Cybrids and Importance of Somatic Hybridization

 

Cybrids or cytoplasmic hybrids:

Sexual hybridization involves fusion of the nuclear genes of both the parents but somatic hybrids involves even cytoplasm from both the parental species in hybrid obtained by protoplast fusion. However somatic hybrids containing nuclear genome of one parent but cytoplasm from both the parents can also be produced.

The cytoplasmic hybrids where the the nucleus is derived from only one parent and cytoplasm from both the parents are referred to as Cybrids. The phenomenon of formation of cybrids is called as cybridization. Cybrids are hybrids only for cytoplasmic traits.

 The approach is time consuming and require several years of crossing plants provides an opportunity to study interparental mitochondrial, chloroplast fusion giving rise to plants with novel genomes.

5.1. Methods to produce cybrids: They are produced in variable frequencies in normal protoplast fusion experiments due to one of the following methods:

1.  Fusion of normal protoplast with an enucleated protoplast. The enucleated protoplast can be produced by high speed centrifugation (20,000-40,000xg) for 60 min with 5-50% percoll.

2. Irradiating (with X-rays or gamma rays) the protoplasts of one species prior to fusion in order to inactivate their nuclei.

3. Fusion between a normal protoplast and another protoplast with a non-viable nucleus or suppressed nucleus.

4.  Elimination of one of the nuclei after heterokaryons formation.

5.  Selective elimination of chromosomes at a later stage.

Application of Cybrids

1. Cybridization has been used successfully to make intergeneric and interspecific transfer of cytoplasm in tobacco, petunia, rice and Brassica species and development of cytoplasmic male sterility.

2. Transfer of plasmogenes of one species into the nuclear background of another species in a single generation, and even in (ii) sexually incompatible combinations.

3.Streptomycine resistance transferred from N. tobaccum to N. sylvestris.

4.  A CMS lines with ‘ Ogura cytoplasm’ a herbicide (Atrazine) resistant line and lines with increased nectar  production have been obtained in Brassica sps. 

(iii) recovery of recombinants between the parental mitochondrial or chloroplast DNAs (genomes), and (iv) production of a wide variety of combinations of the parental and recombinant chloroplasts with the parental or recombinant mitochondria.

 Applications of Somatic hybridization

Novel interspecific and intergeneric crosses which are difficult to produce by conventional methods can be easily obtained.

Most of the agronomically important traits, such as cytoplasmic male sterility, antibiotic resistance and herbicide resistance, are cytoplasmically encoded, hence can be easily transferred to other plant.

Protoplasts of sexually sterile haploid, triploid, aneuploid plants can be used to obtain fertile diploids and polyploidy.

Somatic hybridization minimizes the time taken for cytoplasm transfer to one year from 6-7 years required in back cross method.

Disease and Insect Resistance

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 hybrids were produced by fusion of protoplasts from rice and ditch reed using electro fusion treatment for salt tolerance.

Overcoming Barriers of Sexual Incompatibility

 Sexual crossing between two different species (inter-specific) and two different genuses (intergeneric) is impossible by conventional breeding methods. Somatic hybridization overcomes the sexual incompatibility barriers. Examples are given hereunder: Fusion between protoplasts of potato (Solanum tuberosum) and tomato (Lycopersicon esculentum) has created pomato (Solanopersicon, a new genus). Inter specific fusion of four different species of rice (Oryza brachyantha, O. elchngeri, O. officinalis and O. perrieri) could be done to improve the crop.

 Somatic hybridization has opened new possibilities for the in vitro genetic manipulation of plants to improve the crops.

Recombinant organelle genomes, especially of mitochondria, are generated in somatic hybrids. Some of these recombinant genomes may possess useful features. Somatic hybridization is very useful for plants which is asexual, sterile and that is also beneficial for those plant which has sexual incompatibility with other species.

 Symmetric hybrids can be produced between species, which cannot be hybridized sexually. These hybrids can be readily used in breeding programs for transfer of useful genes to crops or may be useful as new species. The first symmetric somatic hybrid of Citrus was created by protoplast fusion of C. sinens is and Poncirus trifoliata (Ohgawara et al., 1985) and the production of hybrid plants between two sexually incompatible Citrus genera was first reported in 1988, where C. sinens is L. Osb. cv. „Hamlin‟ protoplasts were fused with Severinia disticha (Blanco) Swing protoplasts (Grosser et al., 1988).

Asymmetric somatic hybridization was exploited for transfer of bacterial blight resistance trait from wild Oryza meyeriana L. to Oryza sativa L. ssp. Japonica (Yan et al. 2004). Asymmetric hybridization is very promising as it allows partial genome transfer (Derks et al., 1992; Trick et al., 1994; Liu & Deng, 2002), which may be better tolerated than a whole-genome transfer

 Plants in juvenile stage can also be hybridized by means of somatic hybridization.