New alleles of A.

The alleles A^b and a^p, originating from Ecuador and Peru, respectively, are associated with brown, P‑determined, pericarp color (Emerson and Anderson, Genetics 17:503‑509. 1932). Both alleles are dominant to A (North American origin), which is associated with red pericarp color. Several mutants having intermediate plant color effects and exising spontaneously from A^b have a brown pericarp effect which likewise is dominant to the red of A (Stadler, News Letter 17:2O‑21. 1943). The divergent action of the A alleles of North and South American origin is revealed further in a series of dosage and dominance studies conducted by the author (Microfilm Abstracts 7: No. 1) and is being investigated further using exotic material collected from isolated regions of Peru and kindly supplied by the Pioneer Hi‑Bred Corn Company, Johnston, Iowa. Some results of the preliminary work are reported here.

1. Dominance effects of Peruvian alleles associated with full purple‑aleurone color (A‑P). Small progenies from individual, open-pollinated, Peruvian ears were planted at Columbia, Missouri, in 1945 and crosses were made on aa and on A‑. The progenies of the A crosses with those Peruvian plants which were shown to be homozygous for alleles determining full-purple aleurone color were planted at Ames, Iowa, in 1946. Since the A- plants in the 1945 crosses were either AA or Aa, two kinds of progenies were expected; designating the A‑P alleles carried by any individual Peruvian plant as A‑P₁ and A‑P₂ these progenies were expected to contain plants of the following genetic constitutions:

Cross (1945) Types in progeny (1946)

A/A x A-P₁/A‑P₂ A/A-P₁; A/A-P₂

A/a x A-P₁/A-P₂ A/A-P₁; A/A-P₂; a/A‑P₁; a/A‑P₂

Both types of progeny afford a test of the dominance effects of the Peruvian alleles, the first in compounds with the A allele and the second

in heterozygotes with both the A and a alleles. Crosses were made on

individual plants within progenies using aa Dt Dt plants as a pollen

source. Progeny type was thus distinguished by the presence or absence

of dots and this was also the basis for distinguishing A/A‑P from a/A‑P

plants within progenies of the second type. Seven such progenies representing the test of A‑P alleles of separate origins in Peru were classified for periarp color; the available data are summarized in the following tables.

		A/A - P
Family	Cross	red	brown
117	A/A x A‑P/A‑P	4	7
119	Same	9	14
120	Same	20	0
		A/A - P		a/A - P
Family	Cross	red	brown	red	brown
118	A/a x A-P/A-P	3	4	2	3
122	Same	4	5	2	1
123	Same	0	3	0	3
124	Same	5	0	2	0

In spite of the small numbers involved in these progenies it is obvious that the A‑P alleles of isolated origin are not similar in their effects on pericarp color. Moreover, in the cases of four of the seven progenies (all excepting families 120, 123 and 124) the two A‑P alleles associated in individual Peruvian plants show contrasted behavior. The data suggest that A‑P alleles, so far as these progenies represent them, are of two types: One determining red pericarp color and indistinguishable from A; the other determining brown pericarp color and having an effect completely dominant to that of A. There is no evidence for the existence of an A‑P allele having a brown pericarp effect which is recessive to A, unless it be found that the progenies of the red pericarp types in families 117, 119 and 120 segregate ears showing brown pericarp color.

2. Dominance effects and response to Dt among Peruvian mutants of the a^p and a type. Some of the Peruvian plants which were crossed to A tester in 1945 were not homozygous A‑P; six of the test cross ears gave 50:50 ratios for purple; colorless aleurone and two gave 50:50 ratios for purple; pale aleurone. In each of these eight cases, some of the seeds having colorless or pale aleurone showed dots. Since the tester parent was a^dt a^dt RR CC DtDt in constitution, the presence of these dots establishes with certainty that the colorless and pale seeds are due to mutant alleles at the A locus; if a dominant dilution factor or a recessive factor other than a were responsible for the dilution effects the seeds would be expected to be without dots. This apparently is the first report of the occurrence of recessive a in South American material; since five of the six Peruvian plants which were found to be heterozygous for a were of separate origin this mutant probably is widely distributed in Peruvian material. It is likely that these types failed to be recognized earlier because of the frequent occurrence in Peruvian material of the recessive forms of the genes R and C, which complement A in pigment production and because they may not have been studied in backgrounds providing the Dt gene which is specific for a.

The action of the pale mutants (designted a^p‑P) was studied further in progenies providing the combinations a^p‑P/a and a^p‑P/A. In the cases of both pale mutants, the combinations with recessive a were invariably associated with brown pericarp color, as were those with A. To test the response of the a^p‑P alleles to the Dt gene, crosses were made between a^p‑P/a and the tester a^dl a^dl Dt Dt (the a^dl gene does not mutate under influence of Dt). Without exception the pale seeds (a^p-P/a^dl, Dt) on ears from these crosses were without dots, whereas colorless seeds (a/a^dl, Dt) on the same ear were dotted. Hence, both a^p‑P alleles are similar to a^p in their pericarp color effects and response to Dt, though they may differ from each other and from a^p in the matter of their determination of plant and aleurone pigmentation.

Similar studies are in progress with the six Peruvian a mutants (designated a-P). The liniited data which are available at the time point to a divergence in type of action within the a‑P group as well as between members of that group and recessive a. All six members are associated with brown pericarp color as determined in heterozygotes with a. Dominance effects in compounds with A have been determined for only two of the six mutants but in both cases there is complete dominance over the red effect of A. This is the first knowledge of an a allele which is associated with colorless aleurone and brown plant color, in which respects it is recessive to A, and yet shows complete dominance to A in its effect on pericarp color. Of the four a‑P mutants tested for response to the Dt gene, one proved to be dottable, the other three being without response. The two mutants mentioned as showing dominance to A in pericarp color effect do not respond to Dt. Except for the products of X‑ray and ultraviolet treatment there are no past reports of a mutants which fail to respond to Dt; Rhoades (News Letter 15: 6. 1941) describes an a mutant which is indistinguishable from a with the exception that it shows much reduced response to Dt, but this allele, unlike the a‑P alleles, is recessive to A in pericarp color effect. The lack of response to Dt reported here for three naturally occurring a‑P mutants suggests that the failure to dot in the presence of Dt is not a valid criterion of deficiency at the A locus.

The evidence reviewed here adds to an already complex picture of gene action at this locus. Most significant, from this standpoint, is the evidence on the extreme antimorphism of at least two of the a‑P alleles. The antimorphic effects of certain of the A alleles have been reviewed previously (Microfilm Abstracts 7: No. 1). The evidence is not in suppprt of certain hypotheses, notably those of Wright and Stern, which have been advanced to explain antimorphic effects. It is suggested that the antimorphic behavior of the alleles of A may be explained on the basis of an hypothesis which holds a single gene capable of entering into two different reactions. It is the purpose of further investigation of the Peruvian alloles reported on above to provide additional tests of this hypothesis.

J. R. Laughnan