The color of music - pigment notes
--E. Derek Styles
The American Dream. Although not everyone can be born a queen, king, prince or princess (who would want to be, nowadays?), in theory any citizen should be able to rise (?) to become a president. True, a certain amount of native ability is required, but is insufficient if the more basic necessities of 'resources, friends and helpers' are lacking. Thus it is for presidential aspirants, and thus it is for their genes, and of course thus it is also for the pigmenting genes of maize. Presidential hopefuls and pigmenting genes remain untested potentials until all the basic requirements for their expression are present. In theory, inherent differences in potential can be evaluated by offering all the same opportunity, for example, different R or P alleles can be compared against the same genetic background. This is at best a limited evaluation, and might be equated to finding differences in presidential potential by having everyone live in the same suburban environment, and limiting all to an adequate but minimal income. A different approach might be to offer everyone unlimited 'resources, friends and helpers' and evaluate differences in presidential potential in the 'best of all possible worlds'. For maize pigment genes, this would mean comparing pigment potentials of different alleles when every other genetic and environmental factor favors maximum pigmentation. Ideally, there should be no duplicate factors present (only one candidate at a time per party being evaluated), and it should be recognized that evaluation of current worth is at best temporal, as it has been achieved and can be further altered by 'experience'.
The above wandering waffle is by way of introducing and justifying my current approach to the study of the flavonoid pigment genes in maize. Clearly it will soon be possible to recognize differences at known loci in precise molecular detail, and at that time there may be a terminology problem in using the very word 'allele', especially if a multitude of differences are recorded at a single locus. It is quite realistic to expect reports of differences in large sequences of bases; differences in single base pairs and in numbers of triplet repeats, and differences in methylation patterns, etc., in the near future. At that point the limiting factor will be in our ability to recognize the effects of such differences, if any, at the phenotypic level. Restrictions on the expression of an allele's potential will limit our ability to recognize the effect of a specific change at the base level. It will be an advantage, therefore, to arrange for the 'best of all possible worlds' when attempting to evaluate the potential of a specific allele, meaning that all genetic and environmental factors should favor maximum pigmentation, so that the only limiting factor is the particular allele being evaluated.
P-locus phenotypes. Allelic symbols for the P locus offer only broad categorizations, and the collection of P locus factors incorporated into the inbred strain 4County63 by R. A. Brink and sent to the National Seed Storage Laboratory in Fort Collins, Colorado (see descriptions in MNL40:149-160, 1966) demonstrate that P alleles actually form a continuous series in terms of cob and pericarp pigment when compared in that particular background. The unstable factor for orange pigment (Ufo1) found by Dr. Charles Burnham (see MNL61:100) appears to 'release' the tissue specific controls of P-WR alleles so that pigments normally restricted to the cob are produced not just in the cob, but also in the pericarp, culms, leaves, and other plant parts. By itself, Ufo1 has no expression, so that P-WW alleles remain unpigmented even when Ufo1 is present. The expression of an intermediate P-allele, determining a grainy type of pericarp pigment (from the 4Co63 collection, specific designation unfortunately lost, but probably 65-CFS-364), is not altered by the presence of Ufo1. This 'grainy' allele occasionally mutates to a sectored form, with irregular sectors in the cob, husk and pericarp. This sectored form yields P-RR alleles at high (~10%) frequency, and these P-RR plants are very darkly pigmented in husks, cob, pericarp and even culms, in fact they are among the strongest P-RR alleles in my stocks. The sectored form is distinct from P-VV or P-MO alleles, not only in the type of sectoring, but also in the very dark and clear sectors consistently seen in the husks and cobs. Homozygous sectored forms breed true except for mutation to P-RR. Heterozygous P-WR/'P-sect' plants yield P-WR, 'P-grainy', 'P-sect' and some P-RR progeny. If, as this evidence seems to suggest, the grainy phenotype results from a controlling element at an otherwise very strong P-RR allele, this would explain why, in its controlled state, the allele appears not to respond to Ufo1, and it is like P-MO and P-VV in this respect.
Pith pigment. In line with attempts to get the maximum expression of R and P alleles, I have been particularly interested in those alleles or combinations of enhancing factors that have strongly pigmented piths. Interestingly enough, the 'grainy' P allele described above is easily distinguished from other P alleles by its orange pith pigment. The pith of the sectored form is white with dark red sectors, and the secondarily derived P-RR allele has a solid dark red pith to go with the other darkly pigmented tissues.
R alleles that determine anthocyanins in the pith (with other 'appropriate' non-allelic friendly helpers) include the Group-D R alleles, and R-nj-6, a compound allele derived from R-st:nj (stippled crown). Pl is a required but not sufficient factor for pith anthocyanin, and the enhancing factor a3 definitely favors pith pigmentation by the above listed alleles, although it is not an absolute requirement. Many of the lines with strong pith anthocyanin also carry the recessive aleurone enhancer in, even though selection was made on the basis of pith pigment rather than for aleurone enhancement.
The aleurone intensifier in. If in can enhance anthocyanin production in the pith, does it have an enhancing role in other tissues also? Some miscellaneous observations on possible effects of the in factor may be helpful or misleading, depending on whether the observations hold up with further testing: 1) Red pollen is typically produced by some (but not all) Pl r-ch (or R-ch) strains, but some strains of non-cherry red anthered R alleles (with Pl) can also produce red pollen. Some (but not all?) of our red pollen producing non-cherry strains also carry in. 2) Some c2 R-r Pl have purple anthers indistinguishable from sib C2 R-r Pl plants. I am not sure how typical this is, but so far I have noted this only in c2 Whp strains that also carry in. 3) Some r-g colorless aleurone strains, and even some c2 whp colorless aleurone strains, have a 'dirty' pericarp, that if it were covering a colored aleurone, might well be called a 'sheen'. There seems to be a more than accidental correlation between at least one type of 'dirty pericarp' and the presence of the in factor. Unfortunately the correlation is not one hundred percent. Perhaps another factor such as Pl may be required? 4) The 'pericarp sheen' typical of in appears not to contain flavonoids, at least we have not been able to isolate flavonoids when attempting to extract the 'sheen' from the pericarp.
Although the above observations should be treated with caution until they have been tested properly, I offer them in the hope that they may add or arrange a few pieces in the larger puzzle. To my mind, they would make the most sense if in determined a membrane characteristic and affects, among others, the semi-permeable membrane that separates the pericarp and the endosperm tissues.
The salmon silk factor sm. According to Anderson (NY Cornell Univ. Exp. Sta. Mem. 48:539-554, 1921), the intensity of the salmon silk color is related to that of the pericarp: 'A sm P' have salmon silks and 'A sm p [possibly P-WR?]' having brown silks. There was a report in the 1960 Newsletter (Kramer, HH, MNL34:111) of a salmon silk character appearing in progeny from UV treated pollen that showed good expression in the absence of red pericarp, but it was not stated whether the P allele was P-WW or P-WR. In my experience, sm expression is more variable with P-WR, but is usually good to adequate with only the occasional line showing 'brown' rather than 'salmon' silks. Occasional families show a dominance of the sm factor, but I have never been able to isolate the factor or factors that allow the dominant expression. Until a few years ago, I had never found an exception to the rule that P locus activity in the form of a P-RR or P-WR allele was required for sm expression. I now have P-WW sm stocks that consistently give good salmon silks. The sm allele is one that I obtained from the Coop stocks in 1973, and it has never shown any deviation in its expression to this point, and I have no reason to suspect that it has changed. I have derived r-g P-WW sm and bz2 P-WW sm lines from the original P-WW sm family, to confirm that the silk color is not derived from 3-hydroxy anthocyanins controlled by the R locus. Phenotypically, the P allele is unremarkable, with no evidence of any cob or pericarp pigment, but I have yet to test it with other sources of sm. It is possible that this particular P allele is unique in being active only in the silks, and that will be my next test in trying to track down the reasons for this apparent anomaly in sm expression.
to the MNL 67 On-Line Index
Return to the Maize Newsletter Index
Return to the Maize Genome Database Page