Somaclonal Variation - For clonal propagation or micropropagation as discussed above, shoot tips or axillary buds are used for direct propagation on culture medium without intervening callus phase. It has been shown in recent years, that regeneration from callus, leaf explants or plant protoplasts leads to the generation of considerable variation, described as somaclonal variation.

This variation includes aneuploids, sterile plants and morphological variants, sometimes involving traits of economic improtance in case of crop plants. This variation in cultures was ignored for long time, but during the last decade it received increased attention and excitement, in view of its potential in crop improvement programmes.

In several crops R₀, R₁ and R₂ progenies were analysed for genetic analyses and 3 : 1  segregation leading to the isolation of true breeding variants was observed. Although the details of the genetic basis of somaclonal variation .in most crops are still unknown, variation in structure and number of chromosomes has been suggested to be one possible basis.

Polyploidy, aneuploidy, translocations, inversions and deletions have been reported in several cases. Meiotic crossing over involving symmetric and asymmetric recombination could also be responsible for a part of the variation observed in the regenerated plants.

(ii) In the second approach selection is exercised at the phenotypic level in regenerated plants.

The former approach of selection at the cellular level has the advantage of screening millions of cells with a relatively little effort and resources.

But this method can not be used for traits like flower characters or grain yield. Further, the phenotype expressed by selected cells may not be expressed by regenerated' plants. Therefore, the characters for which selection is exercised at single cell level, need to be checked in the regenerated plants.