Genetic analysis of instability inPetunia hybrida : 3. Periclinal chimeras resulting from frequent mutations of unstable alleles.

InPetunia hybrida frequent mutations of unstable alleles give rise to different types of periclinal chimeras. If genes expressed in the epidermis, such as the geneAn1 for flower colour, are concerned, mutations in the dermal layer of the shoot apex will result in changes in the phenotype but not in the offspring. Mutations in the subdermal layer will not lead to an altered phenotype, but to changes in the sporogenous tissues and, thus, to deviating segregations in progenies. Therefore, in crossing experiments with such an unstable mutant, it is always necessary to take the possibility into account that the plant may be a chimera, so as to prevent an incorrect interpretation of the recorded segregational ratios. Mutations of unstable alleles expressed in the mesophyll, such as geneYg3 for leaf colour, also give rise to chimeras. In such instances, however, a change in phenotype always involves a change in segregational ratios as well, since both the mesophyll and the sporogenous tissues are derived from the subdermal layer of the shoot apex.