1. Introduction
Plants generally exhibit cytogenetic and genetic variations which help the plant breeders in crop improvement. When such variants arise through the cell and tissue culture process using any plant portion as an explant material, variations arising are termed as somaclonal variations . 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). Larkin and Scowcroft (1981) proposed a general term ‘Somaclonal variation' to describe genetic variation in plants regenerated from any form of cell cultures. 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.
The basic cause of these variations may be attributed to changes in karyotype (chromosome number and structure), chromosome rearrangements, somatic crossing over, sister chromatid exchange, DNA amplification and deletion, transposable elements and DNA methylation. Somaclonal variation can be characterized based on morphological, biochemical (isozymes) and DNA markers such as, Random Amplified Polymorphic DNA (RAPDs), Restriction Fragment Length Polymorphism (RFLPs) and Inter-Simple Sequence Repeats (ISSR).
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.
There are different approaches (steps) to create somaclonal variations, which include:
- i. Growth of callus or cell suspension cultures for several cycles.
ii. Regeneration of a large number of plants from such long term cultures.
iii. Screening for desirable traits in the regenerated plants and their progenies. For example, in vitro selection to select agronomically desirable somaclones for tolerance to various biotic and abiotic stresses, herbicides, high salt concentration and extremes of temperature.
iv. Testing of selected variants in subsequent generations for desirable traits.
v. Multiplication of stable variants to develop new breeding lines.
To be of commercial use, a somaclonal variant must fulfill certain basic requirements:
- i. It must involve useful characters.
ii. It should be superior to the parents in the character(s) in which improvement is sought.
iii. The improved character(s) must be combined with all other desirable characters of the parent, and
iv. The variations must be inherited stably through successive generations by chosen means of propagation.