A highly mutable allele of A1.
A new a1 like allele, which is extremely mutable in the presence of the gene Dt but like a1 is stable with dt dt, was found as a single seed on a selfed ear of A1/a1 Dt Dt. It can best be described by comparison with a1, as reported by Rhoades (1941).
In the presence of Dt, the more mutable a (am) mutates to A1 alleles in all tissues which can be tested, giving many dots on the aleurone, a few red pericarp streaks with P, and many purple plant sectors with B and Pl. When the aleurone dots of am and a were compared, on sib seeds of am as as Dt Dt Dt and a as as Dt Dt Dt constitution, the following differences were evident. The a seeds had an average dot number of 47 while the am seeds had 1500± (this count is the nearest estimate because the am dots were so frequent that many adjoined and could easily have been miscounted). The standard a dots were circular, regular in outline, relatively uniform in size, and mostly rather dark. The am dots, on the other hand, were angular, irregular in outline, and ranged widely in size, from single colored cells, through all intermediate sizes up to whole seed cases. Four cases have been found where the colored sectors included 1/8 to 1/4 of an ear. Most of the am dots also had a sharply defined border, and were distinctly lighter than full colored (including, however, some very dark and sone very light ones). There was also a rather high frequency of colorless non dotted partial seed sectors representing somatic mutations to as.
These differences in frequency, size, color, etc., of aleurone dots, have a parallel occurrence in other tissues (such as sheath, husks, and anthers), when compared in sib am am Dt Dt and a a Dt Dt plants. The sectors conform to cell lineage and are so frequent in am Dt plants as to give a blending effect on the sheath. The pericarp, however, shows little difference, both genotypes having a few small red sectors on each brown pericarp ear.
A great number of independent mutants of diverse types have been obtained from am through anther sectors, ear sectors, and single seed cases. Most of these have not been tested, and in many cases exist only as single seeds.
Eleven colored mutants obtained from anther sectors have been tested for plant and pericarp color. These included two A's, three Arb's, five abr's, and two Alt's. This sample, though small, is in contrast with the twenty nine mutants obtained from a, by Rhoades, which included 27 A's, 1 Arb, and 1 Abr. It indicates that the mutants from am group about a level near Abr and are considerably below the level of most of those from a. This agrees with the previous observation that am dots are mostly less than full colored and that there is little difference in the frequency of red pericarp sectors from a and am.
The majority of mutant alleles seen have been stable. Tests of the lighter A's, for dotting, in the presence of Dt, showed complete stability in all but two cases. These two single seeds from a cross of as as Dt Dt by am am Dt Dt had a typical solid pale background (one was very dark) with frequent (180+) clear dark dots and with a few colorless dots. One of the colorless sectors was large enough to include three full colored dots within its borders, indicating a possible mutation back to a mutable form of a.
The full colored mutants could not be tested for dotting but their germinal mutation rate to a could be measured. A test of five of these extracted A's gave no a mutants in 47,755 gametes.
Five large ears, two from one as mutant and one each from three others, were carefully examined under a microscope, and not a single colored cell was found.
There occurred also a second type of unstable allele. Nine independent, less mutable am seeds (designated am‑) which had 150–300 dots on each instead of the usual 1500± have been found. These, however, could be mutants of Dt to dt in the female germ cells giving rise to a dt dt Dt endospem.
To determine the frequency of occurrence of the various mutant levels seen, a cross of am am Dt Dt by as as Dt Dt was made. The ears were examined for single seed cases, avoiding grouped mutants which would indicate a sector. Of 15,000 gametes tested, 57 were A (this included the classes A, Arb, Abr, and some of Alt, all of which would appear as fully colored seeds in this cross), 4 were clearly Alt, 2 abr, 7 am‑, and 20 as.
M. Gerald Nuffer