Somaclonal variation in potato cv. Bintje: Mosaic composition of protoplast calli and segregation of various phenotypes after vegetation propagation

The occurrence of spontaneous variation among plants regenerated from in vitro culture of cells or protoplasts ('somaclonal variation') in a number of crops, including potato, has stimulated great interest in recent years from the point of view of using it in plant-improvement programmes. The plant regeneration process from shoot-culture-derived protoplasts of the Dutch potato cultivar Bintje makes it possible to recover a wide array of variants, including undesirable gross-aberrant types. After tuber propagation it was found that some of the normal and variant protoclones (regenerated plants) segregated or showed new types, probably due to chimerism. For plant-breeding purposes, it is highly desirable to know whether or not small groups of genetically different cells present in heterogeneous calli actually express in regenerated plants, or appear in later generations of vegetatively propagated material. The analysis carried out in this regard on the origin of 'segregating protoclones', through characterization of protoplast calli, protoclones and tuber progeny, suggests that a high frequency of protoplast calli are mosaic in composition, consisting of groups of variant (presumably mutated) cells. Two types of mosaicism can be generally distinguished. (1) Fine-grade mosaicism, occurring because small groups of variant cells do not affect the phenotype of regenerated shoots (plants), but form new phenotypes among tuber progeny. (2) Coarse mosaicism, showing the occurrence of large groups of variant cells in a given callus regenerating variant/chimeric shoots. A majority of the mosaic calli gave rise to variant protoclones and mixtures of normal and variant protoclones. The analysis of second-generation tuber progeny showed that part stabilized in phenotype and part segregated again into various phenotypes, indicating the persistence of at least some chimerism. (author)