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Asexual Reproduction

Reproduction is a characteristic of a living organism. In multicellular organisms, reproduction is the production of progeny which possess features which are similar to those of parents. This is known as sexual reproduction and involves the fusion of male and female gametes to form a progeny.

Asexual reproduction is also a means of reproduction in some organisms. Fungi spread easily by the production of asexual spores. In organisms like yeast and hydra budding is the way of reproduction. In flat worms like
Planarians reproduction is by fragmentation. Unicellular organisms like bacteria and unicellular algae reproduce by increasing the number of cells by cell division. 

Most of the prokaryotes reproduce asexually without the formation of gametes, while processes like conjugation, transformation and transduction are regarded as sexual reproduction. In multicellular organisms, asexual reproduction is very rare. Asexual reproduction may have short term benefits with rapid growth in population a stable environment while sexual reproduction allows more rapid genetic diversity in generations and also allows adaptation to changing environments.  

 

Asexual Reproduction Definition

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Asexual reproduction is a form of reproduction by which a single parent produces a progeny and the genes inherited are from that parent only. The offspring here are usually produced by mitosis. In this type of reproduction, the process of meiosis, reduction of chromosome number and fertilization does not takes place. The offspring of asexual reproduction will have exact copies of the genes of the parent. Asexual reproduction is also referred to as agamogenesis. Agamogenesis is the type of reproduction which does not involve the fusion of gametes. This type of reproduction is the primary form of reproduction for single-celled organisms like archarea, bacteria and protists. Plant and fungi also show asexual reproduction. 

Characteristics of Asexual Reproduction

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Characteristics of asexual reproduction:
  • Asexual reproduction involves only one parent.
  • All the offspring are identical to the parent. 
  • Asexual reproduction does not involve the process of meiosis and fertilization, hence the process does not require a mate for reproduction. 
  • This type of reproduction is seen in lower forms of organisms. 
  • Asexual reproduction does not require time and energy to be spent on seeking a receptive mate. 
  • It is efficient method as large number of offspring are produced quickly, this enable animals to take advantage of favorable environmental conditions. 
  • Asexual reproduction is sometimes known as cloning.
  • There are many types of asexual reproduction like vegetative propagation, fission, budding and fragmentation. 
  • All the forms of sexual reproduction are the variations of process of mitosis. 

Types of Asexual Reproduction

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The types of asexual reproduction are of several types like fission, budding, fragmentation and vegetative propagation.

Fission

  • Fission is a type of asexual reproduction where there is splitting of an individual to form two individuals this is known as binary fission or more may also produce more than two individuals known as multiple fission, all the progeny are approximately of the same size. 
  • This is a common form of reproduction in single-celled organisms and bacteria. 
  • Eukaryotic organisms like some protists and unicellular fungi also divide by fission. These organisms divide through binary fission and most of them are capable of sexual reproduction. 
  • Multiple fission occurs in protists like the sporozoans and algae. 
  • The parent organism nucleus divides many times by the process of mitosis producing several nuclei. The cytoplasm is then separated creating multiple daughter cells.
Illustration of Binary Fission

Budding 

  • In this mode of asexual reproduction, offspring buds off from the body of the parent cell. 
  • The genetic material of the parent cell is divided equally but there is unequal division of the cytoplasm. 
  • Hydra reproduces by budding. 
  • The buds formed on the parent body of the hydra matures and eventually breaks away from the parent organism.  
Budding in Hydra

Vegetative Propagation

  • It is a type of asexual reproduction and is seen in plants where progeny are formed without the production of seeds or spores by the process of meiosis or syngamy.
  • In this type of reproduction plants grow from vegetative parts of the plant like the roots, stems and leaves.
Vegetative Propagation

Spore formation 

  • During the biological life of many multicellular organisms spores are formed and this process is known as sporogenesis. 
  • These spores formed during sporic meiosis in some plant and algae grow into multicellular individuals without the event of fertilization. 
  • These haploid spores give rise to gametes through the process of mitosis. 
  • This process of spore formation and gamete formation occurs in separate generations of life cycle and is known as alteration of generation. 
Pictorial Representation of Spore Formation

Fragmentation 

  • Fragmentation is a type of asexual reproduction where a new organism grows from a fragment of the parent organism. 
  • The body of the parent breaks into distinct pieces and these fragments grow into a full and mature individual. 
  • This type of asexual reproduction is seen in many organisms like annelid worms, sea stars, and some fungi and plants. 
Fragmentation in Spirogyra

Gemmules (Internal Buds)

  • This form of asexual reproduction is seen in sponges where a parent organism releases a specialized mass of cells that develops into offspring. 
Gemmule

Regeneration 

  • In the regeneration type of asexual reproduction, if a piece of parent organism breaks into distinct pieces, each piece develops and grows into a completely new individual. Example: Planarians.
Regeneration in Planaria

Agamogenesis 

  • Agamogenesis is a type of reproduction in which the reproduction does not involve a male gamete. Types of agamogenesis are parthenogenesis and apomixis. 
  • Parthenogenesis is a form of reproduction in which a egg that is unfertilized develops into a new individual. 
  • This type of agamogenesis occurs naturally in many plants and invertebrates like water feas, aphids, stick insects, etc. and in vertebrate like some reptiles, amphibians and rarely in birds. 
  • Apomixis is the formation of new sporophyte without the process of fertilization. It is usually seen in ferns and flowering plants and is a rare process in seed plants. 

Advantages of Asexual Reproduction

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Advantages of Asexual reproduction are as follows:
  • This type of reproduction enable organisms to reproduce without a mate. 
  • It does not require the time and energy that takes to search a mate. 
  • It results in the reproduction of large number of offspring rapidly. 
  • Like in plants, it enables to spread and colonize an area in short period of time. 
  • Animals that are confined to one particular place and unable to look for a mate reproduce asexually. 
  • Stable environments with very little change are favorable for organisms to reproduce asexually. 

Disadvantages of Sexual Reproduction

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Disadvantages of Asexual reproduction are the following:
  • In this type of reproduction the offspring produced are genetically identical to each other and to the parent. 
  • This causes no or very little genetic variation within a population. 
  • Any mutation in the parent cell, can cause harmful effects on the survival ability of the offspring. 
  • If there is harmful mutation in the organisms, environment changes could be deadly to all the individuals. 

Asexual Reproduction in Animals

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Asexual reproduction is less common in animals. It is often seen in simpler animals like hydra. Animals reproduce asexually by many different methods. The methods are budding, parthenogenesis, gemmule formation and polyembryony. 
  • Budding is the process where the parent cell buds off forming daughter organisms. 
  • In parthenogenesis eggs are produced by the females develop into adult organisms without undergoing the process of fertilization. 
  • Gemmules are internal buds, gemmules are cell masses that are released from their body and these cells develop into independent offspring. 
  • Polyembryony is a condition in which a single egg leads to the development of two or more embryos.

Asexual Reproduction in Plants

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Plants show two main types of asexual reproduction, in which new plants are produced that are genetically identical to the parent individual. Plants reproduce asexually by vegetative reproduction and apomixis.
  • In vegetative reproduction a vegetative part of the original plant like the roots, stems and leaves and the new plants grows from these parts of the plant. 
  • Apomixis in plants are similar to parthenogenesis in animals. The seeds produced by apomixis are involved in the formation and dispersal of sedds that are not originated from the fertilization of the embryos.  
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