Genetics of Aleurone Color
It is generally accepted that the presence of anthocyanin in the aleurone is due to the presence of certain alleles of the locus: A1 -- A2 -- C -- R. But, as I have pointed out elsewhere, the action of the genes at these four main loci is conditioned by the coordinate action of the modifier complex. This could be shown by several selection experiments.
A line of red brittle, originally from Cornell, served to demonstrate that by selection, completely colorless ears may be obtained. In the original line occasionally a colorless grain occurred, and it was possible, by selection for higher number of colorless grains and for paler color of colored ones, to extract a line which was completely colorless. When backcrossing to colored lines, no clear segregation could be obtained.
Some of the brittle kernels of the original line appeared to be nearly black, which was attributed to the effect of an intensifier absolutely linked with bt, or to the action of the respective bt allele itself. All selection against this factor was useless. In the extracted colorless lines there still appeared a segregation for a recessive gene, producing deep black brittle kernels. Thus a gene which in the original material was only an intensifier and as such difficult to analyze and classify, became in the extracted lines a recessive determiner of anthocyanin color.
Since no crossing over has been observed so far, we suppose that the original line contained two alleles of bt: the ordinary bt without effect on aleurone color and the new allele btr which causes a deep black color and which is epistatic, when homozygous, over the modifier complex which dominates otherwise the action of ACR. In formulas, we represent the situation:
|bt||bt||A1||A2||C||R||+ original modifier =
or red (pr pr)
|black (Pr or pr pr)|
|bt||bt||A1||A2||c||R||+ extracted modifier
|btr||btr||A1||A2||C||R||+ " " =||black|
The opposite result was obtained in "Chavantes" which as mentioned above has probably the constitution: A1 A2 Ci R where Ci represents a dominant inhibitor at the C locus. Pale purple (Pr) or red (pr pr) kernels occurred in the original material and, by selection, ears could first be extracted which segregated colored kernels in various proportions until finally fully colored ears appeared.
A corresponding situation was found in "Diamantino III" where a sharp segregation occurred for black or orange kernels. But black grains gave ears which segregated for a recessive orange while orange kernels gave ears segregating for a recessive black. The classification was generally easy, but the ratio colored : colorless did not correspond to any standard Mendelian ratio.
It is remarkable that some lines segregate normally in some crosses, and show the modifier effect in others. Thus a "Golden Bantam", when crossed to a cc sh sh - test line was shown to be AA CC rr giving a 9:7 ratio in F2, but crossed with the red-brittle line a mono-factorial segregation was obtained only in part of the offspring and a selection for both low and high ratios of colorless was successfully carried out.
These results may be summarized in the following form:
There are some lines where the modifier complex is well established and in balance with the determiners, not interfering with their action. Such lines give sharp segregations with normal Mendelian ratios.
Other lines have an unbalanced modifier complex and here selection experiments may give positive results. Thus it was possible to shift the color from red to white in the red-brittle line and from white to purple or red in "Chavantes".
The experiments are being continued and it is hoped that eventually a more complete understanding of the physiological action and interaction of determiners and modifiers may be obtained.
The selection line of "Chavantes" was very instructive in showing that we must distinguish between modifiers which act as plant characters and others which are evidently only aleurone characters. It may at first seem strange that aleurone characters may be dependent upon genes of the mother plant, and not only upon their own genes. However the effect of plant genes upon the endosperm seems to be quite general. The difference between flint and dent, between round or pointed kernel, to a large extent the difference between flint and floury, are inherited as a plant character. Now, if sporophytic genes control the type and distribution of starch in the kernel, there is no reason why one should not accept the same for the formation of anthocyanin.
F. G. Brieger and George O'Neill Addison