 
 
Advantages and Disadvantages of Asexual Reproduction
 
 
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