The below mentioned article provides a quick notes on Somatic Hybridisation.
Sexual hybridisation n higher plant is a valuable tool for the conventional plant breeding to improve cultivated crops. It involves the artificial cross-fertilisation between the genetically dissimilar individuals to combine several desirable traits present in different varieties into one single variety.
Unfortunately, conventional hybridisation is limited to only very closely related species. Until recently, sexual hybridisation was a total failure for distantly related plant species as well as sexually incompatible plant species.
Therefore, if a system could be developed for transferring genetic information between widely different plant species, it would provide the basis for a technology to overcome the limitation of conventional sexual hybridisation.
In protoplast technology, from any two genotypically different plants, protoplasts are isolated from the somatic cells (diploid) and are experimentally fused to obtain par-asexual hybrid protoplasts. The hybrid protoplast contains heteroplasoic cytoplasm and two fused parental nuclei.
The fused protoplast is grown in vitro with an aim to obtain a hybrid plant. So the in vitro fusion of plant protoplasts derived either from somatic cell of same plant or from two genetically different plant is called somatic hybridisation. Sometimes the protoplasts from vegetative cell and gametic cell are fused and such fusion is called somato-gametic hybridisation.
Protoplast fusion and somatic hybridisation in plant are based on the following basic points:
1. Protoplast isolation.
2. Fusion of protoplast, obtained from the different species, to produce viable heterokaryons.
3. Cell wall regeneration by the fused product.
4. Fusion of nuclei.
5. Division of hybrid cells and their subsequent growth.
6. Identification, selection or isolation of hybrid cell.
7. Induction of organogenesis in the callus tissue derived from hybrid cell.
8. Raising mature plants from regenerated shoots.
Experimental fusion of plant protoplasts was reported by Kiister (1909) and Michel (1937). Although fusion products were obtained, they failed to survive because of shortenings in the understanding of environmental, nutritional and hormonal requirements of isolated protoplasts.
After protoplasts became available in large quantities with the aid of enzymes, renewed attempts were made for protoplast fusion to obtain somatic hybridisation.
Limitation of Somatic Hybridisation:
It was once suggested that hybrids would be of great value in crop improvement. But the experimental reports are not very encouraging. At present, techniques for selection and manipulation of somatic hybrid cells and regeneration of hybrid plants from them is limited to a few special cases where they can be manipulated very easily in culture.
So far, the production of somatic hybrid of agronomical important plants are not possible.
The main objective of protoplast fusion and somatic hybridisation was to overcome the pre-fertilisation barrier to sexual incompatibility or any genomic incompatibility. Therefore, it would be undoubtedly expected to achieve very wide crosses through protoplast fusion and it will solve many problems relating to crop improvement.
But, practically, inter-generic crosses between widely related plants, which are not compatible sexually—are not possible. Few interspecific somatic hybridizations where plants are sexually compatible or incompatible due to natural reproductive isolation are achieved. In certain wide crosses, elimination of chromosomes from the hybrid cell is another limitation of somatic hybridisation.
So, desirable hybrids are no longer available. In protoplast fusion experiment, the percentage of fusion between two different parental protoplasts is very low. Although some attempts has been made to increase the percentage of fused cells, still it is also a limitation of somatic hybridisation.
Lastly, for hybrid identification, selection and isolation at the culture level, there is no standardised method which is applicable for all material.