Somaclonal variation

Potato is potentially a good model crop for selection of improved lines generated through somaclonal variation from which novel variants can arise. Sexual crosses are not always possible in potato due to sterility problems or lack of flowers and, as already stated, the genetics of tetraploid inheritance is problematic. Potato is easily regenerated in tissue culture (although as stated previously some cultivars are more recalcitrant than others) and is vegetatively propagated from tubers. However, somaclonal variation may produce undesirable effects following targeted genetic transformation events, thus modifying phenotype and agronomic performance independently of any effects induced by insertion of the target gene(s).

The in vitro regeneration process required to produce GM potato lines involves: (a) establishing de-differentiated cells from tissue or organ culture under defined conditions: (b) proliferation for a number of cell generations: and (c) subsequent plant regeneration under in vitro conditions (Karp 1990). Somaclonal variation in regenerated plants is generated duringth in veitro culture stage and particularly during de-differentiation. This is accompanied by increased frequency of chromosomal abnormalities with time in culture. Genetic changes also occur in plant tissues and cells in vivo due to mutations, endoreduplication, chimeras etc (see Kumar, 1994 and references therein for a comprehensive analysis of the origins of somaclonal variation). Genetic variation in plants regenerated in vitro can therefore be derived from in vivo and in vitro events. The contributions of in vivo and in vitro modifications are dependent on parameters including genotypic background, culture conditions, etc.

Somaclonal variation is uncontrollable and unpredictable in nature and most variation is of no apparent use. However, stability of any useful somaclones produced may not be a problem. Morphological changes observed range from gross abnormalities to minor and more subtle modifications. There is distinct genotypic variation in the frequency of somaclonal variants that might arise. Thus selecting GM potato lines for commercialisation which have the desired impact but in which other traits are not significantly modified by the tissue culture process will require the production of several hundred independently transformed lines and full and effective field selection using criteria that breeders would normally impose. This will be in addition to the testing of a range of constructs, promoters, targeting sequences, etc. where relevant.

Compliance, in risk assessment exercises, with the need to demonstrate ‘substantial equivalence’ of a GM line with the parent from which it is derived should take into account compositional variation that might be induced by somaclonal variation in species such as potato, and not only from the expression or insertion of target genes. The concept of substantial equivalence is that the GM line to be marketed should be compositionally the same as the parent line from which it was derived, but with the exception of any modification expected by inserting the gene of interest. This has to be assessed using several growing sites over more than one year. From a personal perspective the value of substantial equivalence should be combined with knowledge of natural genetic variation in the compositional status of cultivars already in production and on sale. For this reason extensive databases which clearly demonstrate the ranges of metabolite concentrations that might be expected in species of crop plants will have great utility. A scenario can be envisaged in which the composition of a GM line may differ from its parent but fall well within the range expected of cultivated potato.