Here is an essay on ‘Somatic Breeding’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Somatic Hybridizations’ especially written for school and college students.
Essay on Somatic Hybridization
Essay Contents:
- Essay on the Meaning of Somatic Hybridization
- Essay on the Features of Somatic Hybridization
- Essay on the Types of Somatic Hybridization
- Essay on the Steps of Somatic Hybridization
- Essay on the Uses of Somatic Hybridization
- Essay on the Achievements of Somatic Hybridization
- Essay on the Limitations of Somatic Hybridization
Essay # 1. Meaning of Somatic Hybridization:
Somatic hybridization refers to crossing of crop plants through fusion of somatic cells. In this method somatic cells are used for hybridization and the sexual process is bypassed. The fusion of somatic cells takes place through protoplasts. Protoplasts are nothing but naked cells or cells without cell wall.
Somatic hybridization permits hybridization between any two plant species irrespective of their taxonomic relationship. In other words, it breaks barriers of cross incompatibility among plant species and makes incompatible crosses possible. In future it is expected to develop into a powerful tool for practical plant breeding.
Essay # 2. Features of Somatic Hybridization:
Main features of somatic hybridization are listed below:
1. Hybridization involves somatic cells rather than sexual gametes. Thus sexual process is bypassed.
2. This method involves tissue culture which is essential for regeneration of somatic hybrids into whole plants.
3. For regeneration of protoplasts specific nutrient medium is required which differs from species to species.
4. In this method, there is equal contribution of cytoplasm from both parents, whereas in sexual hybridization cytoplasm is strictly contributed by maternal (female) parent.
5. It overcomes the barriers of cross incompatibility and permits hybridization between any two plant species whether they are related or unrelated.
6. Hybridization between two diploid species leads to production of allotetraploid hybrid, whereas sexual fusion between two diploid species leads to production of diploid hybrid. Thus, somatic hybridization differs from sexual hybridization in several aspects.
Essay # 3. Types of Somatic Hybridization:
Somatic hybrids can be obtained between sexually compatible as well as incompatible species. Somatic hybrids can be classified on the basis of taxonomic relationship and chromosome complements received from the parental species.
Based on taxonomic relationship of species involved in the hybridization, somatic hybrids are of three types, viz.:
i. Interspecific Hybrids:
Hybrids between two different species of the same genus are referred to as interspecific hybrids. Such hybrids have been obtained in tobacco, potato, carrot, Datura, Petunia, etc. Somatic hybrids have been obtained in sexually compatible as well as incompatible species of tobacco and Petunia and between incompatible species of Datura.
ii. Intergeneric Hybrids:
Hybrids between two different genera of the same family are known as intergeneric hybrids. Somatic intergeneric hybrids have been obtained between tobacco + tomato, Potato + tomato, Datura + Atropa belladona, Arabidopsis + Brassica and several other plant species.
iii. Intertribal Hybrids:
Hybrids between plants of two different tribes are called intertribal hybrids. Such hybrids can only be produced via, somatic hybridization. Such crosses exhibit more asymmetric hybrids than interspecific and intergeneric hybrids.
Based on combination of chromosomes and cytoplasm from parental species, somatic hybrids are of three types, viz.:
(i) Symmetrical hybrids,
(ii) Asymmetrical hybrids, and
(iii) Cybrids.
These are defined below:
(i) Symmetrical Hybrids:
Those somatic hybrid cells or plants that contain all chromosomes of both the species involved in the fusion are referred to as symmetrical hybrids. Such hybrid have been obtained in tobacco, carrot, Petunia, etc.
(ii) Asymmetrical Hybrids:
Those somatic hybrid cells or plants that contain complete somatic complement of one species and only a part of somatic complement of another species are called asymmetrical hybrids. Such hybrids have been obtained in tobacco, potato, Petunia and several other intergeneric and intertribal hybrids.
(iii) Cybrids:
Those somatic hybrids that involve normal protoplast of one species and nucleusless protoplast (cytoplast) of another species are known as cybrids. Such hybrids are useful in transfer of CMS from one species to other.
Essay # 4. Steps of Somatic Hybridization:
Somatic hybridization consists of four important steps, viz.:
i. Protoplast Isolation:
Protoplasts are isolated mainly by two methods, viz. mechanical and enzymatic. Mechanical method is rarely used, because by this method protoplasts are obtained in small quantity and cells are broken. However, this method eliminates adverse effects of enzymes on the protoplasts.
The enzymatic method is widely used and has two advantages over mechanical method. Firstly, protoplasts are obtained in large quantity. Secondly, the cells are not broken. Sometimes, both methods can be combined to retain merits of both techniques. In combined method, first cells are separated mechanically and then cell wall is dissolved enzymatically.
Removal of cell wall makes possible fusion of the protoplast of diverse origin and also uptake of foreign DNA. Protoplasts can be isolated from any part of the plant, viz., roots, leaves, pollen grains, pollen mother cell, callus cultures, etc. Generally, protoplasts are isolated either from mesophyll tissues or from callus cell suspense’s.
The isolation of protoplasts from mesophyll tissues involves four main steps, viz.:
(i) Sterilization of leaves,
(ii) Peeling off the epidermis,
(iii) Enzymatic treatment, and
(iv) Isolation and cleaning of protoplasts.
ii. Protoplast Fusion:
Two things are essential for protoplast fusion technology; protoplasts must be isolated in large quantities and isolated protoplasts should have the ability to regenerate into new plants.
Crop plants in which protoplasts can be isolated from mesophyll cells and callus culture are given below:
i. Mesophyll Cells:
Tobacco, Datura, Cucumber, Rapeseed, Petunia, Field pea, French bean, Cowpea, etc.
ii. Callus Cells:
Carrot, Sugarcane, Barley, Rice, Wheat, Soybean, Upland cotton, etc.
The fusion of protoplasts involves three main steps as given below:
i. Plasma membranes of two or more protoplasts come into close contact.
ii. Fusion of membranes at small localized regions making bridge between protoplasts.
iii. Extension of cytoplasmic bridges and rounding off of the protoplasts forming spherical homokaryons or heterokaryons. After fusion, mixing of the cytoplasm of two cells occurs. There is equal cytoplasmic contribution from both parents. The culture requirements of protoplasts are similar to those of single cells.
Two methods are used for fusion of protoplasts, viz. chemical and electrical. Chemical method involves treatment of protoplasts with polyethylene glycol (PEG). This induces agglutination of the protoplasts and their fusion occurs after dilution of PEG with a solution containing high concentration of calcium ions at high pH.
The frequency of fusion varies from 1 to 20% depending upon cell types and fusion conditions employed. In electrical method, electric current is passed in the culture medium to induce fusion. This method leads to high frequency of protoplast fusion than chemical method and it has less harmful effect on protoplasts than chemical method.
iii. Selection of Hybrids:
After fusion, there is a mixture of parental types, homokaryons and heterokaryons. Union of protoplasts of the same species leads to the development of homokaryons. Union between protoplasts of two different species gives rise to heterokaryons. Generally, desired fusion products represent less than 10% of the total fusion mixture.
Various methods are used for identification of heterokaryons or desired fusion products. Hybrid cells can be identified by hybrid vigour at the callus stage, by morphology which is often intermediate between the parents. Biochemical and molecular techniques such as iso-enzyme pattern, RFLP (restriction frequent length polymorphism) and PCR (polymerase chain reaction) can also be used for identification of hybrid cells.
iv. Culture of Hybrid Cells and Characterization:
Hybrid cells are cultured in a medium which is highly enriched with organic compounds. Growth hormones, such as auxins and cytokinins are always required in the culture medium. Murashige and Skoog’s medium with suitable modification is used for culturing hybrid cells.
In culture medium, protoplasts regenerate a new cell wall within 2-4 days and cell division starts within 2-7 days depending upon the plant species. Plants are regenerated from hybrid tissues. Regenerated hybrid plants are characterised for highly heritable characters such as shape, size and colour of leaf, stem, flower etc. and stem surface (hairy or smooth).
Essay # 5. Uses of Somatic Hybridization:
There are several potential uses of somatic hybridization in plant breeding.
Some potential uses are briefly presented below:
1. Overcomes Cross-Incompatibility:
It overcomes barriers of cross incompatibility and makes possible hybridization between diverse plant species (distant crosses). It permits gene transfer between distantly related species and unrelated species. Thus, it is a powerful tool for modification and genetic improvement of polygenic character.
2. Permits Modification of Cytoplasmic Genes:
In somatic hybridization, cytoplasm is contributed equally by both the parents. Hybrid contains a mixture of two fusion partners. Thus modification of chloroplast and mitochondrial DNA is possible by this method.
3. Production of Somatic Hybrids:
Somatic hybridization is useful is production of fertile somatic hybrids in different crop plants.
4. Production of Allotetraploids:
Somatic hybrids contain diploid chromosome complements of both the parent species. Thus somatic hybridization is useful in the production of allotetraploid plants in a single step.
5. Conservation of Heterosis:
In somatic hybrids, segregation does not occur. Therefore, heterosis can be easily conserved in such crosses.
6. Development of Transgenic Hybrids:
Somatic hybrids are produced through protoplast fusion. Genetic transformation of hybrid cells will lead to development of transgenic hybrids.
7. Transfer of CMS:
It is also useful in transfer of cytoplasmic male sterility from one species to other.
Essay # 6. Achievements of Somatic Hybridization:
Somatic hybrids have been developed in four major families of crop plants, viz., Brassicaceae, Fabaceae, Poaceae and Solanaceae. Interspecific hybrids have been obtained in Nicotiana, brinjal, potato, tomato, Brassica, Medicago, soybean sugarcane, Datura, Petunia, Sorghum and several other crops.
Intergeneric hybrids have been obtained between tomato + potato, tomato + tobacco, Festuca + Lolium, sugarcane + bajra, wheat + bajra, Brassica + Eruca. Brassica + Sinapsis, etc. Intertribal hybrids have been developed between Brassica + Arabidopsis and Brassica + Lesquerella fendleri (Table 30.2).
Somatic hybridization is a potential method for breaking the barriers of cross incompatibility among various interspecific and intergeneric hybrids. It permits transfer of genes between species and also between genera. This technique has been used for transfer of desirable genes from wild species to the cultivated species and also between two genera and different tribes.
Generally disease resistance, quality, adaptation, drought resistance have been transferred from wild species to the cultivated species. Moreover, this method is also useful in transfer of cytoplasmic male sterility from one species to other. Two new crops, viz., pomato (potato + tomato) and Raphanobrassica (Raphanus sativus + Brassica oleracea) have been developed by somatic hybridization (protoplast fusion). However, both have combinations of undesirable characters.
Essay # 7. Limitations of Somatic Hybridization:
1. Somatic hybridization can be used in those species only where regeneration of plants is possible from protoplasts i.e., protoplasts are totipotent.
2. Somatic hybrids are fertile between closely related species. In other words, interspecific somatic hybrids are generally fertile and intergeneric somatic hybrids are usually sterile.
3. Interspecific somatic hybrids are mostly polyploid and often contain many unwanted traits derived from the wild species. Several backcrosses with cultivated species are required to eliminate undesirable characters.
4. Generally, intergeneric and intertribal hybrids exhibit high degree of sterility and cannot be backcrossed with the cultivated species.
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