4. Mutation to unstable alleles.
The unstable mutants of group A, as far as they have been studied, give substantially the same data and are therefore considered collectively. In each instance the mutant arose as a single plant in progenies that were screened for mutations. They were detected by the absence of coloration on the first emerging leaf tip, an area regularly colored in the rch stock. During seedling growth sectors for anthocyanin were observed on the epicotyl, coleoptile, leaf sheath, and blade. The appearance of the sectors was such as to indicate that the colored areas were related in cell lineage. Sectors of independent lineage were found on the same seedling in many cases. At flowering, some sectors were found that extended to the tassel. Branches with colored glumes threw anthers that were full purple to variegated in color, while green glumes outside the sector yielded anthers wholly green in color, or at best very faintly stippled with an occasional speck of anthocyanin. Where plants carried the gene Pl a bizarre sectoring of the pericarp was observed, similar in general aspect to the phenotype given by occasional ears of the factor Pv.
In the F2 progeny of the selfed mutant (rch-Sx/rg) approximately ¼ were wholly green. Presumably, these represent the rg homozygotes. Among the remaining plants three classes of phenotypic effect were observed; those fully rch in phenotype and indistinguishable from the original parental allele, those similar in pigment pattern to rch but with a much reduced level of effect, and those which were sectorial in appearance. The size and frequency of sectoring varied widely in different plants.
The newly derived rch and the intermediate, now designated rch-lt, show normal transmission rates in both male and female germ lines. Progeny tests totaling some 75 plants for either derivative gave no evidence of sectoring. Tentatively, they are presumed to be reverse mutations from an unstable allele similar in phenotype to rg. In the one case, the reversion is complete, and in the other only partial. Partial reversions constitute an available source of now intermediate mutations. The data is not sufficient to indicate whether these intermediate reversions are all alike or whether they represent different levels at which the unstable form can become fixed.
The behavior of variegated plants on outcrossing is instructive as to the nature of the sectoring process. A particularly favorable tassel sector was found in an rch-S3/rg plant. An outcross test to sib ears of the Rr-Catspaw/rg strain was made using pollen from red anthers within the sector and also from green anthers just outside the sector. Among 70 plants grown from colored and colorless seed, from the cross involving green anthers, only two were found to be variegated. Sectors on these plants were small, infrequent, and not observable at all stages of growth. In a progeny of similar size and constitution, from the cross made with pollen from red anthers, 29 of the 70 plants showed sectoring or full rch color. By far, the majority of plants were sectorial rather than self-colored. There can be little doubt that the sectoring is a cell-specific phenotypic indicator for an altered state of the locus.
Tests of the male and female transmission rate for the sectorial quality show that different plants diverge widely in the extent to which they transmit this phenotype to their progeny. In one case, of 66 plants, half of which were expected to show the sectorial phenotype, only one sectorial was found in the test of female transmission and three in the test of male transmission. In a second instance, in a total of 163 plants, where half were expected to show the altered phenotype, 77 were found to be either rch or sectorial. The data also suggest that the rate of complete reversion is somewhat higher in the pollen than it is in the female germ line.
All data considered, it would seem as though a variable factor exists which controls both the time and frequency of mutation at the newly established unstable locus.
Seymour Fogel - Brooklyn College, Brooklyn, N.Y.
Elena Perak - Buenos Aires, Argentina