Molecular Basis of Somaclonal Variation
Variants may arise as a result of more subtle changes due to single gene mutations in cultures which have cells apparently showing no karyological changes. Recessive mutations are not detected in RO plants, but express in RI progeny. The RI progeny segregates in Mendelian 3:1 ration for the trait of interest. This further confirms the mutant nature of the variant. Somaclonal variants for single recessive gene mutations have been reported in respect of maize, Nicotina sylvestris, rice and wheat. In some cases specific genetically marked strains have aided in evaluation of plants regenerated from cell cultures.
Changes in cytoplasmic genome have also been observed in somaclones. Gengenbach et. al. (1977) were succeeded in selecting resistant plants in maize combining resistance to toxin produced by Drechslera maydis with the cms T- trait. Both these traits are controlled by mtDNA. Another aspect of single gene mutation responsible for Somaclonal variation relates to transposable elements. Clourey and Kemble ( 1982) detected variation as a result of insertion of plasmid- like DNA in the mitochondrial genome of cms maize cell cultures.
Somaclonal variation may also be molecular changes caused by mitotic crossing over in regenerated plants. This could include both symmetric and asymmetric variation. Single gene mutations by MCO may constitute s unique mechanism of inducing new genetic variations. Small changes in the structure of chromosomes could alter expression and genetic transmission of specific genes, such as deletion or duplication of a copy of gene or gene conversion during repair processes.
Revent studies have demonstrated that changes in organelle DNA, isozyme and protein prodiles correlate with the occurance of Somaclonal variation in plants (Wheat, Rice, Potato, Maize, Barley, and Flax) . somoclones of wheat show alteration in gliadin profiles, nor loci, and qualitative as well as quantitative differencesin rDNA.