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SOMATIC HYBRIDIZATION&CYBRIDSSOMATIC HYBRIDIZATION Development of hybrid plants through the fusion of somatic protoplasts of two different plant species/varieties is called somatic hybridization. This is a non conventional genetic procedure involving fusion between isolated protoplast under in vitro condition and subsequent development of their product (heterokaryon) to a hybrid plant.
Somatic hybridization technique1.Isolation of protoplast
2.Fusion of the protoplasts of desired species/varieties.
3.Identification and Selection of somatic hybrid cells
4.Culture of the hybrid cells
5.Regeneration of hybrid plants
Protoplast Fusion(Fusion of protoplasts of two different genomes)Spontaneous Fusion2. Induced FusionIntraspecificIntergenericElectrofusionMechanical FusionChemofusion
Identification and Selection of somatic hybrid cells 1.Selection of somatic hybrids by culturing them on such a medium on which only somatic hybrids can grow.(Smith et al.1976).
2.Complimentary selection of somatic hybrids on specific culture medium.(Melscher & Labib 1974).
3.Mechanical isolation by visual means and knowledge of identification of somatic hybrids.
4.Morphology of the plant after regeneration.
Identification and Selection of somatic hybrid cellsHybrid identification- Based on difference between the parental cells and hybrid cell with respect to Pigmentation Cytoplasmic markers Fluorochromes like FITC (fluoroscein isothiocyanate) and RITC (Rhodamine isothiocyanate) are used for labelling of hybrid cellsPresence of chloroplastNuclear stainingHeterokaryon is stained by carbol-fuschin, aceto-carmine or aceto-orcein stain
Genetic complementationPhytotoxins (soybean resistant to Hm T toxin whereas Zea mays sensitive to it)Specific amino acid (Conavalin present in soybean but not in sweet clover, alfalafa)Auxin autotrophy (N. glauca and N. langsdorffi)AntibioticsAuxotrophic and metabolic mutantsChromosomal analysisHerbicidesCulture of the hybrid cells Hybrid cells are cultured on suitable medium provided with the appropriate culture conditions.Iscove's MDM with 25 mM Hepes.DMEM/F-12 with 15 mM HEPES.Alpha MEM with NucleosidesRPMI 1640 Medium
Regeneration of hybrid plants Plants are induced to regenerate from hybrid calli. These hybrid plants must be at least partially fertile, in addition to having some useful property, to be of any use in breeding schemes.
Caipira sweet orange + Rangpur lime
May be complete in two to several days
Although protoplast in culture generally start regenerating a cell wall within a few hours after isolation.
Protoplast lose their characteristic spherical shape once the wall formation is complete.
Regeneration of cell wall can be demonstrated using Calcalfluor White ST fluoresecent Stain or Tinapol solutionCell Wall RegenerationSymmetric hybrids: These contain the somatic chromosome of both the parental species. These are very significant as they show all the properties exhibited by parent species.
Asymmetric hybrids: These are those hybrids which preserve the genetic material of one parent organism. The chromosome content of other parent species is lost.Advantages of somatic hybridization1. 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 species.
2. Hybrids can be produced even between such strains, which are completely sterile, e.g., monoploids.
3. Cytoplasm transfers can be affected in one year, while backcrossing may take 15-16 years. Even where backcrossing is not applicable, cytoplasm transfers can be made using this approach.
4. 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.
5. Recombinant organellar genomes, especially of mitochondria, are generated in somatic hybrids and cybrids. Some of these recombinant genomes may possess useful features.Advantages of somatic hybridizationProduction of novel interspecific and intergenic hybrid Pomato (Hybrid of potato and tomato)
Production of fertile diploids and polypoids from sexually sterile haploids, triploids and aneuploids.
Transfer gene for disease resistance, abiotic stress resistance, herbicide resistance and many other quality charactersAdvantages of somatic hybridizationProduction of heterozygous lines in the single species which cannot be propagated by vegetative means
Studies on the fate of plasma genes
Production of unique hybrids of nucleus and cytoplasm
Disadvantages of Somatic hybridization1. Techniques for protoplast isolation, culture and fusion are not available for many important crop species like many cereals and pulses.
2. In many cases, chromosome elimination occurs from somatic hybrids leading to asymmetric hybrids. Such hybrids may be useful, but there is no control on chromosome elimination.
3. Many somatic hybrids show genetic instability, which may be an inherent feature of some species combinations.
4. Many somatic hybrids either do not regenerate or give rise to sterile regenerates. Such hybrids are useless for crop improvement. All interfamily somatic hybrids are genetically are unstable and/or morphologically abnormal, while intergeneric and intertribal hybrids are genetically stable but produce abnormal and/or sterile plants or only teratomata.Disadvantages of Somatic hybridizationPoor regeneration of hybrid plants
Non-viability of fused products
Not successful in all plants.
Production of unfavorable hybrids
Lack of an efficient method for selection of hybrids
No confirmation of expression of particular trait in somatic hybridsCYBRIDPlants or cells witch containing nucleus of one species but cytoplasm from both the parental species . Cytoplasmic hybrids.Involves fusion of two protoplasts (donor and recipient) in which donor nuclear genome is eliminated while its plastome and/or chondriome are merged with that of the recipient.For production of a cybrid cell uses two parental cells but should be one complete cell (cytoplasm with nucleus ) and other should be without nucleus or in active nucleus
Suppression of nuclear genome during fusion.(uses gamma rays and X-rays )
Selection of the fusion products that have only the extranuclear genomes of a donor.
CybridizationPossibility of injection of nucleus of A to B.19Aims of cybridizationTo obtain biological information on intra- and intergeneric transfer of organelles. To combine cytoplasm gene of one species to anther species witch have nucleus with cytoplasm.This is also a way for those species witch dont perform to sexual reproduction with each other, so this way provided to facilitate to make a desire species with combination of both species.UsesInteraction of nuclear genome and plastome and/or chondriome of another species.Phylogenetic limit of compatibility between nucleus and plastome and/or chondriome.
To replace organelles such as chloroplast and mitochondria with that of another genotype.UsesChloroplasts replacement in Brassica confers resistance to herbicides such as Atrazine.CMSBenefitsTransfer of plasma genes of one species to nuclear background of another species.Production of wide range of genetic variations, sexually incompatible combinations.Useful for sterile plants.Mitochondrial gene also combined with chloroplast gens of another species.Plants like medicinal plant, crop plants, sterile pants, or many other valuable species also included in this.