Asexual reproduction

Introduction to reproduction in organisms

"Reproduction is defined as the formation of offspring by the fusion of parental cells or gametes either by sexual contact or by asexual means".Asexual reproduction does not involve fertilization with the help of male and female sexual organs. Reproduction is the most essential process of life, without which organisms cannot exist. It is the law of nature to survive and develop the species.  Reproduction pattern, frequency, and mechanism varies between species, age groups, geographic areas, etc. Plant reproduction typically takes place by asexual mode but some of the plants adapt other types of reproduction. In most cases, animal reproduction is sexual in nature with few exceptions. Sexual reproduction allows the formation of new variants across generations depending upon the evolution and genetic processes. This brings a greater survival advantage for most of the species. In this chapter, we will be learning about the general principles underlying the reproductive processes of flowering plants and humans with some examples.

Table Of Contents

1. Introduction to reproduction in organisms

2. Types of reproduction

3. Asexual reproduction

    3.1 Advantages of Asexual Reproduction

    3.2 Disadvantages of Asexual Reproduction

    3.3 Differences between sexual and asexual                                                reproduction

     3.4 Types of Asexual Reproduction

     3.4. 1 Binary Fission

     3.4.2. Multiple fission

     3.4.3. Budding

     4.3.4. Spore formation

     3.4. 5. Regeneration

     3.4. 6. Fragmentation

 

Types of reproduction

 

 

Asexual reproduction

Asexual reproduction doesn’t involve the combination of 2 parents instead, an offspring arise out of a single organism from which the exact same genetic material is broken down and distributed into progeny. Asexual reproduction doesn't involve the fusion of gametes unlike in the case of sexual reproduction. It is the most primitive form of reproduction because the history of evolution suggests that the majority of microbes have practised this method since ancient times. Most of the prokaryotes ( unicellular) such as algae, fungi, viruses, and bacteria are reproduced by this model. Many plants and some of the animals do reproduce by asexual means. It is still not clearly understood why certain organisms follow asexual reproduction and a few follow sexual reproduction modes. Asexual reproduction has its own benefits but the major drawback here is that this method will only benefit the species by rapid multiplication with little to no stability of organisms.  On the contrary, sexual reproduction is the great driver of genetic diversity, which allows organisms to develop stability, longevity, variety, and uniqueness.  

 

Advantages of Asexual Reproduction

1. It is quick and rapid multiplication mode with the resulting progeny in huge numbers. On the contrary, there is the least chance of survival of the whole colony but a decent number of offspring can grow up. Microbes like bacteria use this mode of reproduction where they rebuild a population from a tiny piece of a cell.

2. These methods need neither the set of 2 genders nor a complete ideal situation for fertilization as in the case of sexual reproduction.

3. Artificial mode of reproduction is easy in the case of asexual reproduction when compared to sexual mode. For example, plants and sea creatures can be simply cut a part of themselves to replicate too many.

4. Asexual reproduction can transfer positive genetic influences through generations.​

 

Disadvantages of Asexual Reproduction

1. There is a lack of diversity in the progeny because there is only one kind of genetic material being shared with the next generation unlike in sexual reproduction where father and mother are equally sharing their traits. 

2. There is poor resistance and stability members because these organisms cannot adapt against the adverse effects posed from diseases, nutritional issues, environment, etc. One of the best examples is the Irish Potato Famine.

3. Some unicellular organisms have a great chance of a higher rate of errors while copying their genetic sequences.

4. Because of the nature of rapid multiplication of organisms by asexual means, population numbers can be difficult to control this is true especially in the case of infections caused by microbes.

5. Asexual organisms are inert and statics there are limited movement capabilities that make brings in a state where they can't escape from worst situations of environment and predators like other animals.

 

Differences between sexual and asexual reproduction

CRITERIA

ASEXUAL REPRODUCTION

SEXUAL REPRODUCTION

Parent cell involvement

Happens without any fusion between the father and the mother.

There will be a fusion between the 2 matured, germ cells derived from the 2 opposite genders.

Types

Vegetation, binary fission, budding, fragmentation and spore formation.

Syngamy and conjugation.

Number of organisms or parents involved

One parent (uniparental).

One male and one female (biparental)

Cell division type

Mitosis only

Both mitosis and meiosis.

Involvement of sex cells

Somatic or peripheral body cells of parents.

Germ cells or gametes of parents.

Class of species involved

Ii occurs in lower invertebrates, chordates, and plants having a simple organizational structure.

Occur in higher plants and all animals.

Fertilization

No fertilization.

Fertilization takes place.

Time is taken to complete

Multiplication is very rapid and in lesser time.

Multiplication is not rapid, instead, it is a  very long-term process.

Offspring

The number of offspring may vary from two to many.

Some of the offsprings produced are less in number comparatively.

The offspring are genetically similar.

The offspring produced are genetically different from their parents.

 

 

Types of asexual reproduction in plants

There are various types of asexual reproduction in plants namely, Fission, Budding, Vegetative propagation, Spore formation, Fragmentation, Layering, multiple fission and also the artificial methods like grafting. Few important modes of sexual reproduction will be discussed in this part. In asexual reproduction, some of the cells of the parent plant undergo mitotic cell division that helps to generate two or more new organisms. The offspring produced with such a process will be not only identical to one another but they are almost the copies of their parent. This genetic and morphological similarity between the parents and the offspring produced during asexual reproduction is explained by a concept called a clone. A clone is an identical unit of fused products forming similar characteristics like parent. Asexual reproduction is a very common reproductive method in plants and also in some single-celled organisms, where the tissues and cellular organization is simple and limited. If we take an example, in Protists and Monerans, the organism or the parent cell divides into two to give rise to new individuals as shown in the image below.

 

1. Binary Fission

In binary fission, the organisms undergo a cell division within themselves with no participation of 2 genders. A binary Fission is when the parent cell divides into two pieces by forming a cleft in the middle of the cell. It is possible only if the parent body has reached a complete maturity point. The parent cell undergoes self-destruction after this process, leaving behind two new organisms. Members of the Kingdom Fungi and simple plants such as algae reproduce through special asexual reproductive structures as shown below. 

Binary Fission of amoeba-Souce NCERT

 

2. Multiple fission

Multiple fission is also a process of asexual reproduction in which parent cell splits to form many new organisms. This happens when the cyst is formed around a unicellular organism. Inside this cyst, the nucleus of an organism breaks in many smaller nuclei. When the favourable conditions come, the cyst breaks and many daughter cells inside it are released. Plasmodium undergoes the process of multiple fission.

Multiple Fission -Image from NCERT

 

3. Budding

A bud refers to a small outgrowth budding is a process where a tiny bud grows on the body of parent organism and it stays there for some time until the bud reaches a maturity level and slowly gets detached to grow itself into a new organism., Such reproduction is common in hydra and yeast.

Budding of hydra

 

 

4. Spore formation

In this type, the parent plant produces multiple reproductive units called spores. The spores grow inside a case called spore case, eventually the spore case of the plant bursts, and these spores travel by the wind force to land on food or soil to further germinate and produce new plants. Fungi like Rhizopus, Mucor, etc., are examples of spore formation.

  A Rhizopus fungus forming spores

 

 

5. Regeneration

It’s an asexual method of reproduction, where the body of a parent organism gets cut and the small pieces will regrow into new organisms by cellular growth and multiplication, It occurs in both plants and animals. Hydra and planaria undergo regeneration.

Regeneration in Planaria

 

 

6. Fragmentation

It is unique to multicellular organisms in both plants and animals. As the term it indicates the organism breaks into two or more pieces on maturation, it is slightly different than the regeneration because, in the case of regeneration a part of the body is cut into growing a new body but in the case of fragmentation, the whole body is fragmented into pieces to form equal pieces which will eventually grow. Spirogyra is an example of this.

 Spirogyra, undergoing fragmentation

 

 

7. Vegetative Propagation

It’s an asexual reproduction seen exclusively in plants. In this case, parts of the old plants like stems, roots, and leaves will produce some dormant buds under good conditions like moisture and warmth so that they grow and develop to form a new plant. Green grass, Bryophyllum, money plant, potato plant, onion, will use this method.

 

Potato undergoing vegetative propagation( shutter stock)

In addition to these natural methods, some artificial methods like Cutting, layering, and grafting are also useful but these are for commercial purposes and induced artificially by the plant nurseries. All organisms have to reach a certain stage of growth and maturity in their life before they can reproduce sexually.



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