APPLICATION OF SOMATIC HYBRIDS AND CYBRIDS IN CROP IMPROVEMENT
Genetic recombination in asexual or sterile plants:
Protoplast fusion has overcome the impediment of reproduction in haploid, triploid and aneuploid plants. Also, genomes of asexually reproducing plants can be recombined using this approach viz. protoplasts isolated from dihaploid potato clones have been fused with protoplasts of S. brevidens to produce hybrids of practical breeding value.
Genetic recombination between sexually incompatible species:
The incompatibility barriers in sexual recombination at interspecific or intergeneric levels are also overcome by somatic hybridization. Generally, somatic hybrids are used for transfer of useful genes such as disease resistance, abiotic stress resistance or genes of industrial use for e.g. Datura hybrids (D. innoxia+ D discolor, D. innoxia+ D stramonium) show heterosis for scopolamine (alkaloid) content which is 20-25% higher than in parent species and therefore has industrial application.
Cytoplasm transfer: Somatic hybridization minimizes the time taken for cytoplasm transfer to one year from 6-7 years required in back cross method. Also, this method allows cytoplasm transfer between sexually incompatible species. Cybrids have cytoplasm from both parents but nucleus of only one. Nucleus of other parent is irradiated. This approach has been potentially used to transfer two desirable traits – cytoplasmic male sterility (CMS) and resistance to atrazine herbicide, both coded by cytoplasmic genes in Brassica to different crops like tobacco, rice etc. Cybrids also provide opportunity for combining mitochondria of one species with chloroplasts of another species, and of generating recombinant organelles, especially mitochondria. This may be an important objective in case of several alloplasmic lines in which one organelle confers desirable characters, while the other has deleterious effects. An alloplasmic line has nucleus of one species and cytoplasm from a different species. For example Brassica napus alloplasmic lines (produced sexually by repeated back crossing) carrying radish (Raphanus sativus) cytoplasm (Oagura cytoplasm) are male sterile (due to mitochondrial genome) but show chlorophyll deficiency under low temperature (due to chloroplast genoe). Cybrids were produced between CMS (Oagura cytoplasm) B. napus and normal male fertile B. napus. Some of the cybrid regenerants were male sterile but did not show chlorophyll deficiency uder low temperature, these cybrids contained radish mitochondria and B. napus chloroplasts.
Genetic Transformation:
Protoplasts can take up micro-organisms, cytoplasmic organelles, isolated chromosomes and naked or plasmid DNA etc. They can be used for genetic transformation by PEG or liposome mediated deliveries or by electroporation.