Chimerism as the basis for the occurrence of amylose synthesizing clones derived from an amylose-free potato mutant

Earlier described revertants, obtained after irradiation of an amylose-free (amf) mutant which carries a point deletion in the gene for granule-bound starch synthase, were analysed at the DNA-sequence level. Direct sequencing of fragments amplified by the polymerase chain reaction revealed that all investigated revertants carry the original wildtype sequence. It is argued that mutation as the basis for the re-occurrence of wildtype alleles is highly unlikely. The alternative conclusion is reached that the original amylose-free monoploid clone must have been a chimera. Chimerism with wildtype and mutant tissues was actually found in a plant which at a later stage was obtained from the same mutant clone without the use of X-rays. Wildtype cells could have remained in the L2 layer of the original monoploid mutant, which cannot be analyzed for starch composition.