"Reversion" frequency of putative Mutator induced waxy alleles

Twelve plants from a putative Mu induced sector of 13 seeds were scored for wx "reversions" in the pollen. (Please see the previous paper for the definition of "reversion", for a description of the pollen staining techniques and for the standard waxy allele "reversion" frequencies.) The origin of these putative Mu-induced new waxy mutants has been reported in the previous article entitled: "Putative forward mutation frequencies at the Y1 and wx loci in the presence of Mu.

Plants of this sector are heterozygous for new mutant Mu induced alleles and the standard wx allele. This would create a heteroallelic situation in which, in many instances, previous workers have reported results of elevated "reversion" frequencies as compared to those found in homoallelic plants. This elevated rate is thought to be due to intragenic recombination. Such an elevated frequency is not observed here (Table 1). In fact, the frequency in the heteroallelic plants is about half that seen in the wx wx Mu plants (Table 2, previous paper). Thus, the observed "reversion" frequency in these Mu plants heterozygous for the standard waxy and putative Mu-induced waxy alleles can be explained on the basis of the action of Mu on the standard allele alone (Table 2, previous paper). This suggests that the putative Mu-induced waxy allele is not "reverting" at an appreciable frequency nor is there intragenic recombination occurring.

Two plants from single seed mutants from different ears (Table 2) were screened for the Wx "reversions", and it was observed that not only did plant 8109-1 have a higher frequency than any single plant in this study, but it also was higher than has been reported for waxy alleles in any other homoallelic or heteroallelic condition. This high frequency could be due in part to the heteroallelic condition of this plant. But this is probably not the full explanation. Because mutable mutants occur in about 40 percent of all Mu-induced mutants, it could be that the allele of 8109-1 is unstable (mutable) and that this instability is expressing itself in the germ line. The somatic mutability observed for most Mu-induced mutable mutants is very late and is frequently a rare event. Thus, we probably would not recognize a mutable waxy seed visually without staining. Since we did not want to run the risk of impairing the germination of these new waxy mutants, the seeds were not stained with iodine. A selfed ear and an outcross to a y1 Y1 wx wx stock did not reveal any wx seeds, but the number of seeds obtained was small. After this mutant has been increased, more extensive studies of its germ line stability as measured in both pollen and seed can be undertaken.

The other plant, from a single seed mutant 8109.1-1, was observed to be very stable. Why "reversions" to the wx phenotype in this plant did not take place is not known.

In summary, it seems that two classes of putative Mu-induced waxy alleles have been found. Those that appear to be quite stable and those that are apparently germinally unstable.

Table 1.

Solomon K. Sackitey and Donald S. Robertson

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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