SOMACLONAL VARIATION

Somaclonal variation is defines as the genetic variability present among cultured somatic cells.
Plants derived from such cells or progeny of such plants are called somalcones.
The term somaclonal variation was first used by Larkin and Scowcroft in 1981.
Somaclonal variations can be selected for disease resistance, improvement of nutritional quality, adaptation of plants to stress conditions, resistance to herbicides etc.
Somaclonal variation has been observed in plants such as apple, sugarcane, potato, tomato etc.

PROCEDURE FOR ISOLATION OF SOMACLONAL VARIANTS


Isolation of somaclonal variants can be grouped into two broad categories. They are:
1. Screening
2. Cell selection Screening

This involves the observation of large number of cells/plants from tissue culture and detection of variants.
In general R1 progeny (progeny of regenerated R0 plants) are used for the identification of variant plants.
R2 progeny (progeny of R1 plants) are used for confirmation. This has been employed for a number of plants.
Computer based automated cell sorting devices have been used to screen as many as 1000-2000 cells/second from which variant cells can be automatically separated. These variant cells are further regenerated to produce complete plantlets.
This approach is widely used for the isolation of variants which produce high yield and desirable traits.
Also used to obtain cell clones that produce higher quantities of certain biochemical agents.

Cell Selection
In this method an appropriate selection pressure is applied which permits the survival/growth of vibrant cells only during culture.
When the selection pressure allows only the variant (mutagenic) cells to survive, it is called positive selection.
In negative selection, the selection pressure allows only the wild type cells to survive. These wild type cells are later killed by counter selection pressure. The variant cells are rescued by the removal of counter selection agents.
Positive selection approach maybe further sub-divided into 4 categories,

a) Direct selection
b) Rescue selection
c) Step wise selection
d) Double selection

Direct selection
In this method the selection agents kills the wild type cells. The mutant (variant) cells remain unaffected. The mutant cells continue growing and in dividing the medium. This is the most common method employed to obtain variants that are resistant to toxins, herbicides, antibiotics, high salt concentration etc.
Rescue selection
In this method the selection agent kills the wild type cells. The mutant (variant) cells remain alive but do not divide due to unfavourable environment created by the selection agent. The selection agent is then removed to recover the variants. This method is used to obtain variants that are resistant to aluminium, cold temperature etc.

Stepwise selection

In this method the concentration of selection agent is increased in a stepwise manner. Eg, to obtain variants those are resistant to high salt concentration. At first low concentration of salt is added to the medium. Those cells which survive are then subjected to higher salt concentration, those cells which are survive at this concentration are further subjected to higher salt concentration and so on.

Double selection

In this method variants with two traits are selected simultaneously with the same selection agent. E.g.; Selection of a variant, which shows antibiotic resistance (streptomycin resistance) and development of chlorophyll. Here streptomycin resistance is the first trait and development of chlorophyll in the cells is the second trait.

ADVANTAGES OF SOMACLONAL VARIATIONS

1. Somaclonal variations are stable and occur at high frequencies.
2. Somaclonal variations may show novel mutations.
3. Can be performed in all types of cells, ie; vegetatively or sexually or asexually propagated plants.
4. Somaclonal variations may reduce two years the time required for the release of new variety compared to mutation breeding.
5. Only approach for the isolation of biochemical mutatants.
6. It is an effective method.

DISADVANTAGES OF SOMACLONAL VARIATIONS

1. Somaclonal variation is applicable to only those species which can regenerate complete plants
2. Many Somaclonal variants show undesirable features such as reduced fertility, growth rate etc.
3. The variation is not always heritable
4. The variation is generally cultivar dependent
5. Selected clones show unpredictable and uncontrollable variants.

BASIS OF SOMACLONAL VARIATION

A number of factors are responsible for somaclonal variation.
Gene mutations such as translocations, deletions, inversions
Pre-existing chromosomal ploidy in the explants
Number fragmentation at callus induction stages.
Changes in gene expression and gene amplification.

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