1. Some Alleles of R. Detailed phenotypic comparisons were made between R alleles derived from relatively unrelated individual plants. The original stocks were mostly of strains cultivated by various American Indian tribes, specimens of which were supplied by J. H. Kempton. Twenty-two alleles with colored aleurone and colored plant effects (Rr series) were included (abstract in Genetics, 28: 90-91). In addition a number of alleles of the rr series are included in a later parallel study.
The effect of different R alleles upon plant color differs widely, as to both intensity and distribution of pigmentation. Since the associated independent effect upon aleurone color provides a completely linked marker, it is possible to identify even very slight differences, due to the R alleles, as distinguished from the effects of modifying factors.
The series is non-linear, in that various cases occur in which one allele produces distinctly more effect than another upon some tissues and distinctly less upon others. Such cases might be expected to occur if the alleles differ only in the extent of their effect upon some single reaction, for it might be expected that pigmentation would increase with "strength of action" up to a given point and then deline, and that this optimum point might differ in the various tissues concerned. The effects observed do not fit this hypothesis in any reasonably simple form. They suggest rather that the effect of R alleles upon plant color is a complex of two or more types of action, independent in the sense in which the aleurone color effect and the plant color effect are independent.
For a major portion of the plant color effect, however, the reaction of different tissues is quite closely correlated. The Rr alleles may be arranged in a single sequence to represent their effect upon occurrence and intensity of pigmentation in mesocotyl, coleoptile, seedling leaf tip and margin, seedling leaf sheath, mature plant basal sheaths, tassel glume, and anther. For example, the occurrence of seedling leaf tip color marks a level beyond which full anther color is developed and below which anther color is distinctly weak. Full coleoptile and mesocotyl color are reached below this level, though the color of these organs is deeper and more rapidly developed in the types with tip color. Distinct seedling sheath color does not occur until a higher level is reached, and is accompanied by deepened coloration of the tassel glumes and anthers. In their effect upon this character complex the R alleles studied may be regarded as differing merely in level of action, and the varying thresholds of response in the tissues studied provide a sensitive means of detecting differences in the level of action of the alleles compared.
L. J. Stadler and Seymour Fogel