It has generally been considered that a controlled locus is completely stable in the absence of the controlling gene (or at least the instability must be initiated by a controlling locus) but produces a mosaic array of cells with functioning alleles in its presence. During the course of the construction of anthocyanin R self color testers (MNL 53), observations were made on the a locus, which suggest that the a mutable alleles of the Dotted system can function in a very restricted set of cells in the absence of a Dotted gene or at least a conventional Dotted gene.
The a-m (normal frequency of dots), a-m-1 (high frequency of dots) and a-st (stable in the presence of Dt) alleles were converted to lines which were homozygous for R-sc in order to allow anthocyanin production, in the scutellar tissue. The aleurone in all three stocks is completely without pigment, unless the dominant A allele is introduced or unless one of the Dotted loci is present, in which case the a-m and a-m-1 lines exhibit their characteristic response of dots. The unusual feature of these stocks is a halo of pigment in the scutellum around the plumule of the embryo. This effect radiates farther in the a-m-1 material than with a-m but is completely absent in the a-st line. The pigment is most intense adjacent to the plumule and decreases to none within a few cell layers. This expression is not in the form of dots but is a continuous one.
There are several possible interpretations of these observations, which point to some unique properties of the system. Firstly, the "mutable" a alleles may be functioning of their own accord, but their action is limited to a small portion of the scutellum or alternatively to the plumule. If the latter is true, the factors (Pu genes) necessary for anthocyanin expression in the plumule are not present and therefore this tissue is colorless. The pigment precursors would diffuse from the plumule into the scutellar tissue to complete the anthocyanin pathway. If these genes are indeed functioning of their own accord, it is interesting to note that the level of expression is correlated with the frequency of dots as when a Dt allele is present in the genome.
An alternative might be that a highly tissue specific Dotted locus is present in these stocks. Such a locus would be active only in the plumule, producing sectors of A tissue. This would result in the production of anthocyanin precursor which upon diffusion into the scutellum would be utilized in the remainder of the pathway. The correlation between the halo size and dotting frequency would be due to the number of presumptive "dots" in the plumule.
The possibility that the anthocyanin production is due to a duplicate A gene with altered tissue specificity is not favored because of the correlation between the halo size and the "mutability" of the a alleles used.
James A. Birchler
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