Asexual Reproduction in Animals Text

Definition of Asexual Reproduction

 "Asexual reproduction is the mode of reproduction involved in the production of offspring by a single parent."

What is Asexual Reproduction?

  • Asexual reproduction is a method of reproduction that does not include gamete fusion or chromosomal number changes. Offspring’s produce through this method are physically and genetically identical clones of their parents.
  • This mode of reproduction can be observed in both multicellular and unicellular organisms.
  • Due to no gamete fusion or chromosomal number changes, It will inherit the same genes as the parent, with the exception of few circumstances where a rare mutation may develop.

Characteristics of Asexual Reproduction

  • In Asexual reproduction only single parent is involved.
  • Asexual reproduction occurs in a single individual without production or fusion of gametes.
  • Asexual reproduction is a method of reproduction which produces genetically identical clones.
  • Higher animals do not naturally reproduce asexually.
  • It involves mitotic division.
  • Asexual reproduction takes place by several methods like fission, spore formation, budding (gemmation), fragmentation etc.
  • Most common forms of asexual reproduction in animals are fission and budding.
  • The body of a single organism divides into more than one new organism by a process called fission.
  • Fission may be binary or multiple.
  • Binary Fission occurs in Amoeba, Paramecium and Planaria etc.
  • Binary Fission may be longitudinal (Euglena) or transverse (Paramecium and Planaria).
  • During multiple fission, the parental body divides into many daughter organisms (example- Malaria Parasite).
  • A spore is a small reproductive body which is microscopic and unicellular containing a small amount of cytoplasm and a nucleus. Spores are produced by bacteria and protozoa.
  • Hydra may reproduce vegetatively by exogenous budding.
  • Fragmentation is breaking of an organism into two or more parts, each of which grows to form a new individual, example-filamentous algae Spirogyra.
  • For instance, most plants are capable of vegetative reproduction—reproduction without seeds or spores—but can also reproduce sexually.
  • Likewise, bacteria may exchange genetic information by conjugation.
  • Other ways of asexual reproduction include parthogenesis, fragmentation and spore formation that involves only mitosis.

Modes of Asexual Reproduction

  • Cell division is the mode of asexual reproduction in single-celled organisms.
  • In Monerans and Protists, the single cell divides into two daughter cells/individuals.
  • In yeast, the cell division results in a large cell and small cell, called bud, attached to the large cell the bud gets separated and grows into an adult.
  • In Fungi and Algae specialised asexual reproductive units are formed, they are as follows-
  • Zoospores in many Algae and certain aquatic Fungi.
  • Conidia in Penicillium, Aspergillus, etc.
  • Hydra reproduces asexually by budding.
  • In higher plants vegetative reproduction is common; the vegetative propagules are-
  • Runner-Oxalis
  • Rhizome-Ginger
  • ‘Eyes’ on tuber- Potato
  • Bulbil-Agave
  • Offset-Water Hyacinth and Pistia
  • Leaf buds- Bryophyllum
  • Bulbs-Onion, Garlic

Fission

The process of division of one cell/organism into daughter cells/ individuals, is called fission.

Fission is of two types-

  • Binary Fission          
  • Multiple Fission

Binary Fission

In this method, the parent cell divides into two halves and each half grows into an adult. Example- Amoeba, Paramecium.

 

Multiple Fission

  • In this method, the parent cell divides into many daughter cells, each of which grows into an adult. Example-Plasmodium.
  • In yeast, the cell division results in a large cell and small cell, called bud, attached to the large cell the bud gets separated and grows into an adult.
  • In Fungi and Algae specialised asexual reproductive units are formed, they are as follows-
    • Zoospores in many Algae and certain aquatic Fungi.
    • Conidia in Penicillium, Aspergillus, etc.

Budding

  • In this type of reproduction, a bulb-like projection or outgrowth arises from the parent body known as bud, which detaches and forms a new organism. For example, Hydra reproduces by budding. A small protuberance arises from one side of its body, which grows, develops tentacles and gets detached to lead an independent life.

Regeneration or Fragmentation

  • In this type of reproduction, the body of an individual breaks up into two or more parts and each part develops into a complete individual.
  • Examples-Spirogyra, and Planaria.

     

 

Spore formation

  • In lower forms of life like the alga, Chlamydomonas, the protoplast of the cell divides to form 4–8 spores.
  • These being motile are termed as zoospores.
  • When spores are released in the surrounding medium they develop into new plants.

Vegetative propagation or vegetative reproduction in plants

  • Vegetative reproduction (or vegetative propagation) is a form of asexual reproduction in plants in which a bud grows and develops into a new plant.
  • In this type of reproduction, any vegetative part of the plant body like leaf, stem or root develops into a complete new plant.
  • Vegetative reproduction can take place by two methods—natural and artificial.

Vegetative reproduction by natural methods

  • This type of vegetative reproduction can involve roots, stem or leaves.
  • Some common modes of vegetative reproduction are given below-

 By roots

  • The roots of sweet potato and mint bear adventitious buds.
  • When these roots are planted in the soil, new plants are produced.

 By stem

  • In many plants the stem develops buds on it.
  • The part of the stem that bears buds serves as an organ for vegetative multiplication, Example-the modified parts of stem, such as runners of grass, suckers of mint and Chrysanthemum, bulbs of onion and tulip, rhizomes of ginger, corms of gladiolus and Colocasia, and tubers of potato, etc.

 

 

 

By leaves

  • In some plants, Example- in Bryophyllum and Begonia, adventitious buds are developed in the margins of their leaves.
  • When the leaf falls on moist soil, these buds develop into small plantlets, which can be separated and grown into independent plants.

Vegetative propagation by artificial methods

  • Some plants can be propagated artificially.
  • The methods of artificial propagation include grafting, layering, cutting and tissue culture.

Grafting

  • It is the method of obtaining a superior quality plant from two different plants, taking the root system of one plant and the shoot system of another plant.
  • The plant whose root system is taken is called stock.
  • The plant whose shoot system is taken is called scion.
  • The ends to be grafted, of the stock and the scion, are cut obliquely and placed face to face and are bound firmly with tape.
  • The stock supplies all the desired nutrients to the scion.
  • This technique has been used in raising superior quality plants of mango, apples, roses, rubber and citrus.

 

Cutting

  • In some plants like rose, sugarcane, Bougainvillea, etc. this method is used quite frequently.
  • Stem cuttings with nodes and internodes are placed in moist soil which give rise to adventitious roots, and grow into new plants.

Layering

  • Layering is the development of roots on a stem while it is still attached to the parent plant.
  • The stem or the branch that develops adventitious roots while still attached to the parent plant is called a layer.
  • It is a means of reproduction in black raspberries, jasmine (Jasminium), Magnolia, etc.

Tissue culture

  • This is a modern technique of vegetative propagation. In this technique, a small piece of tissue is cut from a plant and is transferred to a container with nutrient medium under aseptic conditions.
  • The tissue utilizes nutrients from the medium, divides and re-divides, and forms a callus.
  • Small portions of this callus are transferred to another medium which induces differentiation and plantlets are produced.
  • These plantlets are transplanted in soil to form an adult plant.
  • Orchids, Chrysanthemum, Asparagus and many other plants are now being grown by using plant tissue culture technique.

Advantages of Asexual Reproduction

  • Only one parent is required
  • It is more time and energy efficient because it does not require a mate
  • The reproduction procedure is quick.
  • In a very short period of time, a tremendous number of offspring’s can be generated.
  • Positive genetic impacts are passed down through generations.
  • It occurs in a variety of surroundings.

Disadvantages of Asexual reproduction

  • It does not result in genetic variation in a population because the offspring are genetically identical to their parents; they are more vulnerable to the same diseases and nutrient deficits. All of the unfavourable mutations are passed down over generations.
  • The species may be limited to a single environment and change in this environmental will be enough to wipe out the entire species.
  •  Since there is just one creature involved, the diversity of the organisms is limited.
  •  They are unable to adjust to changing conditions.

Asexual Reproduction Examples

 

  • In Monerans and Protists, the single cell divides into two daughter cells/individuals.
  • Hydra reproduces asexually by budding.
  • Binary Fission occurs in Amoeba.
  • Spirogyra and Planaria reproduce by regeneration or fragmentation.