08-24-2013, 06:08 AM
Somatic hybridization produces symmetric or asymmetric hybrids, or cybrids. Symmetric hybrids contain the somatic chromosome complement of both the fusion parents. But asymmetric hybrids have the full somatic complement of one fusion parent and a variable number of chromosomes from the other fusion parent. Cybrids represent an extreme case in that the hybrids have the nuclear genome of only one fusion parent but, the cytoplasm of both the parents involved in fusion.
Advantages:
1. 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.
Disadvantages:
1. 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.
Advantages:
1. 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.
Disadvantages:
1. 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.
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