Somatic hybridization which is also known as protoplast fusion, is a type of genetic modification in plants by which two distinct species of pant are fused together to form new hybrid plant with the characteristics of both.
Authors: Abrish Fatima
It involves In-vitro fusion of isolated protoplast to form a hybrid cell. Somatic hybridization of crop plants is a new challenge for plant breeding and crop improvement. By using this technique barrier of species can be overcome, the fusion of protoplasts of genetically different lines or species has been possible.
Fusion products- the hybrid and cybrids
When two nuclei are fused the cells are known as hybrid, which contain genetic material of both the parents. When only cytoplasms fuse and genetic information from one of the two nuclei is lost or inactive cell is known as cybrid and this process is called cybridization.
Recently this technique was carried out using tomato and potato. The protoplast of tomato was fused with the protoplast of potato to form a new hybrid plant called pomato or tomtato.
Somatic hybridization technique:
1. Single cells isolated from the selected plants.
2. Removal of cell wall of the fusing cell with the help of the enzymes like pentinase and cellulose. These enzymes digest the cell wall to expose the naked protoplast.
3. The isolated protoplasts of selected parents are fused to obtain hybrid protoplast on special nutrient medium under aseptic conditions. This fusion is induced by the use of polyethylene glycol (PEG) or by a brief high voltage electric current.
4. The hybrid protoplasts are cultured on suitable nutrient medium, where they regenerate cell wall and begin to undergo division to form new plantlets. These plantlets are the somatic hybrids.
Advantages of somatic hybridization:
Somatic hybrids can be produced between species, which cannot be hybridized sexually. These hybrids can be really used in breeding programmes for transfer of useful genes to crops or may be useful as new species. It opens the possibilities for the In-vitro genetic manipulation of plants to improve the crops. Some of the applications are mentioned below:
• Production of fertile diploids and polypoids from sexually sterile haploids, triploids and aneuploids.
• The organellar genomes such as mitochondria which are recombinant in somatic hybrids and cybrids, which may possess useful features.
• Somatic hybridization has helped in the successful transfer of some disease resistance genes from one species to another. For example in tomato resistance against diseases such as tmv spotted wilt virus and insect pests’ environmental tolerance has been introduced.
• Some environmental factors like cold; frost and salt also pose a challenge for crops. The genes responsible for the tolerance of such factors have been successfully introduced through somatic hybridization, for example, introduction of cold tolerance gene in tomato.
• Quality characters: somatic hybrids for the production high nicotine content and low uric acid have also been developed
Limitations of somatic hybridization:
Fertile and visible seeds are not always produced by plants through somatic hybridization. The production of viable somatic hybrids is not possible in all instances even though protoplast fusion between different species or genus is easy.
Somatic hybridization between two diploids results in the formation of an amphidiploid which is not favorable. That’s why haploid protoplasts are recommended in somatic hybridization.
Disadvantages of somatic hybridization:
• Techniques are not available for protoplast isolation, culture and fusion for many important crop species like many cereals and pulses.
• In a lot of cases, chromosome elimination occurs from somatic hybrids leading to asymmetric hybrids. These hybrids maybe useful, but we cannot control on chromosome elimination.
• Many somatic hybrids show genetic instability, which may be an inherent feature of some species combinations.