Genetic elements inducing gene changes at the pallida locus of Antirrhinum majus.

The pal-rec-low-o is a special repressed state of the pallida-recurrens allele, which normally mutates from the recessive to the dominant condition, giving pigmented Pal spots on corolla lobes. The pal-rec-low-o in the homozygous condition is stably colorless (except for rare mutant spots), but when crossed with the recessive tester strain, pal-tub pal-tub (also stably colorless), the mutation frequency of the particular repressed state of pal-rec (i.e., pal-rec-low-o) increases spectacularly, giving several shifts of varying sizes. The evidence suggests that the activity of the repressed state of the: pal-rec-low-o allele is dependent on the presence of an independently located Pr element, contributed by the pal-tub tester strain. In the absence of such a regulatory element, the repressed allele exhibits stable expression due to the effect of a "repressor" Rp element residing at or near the locus. It has also been shown that the pal-tub regulatory element, Pr, while coming through a phase of heterozygosity, could be changed either by picking up an element from the stabilized colorless "original" strain which, being dominant, suppresses the gene action completely; or a change may take place in pal-tub's own regulatory machinery, which otherwise has characteristics of gene regulation. However, the pal-tub regulatory element, when inactive, can be made trans-active by introducing a fresh regulator into the genome, which may segregate at meiosis.