Somatic cell means body cells i.e., other than the reproductive cells. In this method, the somatic cells of two varieties or species are fused together. For doing so, first the cell walls are removed by a certain enzyme. The plant cells lacking the cell wall are called protoplasts. The protoplasts of the selected parents are made to fuse with the help of certain chemicals. The resulting fused nucleus is called heterokaryon. The fused protoplasts are cultured, they regenerate their cell wall and start dividing to produce plantlets.
Limitations of Somatic Hybridization
There are certain limitations to the use of these types of somatic hybridization since plants regenerated from some of the combinations are not always fertile and do not produce viable seeds. Following are the few problem faced in somatic hybridization:
- Somatic hybridization does not always produce plants that give fertile and viable seeds.
- The lack of an efficient selection method for fused product is sometimes a major problem.
- Somatic hybridization between two diploids results in the formation of an amphidiploid which is not favourable. For this reason, hybridization of two haploid protoplasts is recommended in somatic hybridization.
- It is never certain that a particular characteristic will be expressed after somatic hybridization.
- Production of somatic hybrid plants has been limited to a few species.
- Intergeneric crosses between widely related plants which are not compatible sexually are not possible.
- For hybrid identification, selection and isolation at the culture level, there is no standardized method which is applicable for all material.
- Techniques for protoplast isolation, culture and fusion are not available for many important crop species like many cereals and pulses.
Advantages of Somatic Hybridization
- Symmetric hybrids can be produced between species, which cannot be hybridized sexually. These hybrids can be readily used in breeding programmes for transfer of useful genes to crops or may be useful as new crop species.
- Hybrids can be produced even between such strains, which are completely sterile, e.g., monoploids.
- Cytoplasm transfers can be affected in one year, while backcrossing may take 5-6 years. Even where backcrossing is not applicable, cytoplasm transfers can be made using this approach.
- Mitochondria of one species can be combined with chloroplasts of another species. This may be very important in some cases, and is not achievable by sexual means even between easily crossable species.
- Recombinant organellar genomes, especially of mitochondria, are generated in somatic hybrids and hybrids. Some of these recombinant genomes may possess useful features.
Practical Applications of Somatic Hybridization and Cybridization
- Production of novel interspecific and intergeneric crosses between plants that are difficult of impossible to hybridize conventionally.For example fusion between protoplasts of Lycopersicon esculentum (tomato) and Solanum Tuberosum (potato) created pomato first achieved by Melchers (1978).
- Several interspecific and inter-generic hybrids with disease resistance have been created. Many disease resistance genes (e.g., tobacco mosaic virus, potato virus X, club rot disease) could be successfully transferred from one species to another. For example, resistance has been introduced in tomato against diseases such as TMV, spotted wilt virus and insect pests.
- The genes responsible for the tolerance of cold, frost and salt could be successfully introduced through somatic hybridization, e.g., introduction of cold tolerance gene in tomato.
- Somatic hybridization has helped to study the cytoplasmic genes and their functions. In fact, the information is successfully used in plant breeding programmes.
- Protoplast fusion will help in the combination of mitochondria and chloroplasts to result in a unique nuclear-cytoplasmic genetic combination.
- Somatic hybridization can be done in plants that are still in juvenile phase.
- Protoplast transformation (with traits like nitrogen fixation by incorporating exogenous DNA) followed by somatic hybridization will yield innovative plants.
- Several agriculturally useful traits are cytoplasmically encoded,including some types of male sterility and certain antibiotic and herbicide resistance factors.
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