2. New Alleles of A. As previously noted (News Letter, 1941: 44) the gene Ab mutates spontaneously at a fairly high rate to a type resembling ap. The mutants, identified by the pale aleurone effect, produce plants which like ap produce both anthocyanin and anthoxanthin pigment. Nine of the mutants were checked for the dominant brown pericarp effect present in Ab and aP, and all showed this effect also.
In plant color with B and Pl, the mutants were in general more deeply colored and more reddish than the standard ap. They varied rather widely in degree of redness, ranging from a deep brown to a maroon shade approaching purple at maturity. The original mutants and various others which have occurred in later experiments with Ab, form an apparently continuous series between the two extremes. No mutant of Ab to a colorless aleurone type or to a type producing only anthoxanthin pigment in the plant has been found.
Four representative mutants were selected for further study, to determine whether the differences in expression were due to differences in the mutant alleles. The factor et, an X-ray induced chromosome 3 mutant, located 11 units distal to A, was combined with one of the mutants and also with standard ap, and the phenotypic effects were compared in backcross progenies in which the various alleles could be compared in plant color (with B and Pl) in sib plants. The results show that the four mutants represent distinguishable alleles of A, each producing a mixture of anthocyanin and anthoxanthin pigments but differing in the relative quantity of anthocyanin produced. These are designated mahogany (Ab-m), cedar (Ab-c), chestnut (Ab-ch) and walnut (Ab-w).
The aleurone color of the mutant Ab's described, as identified in et-marked segregations, is paler than that of Ab or A, but not so pale as ap. Seed separation may be made effectively in segregations against either A or ap. There is also a recognizable difference in aleurone color between some of the mutant types, which sometimes is distinct enough for individual classification.
There are some interesting differences in the action of these pale aleurone mutants of Ab and the two pale aleurone mutants at hand which arose from other members of the A series. Alt (News Letter, 1941: 46) is an ultra-violet mutant of A, which has a pale aleurone and reddish purple plant color, yielding anthocyanin and anthoxanthin pigment. Aw is a mutant of a, which occurred as a sector with pale purple anthers in a plant of a Dt B Pl Rr. It also produces pale aleurone and a reddish plant color, yielding anthocyanin and anthoxanthin. Qualitative tests show a distinct difference in the anthocyanin produced by Alt and Aw, on the one hand, and by Ab-m, Ab-c, Ab-ch, Ab-w, and ap on the other.
The pale Ab mutants, like ap, show little or no difference in the aleurone color of homozygous seeds vs. seeds heterozygous for a. Both Alt and Aw, in selfed ears of plants heterozygous for a, show clearly cumulative effects, the heterozygous seeds being distinctly pale and the homozygous seeds often being indistinguishable from full A.
In compounds among the pale Ab mutants and between these mutants and ap, the plant color effect of the redder member is distinctly dominant, and in those cases in which aleurone color is distinguishable the darker type is dominant. Alt produces a redder plant color than the Ab mutants or ap, but the hybrid Alt/ap is intermediate, with a pronounced increase in anthoxanthin content. Alt × ap/a yields progeny of two very distinct types, the Alt/ap plants showing a distinct dominant effect of ap on anthoxanthin production as compared with the Alt/a sibs. This dominant effect of ap is not evident in crosses with A or Ab, so far as the appearance of the plants is concerned. It is evident, however, in crosses with Abr, a Dt-mutant obtained by Rhoades, (News Letter, 1941: 6) which resembles A in plant and aleurone color but does not give red pericarp. In crosses of Abr × ap/a there is a distinct diminution of red and increase of brown in the plant color of Abr/ap vs. Abr/a sibs. A similar effect is shown by cedar, chestnut and walnut, the only Ab mutants tried in this combination. It is wholly absent in Abr × Alt/a, the Abr/Alt plants being indistinguishable from the Abr/a sibs.