Comparison of Xray and UV Induced Mutations of A. Mutations and deficiencies involving the A locus may be identified by seedling examination of F1 plants from the cross a x A B Pl Rr. A very large number of plants of this constitution have been examined following treatment of the male parent with Xrays, and the green seedlings saved for identification of the mutation or deficiency. The majority of such plants turn out to be distinctly defective in growth and to have segregating aborted pollen. A small proportion approximate normal growth, but these also have defective pollen. Among them a few are found with segregating pollen of the subnormal type. Two plants were found in which the A effect had been lost, the plant was of normal vigor, and the pollen was completely normal in appearance. Both plants had the phenotypic appearance of typical a B Pl. They are designated aX4 and aX6. In addition one plant of A B Pl phenotype and normal vigor, but with segregating subnormal pollen, was included in the further tests. It is designated aX1.
In similar progenies of plants from UV treated pollen, the frequency of loss of the A effect is very much lower, as noted in connection with the experiment first described. Such plants may be found, however, by growing large enough progenies of F1 seedlings, and we have so far identified
about fifty of them. Among these, four individuals showed loss of the A effect but fully normal pollen. All of the others had aborted pollen, and in all cases this was empty or nearly empty. Three of the four mutants showed the phenotype of a B Pl. They are designated aU3, aU15, and aU18. The fourth mutant, though green as a seedling, showed faint anthocyanin coloration in later growth and deepened to a light purple at maturity. It is designated Alt.
The chief characteristics of these induced mutants, with reference to the criteria which have been mentioned, are as follows:
(1) Phenotype. Except in the case of Alt no consistent difference has been found in the phenotype of the mutants and that of a. In all six the aleurone is wholly colorless with C R A2. and the plant is typically brown with B Pl. The pericarp is red with A P but has not yet been seen with a P. With aU3 B Pl a considerable amount of purple pigmentation was observed, chiefly in the upper half of the lower leaf sheaths, but similar coloration has been found in a B Pl plants extracted from the same culture. In segregating progenies from amutant/a x a B Pl and amutant/a x ap B Pl, it was not found possible to distinguish the mutant a from the standard a in any of these six cases.
The phenotype of Alt is clearly distinguishable from A, a, and ap in plant color, but it is not always distinguishable from ap in aleurone color. The plant color at maturity (with B Pl) is more similar to A than to ap, and the plant does not appear brown at any stage. The cob is reddish purple. The extracted pigment includes a considerable quantity of anthoxanthin as well as anthocyanin. The purified anthocyanin is distinct from both chrysanthemin (A) and the anthocyanin of ap.
(2) Gametophyte viability. AX1 is transmitted through female germ cells but in reduced proportion, seldom in more than 30 per cent of the expected number. Seeds heterozygous for the variant are reduced in size. There is no transmission of the type through pollen of the heterozygous plant.
aX4 and aX6 show full viability in the female gametophyte, and the seeds are full size. Although the pollen in both these types is fully normal in appearance, transmission of the mutant is reduced in pollinations from heterozygous plants, ordinarily to 25 to 40 per cent of the expected numbers.
Self-fertilization of A/aX6 plants yields no colorless seeds, even though the same pollen used on a C R testers both before and after selfing shows transmission of the mutant aX6. This type therefore appears to be zygotically lethal when homozygous. The same result is obtained with aX4, though the trials in this case are less extensive.
aU3, aU15, and aU18 show full male and female viability and transmission. Alt is also fully viable in male and female gametophytes and regular in transmission.
(3) Relation to Dt. The reaction to Dt is determined chiefly by examination of the aleurone of seeds produced by the cross amutant/ap Dt Dt x adotless Dt Dt in comparison with sister ears of a ap Dt Dt similarly pollinated. Supplementary determinations have been made in other ways.
None of the mutants show regular dotting comparable to that of a. Occasional seeds may show a single dot, but this may be ascribed to the adotless tester as well as to the amutant. Evidence on dotting in the homozygous amutant Dt combination is still scanty and has shown no dots so far.
L. J. Stadler, J. W. Cameron, K. O. DeBoer, Herschel Roman