1. Mutation at the P locus.
A. Variegated pericarp in heterozygous combination with red cob, colorless pericarp (V/WR) shows a higher frequency of mutation in grade of variegation than homozygous variegated (VV). R. A. Emerson reported this fact in 1921. It is confirmed in the present studies with stocks which are believed to be unrelated to those which Emerson tested. The procedure used was similar to Emerson's. V/WR plants were selfed and the offspring classified for (1) grade of variegation and (2) cob color. Variegation grade was scored against a set of standard ears varying from class 1 in which each kernel, on the average, showed a single stripe of red pericarp, to class 7 which is heavily striped. The mean grade of variegation on the V/WR ears was about 1.5 classes, on the average, above the corresponding value for the VV ears.
B. Different colorless (or near‑colorless) pericarp alleles, or genes closely linked with them, differentially stimulate the mutation of variegated to self color in VW plants, as R. A. Emerson reported in 1929. Our procedure was to cross a series of red‑cobbed, colorless (WR) pericarp inbred field corn strains with inbreds Oh4OB and Wisc67C which are white cobbed with colorless or near‑colorless pericarp (WW). These WR/WW heterozygotes were then pollinated by variegated. The test cross offspring carrying the red cob allele are red‑cobbed and those receiving the white cob allele have variegated cobs, and, thus, may be distinguished from each other readily. The red‑cobbed alleles (or genes linked with them) carried by inbreds WF9, WiscM13, Wisc22 and Wisc38 tested in this way against the white‑cobbed Oh40B allele gave definitely lower average grades of variegation and fewer mutations from variegated to self color than did the white-cobbed Oh40B allele. Similar tests involving Wisc67C and a series of red‑cobbed, colorless pericarp alleles from inbreds A334, 111B, 79A, 153, 355, and 143A showed smaller differences. In certain combinations the WW heterozygotes had the higher variegation grade and in other combinations the reverse was true.
C. The self colors (red and orange) at the P locus, in contrast to variegated and mosaic, represent relatively stable alleles. Certain of them, however, mutate in somatic tissue to alleles lower in the dominance series with measurable frequencies. Preliminary studies show that the mutation rates vary widely. The extent to which the differences are due to the P alleles themselves and to modifiers is not known. The procedure followed was to cross the self‑colored stock to a colorless (or near‑colorless) pericarp strain and then to score the kernels on the F1 ears for number of colorless sectors one millimeter wide or larger. Mutation rate is expressed as number of such mutant sectors per 1000 kernels. A self‑red of uncertain origin but believed to have come originally from a farmer’s open‑pollinated variety showed a mutation rate of .02, based on about 880,000 kernels. Two plants sampled from the Strawberry Pop (red pericarp) variety differed greatly. One gave no mutations; the other showed a rate of .41. An orange stock of Cornell origin gave no mutations in about 50,000 kernels classified. Another orange derived from a mutant sector on an ear from a red‑cobbed, white pericarp field corn hybrid showed a rate of .05. No mutations were found among 7000 kernels of a self-red derived by mutation from a “Crow Creek" variegated pericarp. On the other hand, two separate mutations from variegated to self‑red in an unrelated variegated stock gave rates of 1.9 and 3.4 based on about 512,000 and 313,000 kernels, respectively. These latter values contrast with mutation rates of about 10‑70 per 1000 kernels observed in the limited number of variegated pericarp stocks which we have scored for mutations from variegated to self‑red. Most of the mutant sectors on the red and orange ears involved less than a single kernel. Because of this circumstance, data concerning the qualitative character and heritability of the changes are difficult to obtain.
D. Methyl‑bis (b‑chloroethyl) amine applied to pollen in the vapor phase is not effective in altering the mutability of variegated pericarp. Two levels of treatment were testd, the higher of which approached the lethal dosage. The pollen treatments were severe enough to result in numerous partially shrivelled kernels on the ears to which the pollen was applied. Pollen from a waxy VV stock was used, waxy serving as a means of detecting contaminants. The control and treated lots of pollen were applied to a red‑cobbed, colorless pericarp single cross between two dent inbreds. The ears borne by the F1 plants were scored for (1) grade of variegation and (2) number of mutations to self‑red. The respective scores for the controls and the treated series did not differ significantly from each other. The sub‑class in the treated series comprising the plants reared from the partially shrivelled F1 seeds likewise did not differ significantly from the controls in variegation grade and frequency of mutations to self‑red.
E. In order to facilitate their studies on mutation at the P locus the Wisconsin group is interested in securing seed of the mutant types which appear occasionally in commercial corns. Seed from ears bearing patches of mutant kernels are especially desired.
R. A. Brink