2. Mutation to a stable intermediate, rch-V10.
The single mutant in group B, rch-V10, is of considerable interest. It represents what is probably the first mutation to an allele of intermediate effect within the rr series. Though it arose in progeny of a mating effected with ultra-violet irradiated pollen, there is nothing to indicate that it is an induced alteration rather than a coincident spontaneous mutation. Further quantitative studies are clearly needed.
From seedling on, and just prior to flowering, plants carrying this allele in the homozygous state are nearly devoid of anthocyanin in all tissues. At tasseling and dehiscence, however, an intense depostion of pigment occurs in the tips of the staminate glumes. After flowering, a much reduced generalized coloration of the glumes is evident. In addition, considerable color is found in the silks, and also in the pericarp of ears known to carry the dominant form of the gene pl. These tissues are colored to about the same extent in plants carrying either the parental rch factor or the mutant rch-V10, though perhaps somewhat reduced in the latter. Objective tests of this suspected difference in action will be carried out in backcross progenies segregating for both alleles. Thus, rch-V10 represents a mutational change, the effect of which is permissive of coloration in some tissues pigmented by the parental rch gene, but fails to do so in others. It follows therefore that coloration of the silks, pericarp, etc. is indicative of a gene-controlled reaction more or less independent of the action leading to pigment synthesis in other tissues. It was in this sense that independent coloration of the plant and aleurone led to the opinion that the action of Rr was due either to separable, more or less independent components of action of a single reduplicating unit, or to the action of two completely linked genes or subgenes of similar though divergent effect. The mutational origin of rch-V10 seems to indicate that the plant color complex may be resolved into still other components of action.
Analysis of the action for Rr alleles has already shown that color intensity in the silks and pericarp vary independently in some alleles. It is not unreasonable to suppose therefore that single gene mutations might affect pigmentation in the one tissue, and not in the other. Rr alleles which yield wholly green plants but for coloration of the glumes or silks are described in the early literature. An allele yielding a green plant but with cherry pericarp effect has not yet been found. However, in light of the above, one might predict that type alleles of this nature are derivable from rch-V10 or that they may already exist in the pool of allelic variability found in cultivated races.