1. Gene Variability. The study of R alleles which Fogel and I reported in the 1943 News Letter has been continued, with the addition of a series of rr types and with further study of specific modifiers of R action and of environmental conditions affecting it. All or nearly all of the 22 Rr's originally included appear to be distinguishable in their effect upon plant color, but since some of these differences are slight they require confirmation in experiments in which modifier action may be excluded more critically than is possible by repeated parallel backcrossing.
For this purpose we have used colorless aleurone mutants of several of the original Rr alleles, since as previously reported spontaneous mutations of Rr → rr have no appreciable effect upon the plant-color action. For example six Rr alleles (Boone, 997, Cornell, Quapaw, Ponca, and Black Beauty) form a group characterized by rather strong pigmentation, though distinguishable in parallel backcrosses by slight though consistent differences. Colorless aleurone mutants of Cornell and Quapaw were crossed with other members of the group, and backcrossed by rg. This yields progenies in which the Cornell or Quapaw phenotype may be compared with the phenotypes of similar alleles in sib plants, the aleurone color difference providing a completely linked marker. Such comparisons, so far as they have gone, confirm the reality of the small differences observed between members of this group. A similar method may be used for the study of "non-linear" variation in the action of the different alleles (News Letter 1943, page 20), and here the mutant rr's may be supplemented by naturally occurring rr's. We are using the latter chiefly for this purpose.
The alleles of B (News Letter 1943, page 22) appear to be fully as variable as those of R, and since the range in plant-color phenotype is even wider, they may be better suited to the identification of small differences. Among 14 Bw's compared, 6 were selected as standards to represent distinct levels spaced roughly between b and B, and in each of these a stock of B-gl rg was established. These alleles listed in ascending order of effectiveness, are designated as follows:
|1. Bw (Boone)||3. Bw (Clarage)||5. Bw (Lookout)|
|2. Bw (Young)||4. Bw (La Paz)||6. Bw (Seattle)|
Additional Bw's, both from existing stocks and from mutations of various B's, have been crossed each with the standard Bw-gl strains which appear to be just below and above them in effectiveness, and backcrosses of these hybrids will determine their position in the series. For further mutation work, Anderson's In2 (v4 B Gl lg) stock is being extracted in homozygous combination with rg since Bw mutations induced in this stock may be crossed with the naturally-occurring alleles to produce backcross progenies with virtually complete linkage of marker genes.
Miss Elizabeth Somers is making a detailed histological study of the development and distribution of anthocyanin under the action of R and of B.