SOMACLONAL VARIATION AND CRYOPRESERVATION

Plants derived from tissue culture has been variously referred to as somaclones or calliclones or protoclones and the variations displayed by such plants are simply called ‘somaclonal variation’. Variants obtained using callus cultures are referred as “Calliclones” (Skirvin, 1978) while variants obtained using protoplast cultures are known as “Protoclones” (Shepard et al. 1980). According to Larkin and Scowcroft (1986), ‘somaclonal variation is the genetic variability which is regenerated during tissue culture.’

Accordingly, the plants derived from cell and tissue cultures are termed as ‘somaclones’, and the plants displaying variation as ‘somaclonal variants’. Another term suggested by Evans et al. (1984) as ‘gametoclonal variation’ for those variations arising in cell cultures of gametic origin like, in pollen and microspores cultures, to distinguish them from somatic cell derived regenerants. However, generally the term somaclonal variation is used for genetic variability present among all kinds of cell/plants obtained from cell cultures in vitro. Plants regenerated from tissue and cell cultures show heritable variation for both qualitative and quantitative traits. Several useful somaclonal variants have been obtained in large number of plant species such as, potato, sugarcane, banana, tomato etc. Chaleff (1981) labeled plants regenerated from tissue cultures as R0 generation and their successive sexual generations as R1, R2 and so on.

Variations for karyotype, isoenzyme characteristics and morphological variations in somaclones have been commonly observed. Such variations manifest themselves as heritable mutation and persist in the plant population even after transplantation to the field.

Sometimes, phenotypic variations may arise in the progeny of plants regenerated from culture, but after the transplantation to the field, the plants exhibit the parental characteristic during their further growth and development. Such variations are not considered as somaclonal variations.

Steps Involved in Somaclonal Variation

1. Growth of callus or cell suspension cultures for several cycles.
2. Regeneration of a large number of plants from such long term cultures.
3. Screening for desirable traits in the regenerated plants and their progenies. For example, invitro selection to select agronomically desirable somaclones for tolerance to various biotic and abiotic stresses, herbicides, high salt concentration and extremes of temperature.
4. Testing of selected variants in subsequent generations for desirable traits.
5. Multiplication of stable variants to develop new breeding lines.

Characteristics of Somaclonal variants

1. It must involve useful characters.
2. It should be superior to the parents in the character(s) in which improvement is sought.
3. The improved character(s) must be combined with all other desirable characters of the parent, and
4. The variations must be inherited stably through successive generations by chosen means of propagation.

Mechanisms of Somaclonal Variation:

The variations could also arise in tissue culture due to physiological changes induced by the culture conditions. Such variations are temporary and are caused by epigenetic changes. These are non-heritable variations and disappear when the culture conditions are removed. The somaclonal variation may be attributed to either:

1 – Pre-existing variation in the somatic cells of the explant (genetic)

2- Variation generated during tissue culture (epigenetic). Often both factors may contribute.

3-Several molecular basis for somaclonal variation have been proposed, which include changes in chromosome number, point mutations, somatic crossing over and sister chromatid exchange, chromosome breakage and rearrangement, somatic gene rearrangement, DNA amplification, changes in organelle DNA etc.