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7.  Gametoclonal variation

The variation observed among plants regenerated from gametic cell cultures is termed as gametoclonal variation. The concept of gametoclonal variation evolved from that of somaclonal variation. Both somaclonal and gametoclonal variations were detected in cultured cells and regenerated plants for morphological, biochemical characteristics, and chromosome number and structure.

The life cycle of higher plants comprises a sporophytic (2n) and a gametophytic (n) generation. For genetic reasons, it is necessary to distinguish between plants regenerated from somatic (2n, somaclones) and gametic tissues (n, gametoclones), and also between somaclonal and gametoclonal variation.

i.  Unlike somatic cells, which in theory should be homogeneous within any single plant, the gametes are products of meiosis, governed by Mendel's laws of segregation and independent assortment.

ii.  Direct expression of both dominant and recessive mutations in the regenerated plants is a distinct feature of gametoclonal variation. Since plants derived from gametic cells are usually haploid but to retain the fertility chromosome number should be doubled. Chromosome number can be doubled by treating haploid plants with colchicines which is a frequently used mutagen to induce diploidization.

iii.  Clone is defined as a population of a cell or organisms derived from a single cell or common ancestor by mitosis. The concept of somaclonal variation is built on this definition. Variation is induced either during plant development or in vitro cell cultures, in the mitotic process.

On the other hand, the term ‘gametoclonal variation' can be described as the variation among derivatives of gametic cells in culture, or sexual progeny of plants regenerated from gametic cells in culture. These variants are obtained from both meiotic and mitotic divisions.

There are four distinct sources of variations when referring to gametoclonal variation:

• i.  New genetic variations induced by the cell culture procedures.

ii.  Variations resulting from segregation and independent assortment.

iii.  New variation at the haploid level induced by the chromosome doubling, and

iv.  New variation induced at the diploid level, resulting in heterozygosity.