1. Comparison of Xray and Ultra-violet Mutations of A. The origin of the Xray and UV mutants compared in this study, and observation on their phenotypic effects, viability and reaction to Dt, were given in the last News Letter. All three Xray mutants showed more or less reduction in gametophytic viability and were zygotically lethal; all four UV mutants were fully viable, regularly transmitted through male and female germ cells, and readily established as homozygous recessives.
This suggests that the Xray mutants are probably deficiencies too small for cytological identification and too slight in effect to be lethal in haplophase, but it leaves open the possibility that they are alleles of a with lowered viability.
With losses too small for cytological detection, the only proof of deficiency is genetic evidence of the loss of associated loci. McClintock's study of Bm ring-chromosomes showed the possibility of identifying loci n a deficiency through their effects upon tissue within a sector made homozygous deficient by loss or modification of the covering ring.
We were fortunately able to obtain a ring including the A locus. The origin of this ring is an interesting story in itself, but it will not be included here. The ring carries the gene Ab, and its behavior is similar to that described by McClintock. It is maintained in a stock otherwise homozygous for a. Crossed on standard a stocks it gives sectors of a tissue in both the aleurone and the plant.
Ring bearing plants otherwise homozygous for the Xray mutant aX4 were obtained for comparison by crossing and backcrossing as follows:
(1) aX ap x a a Ab-ring
(2) aX ap x aX a Ab-ring
(3) aX ap x aX aX Ab-ring
Cross (1) gives mostly pale and colorless seeds, but also a considerable number of colored seeds, all of which are mosaic for pale or colorless. These are the ring-bearing individuals. Cross (2) yields mosaic colored seeds similarly, but among them there is included a new class in which the mosaic regions are of shriveled, degenerate tissue. These are the aX aX Ab-ring individuals. In cross (3) this class comprises nearly half of the mosaic seeds. The remainder (without degenerate tissue) are all phenotypically ap in the mosaic regions, and represent the aX ap Ab-ring class.
The sectors produced in plants grown from these two types of seed are very different. In the plants with ap the sectors are of wholly normal tissue, lacking only the anthocyanin characteristic of Ab. They include both large and small sectors. In the plants homozygous for aX the sectors are small, and many show reduced growth leading to distorted development of the plant. Their most conspicuous feature is lack of chlorophyll. These sectors, whenever they occur in regions in which anthocyanin develops, show normal anthocyanin. In other words, they do not show the loss of Ab. Very rarely a sector is found with loss of anthocyanin and with no loss of chlorophyll. In four cases we have found narrow sectors showing loss of both anthocyanin and chlorophyll, and each of these occurred as a secondary sector within a larger sector showing loss of chlorophyll without loss of anthocyanin.
We interpret this to mean that the mutant aX4 represents the loss of not only the A factor but also of a separable factor essential to chlorophyll development, and possibly of another essential to tissue survival. If the sectors showing loss of chlorophyll without loss of anthocyanin have the genetic constitution indicated by their phenotype, the separable viability factor must be assumed. The absence of primary sectors showing loss of both chlorophyll and anthocyanin would indicate that simultaneous loss of the two factors is lethal, while the occurrence of sectors deficient for both as a result of consecutive losses would show that the lethal effect is not due merely to deficiency of these two factors. It would therefore have to be ascribed to a separable portion of the ring which is regularly eliminated when A and the chlorophyll factors are lost simultaneously. It is possible however that the sectors are in fact deficient for Ab. Their anthocyanin pigmentation is normal, but since the sectors are small it is possible that this may be a result of diffusion from the neighboring non-deficient tissue. If this is true, the assumption of a viability factor separable from A and the chlorophyll factor is not required.
The description given above for aX4 aX4-Ring plants applies also to the compounds aX4 aX1-Ring and aX4 aX6-Ring. This shows that aX1 and aX6 also lack the associated factor or factors. We have not yet succeeded in producing a plant which could be proven to be homozygous aX1 aX1 Ab-ring or aX6 aX6 Ab-ring. It is possible that both aX1 and aX6 involve more loss than aX4. aX6 is distinctly lower in male transmission than aX4, while aX1 is distinct from both in having visibly defective pollen and no male transmission. The most extreme mutant, aX1, reduces crossing-over between A and Et, though there is no visible indication of deficiency in the pachytene chromosome.
The results indicate that the apparent mutations of A induced by Xray treatment are in fact minute deficiencies. The original series of Xray-induced A-losses from which the mutants were selected included, in addition to obvious extreme deficiencies, several less defective plants with segregating pollen not wholly aborted but distinctly sub-normal in development. aX1 a was a representative of this class. The A-losses with normally developed and partially functional pollen, aX4 and aX6, apparently represent simply the extreme of the continuous series of intercalary deficiencies of varying length induced by Xray treatment.
On the contrary, the UV mutants, aU3, aU15, and aU18, similarly tested with the ring-chromosome, behave precisely as do the standard alleles, ap and a, and their sectors are phenotypically identical with those of standard a.
The UV mutants, unlike the X-ray mutants, appear in the F1 from treated pollen as a class distinct from the deficiencies produced by the treatment. The series of UV-induced A-losses included, in addition to the three mutant a's and the intermediate allele Alt, a large number of extreme deficiencies with distinctly defective growth and aborted pollen, but none of the intermediate type with subnormal pollen. This may be due to the rarity of intercalary deficiencies induced by this agent. Although it is reasonable to assume that intercalary deficiencies may sometimes be induced by UV (since translocations are), it is clear that the UV mutations are much too frequent to be accounted for in the way suggested above for the Xray mutations. If the UV mutants are deficiencies they are deficiencies of a different order. They show no difference from standard a except in their failure to mutate under the influence of Dt. As previously stated (News Letter 1941: 45), this is not convincing evidence against intragenic mutation.
L. J. Stadler and Herschel Roman