Definition: The term reversion in this report refers to the appearance of a seed or pollen grain with the dominant starchy phenotype in stocks that originally had only recessive waxy alleles. Reversion is not synonymous with the term "back mutation" since we have not demonstrated if the mutant waxy or y1 alleles have indeed been restored to an allele responsible for the wild phenotype (i.e., Wx or Y1). For convenience' sake, we will speak of wx and/or y1 "reversions", but-such terminology refers to phenotype only and the foregoing caveat must be kept in mind.
In an isolation plot, y1 y1 wx wx Mu seeds from outcross ears of plants that had been tested and found to have Mu were used as female rows. The pollen rows were y1 y1 wx wx plants without Mu. Both pollen rows (controls) and female rows were screened for yellow waxy (y "reversions") and starchy white (wx "reversions") seeds (See Table 1). The "reversion" frequencies of both y1 and wx are slightly higher in the Mu stocks. If Mu is acting at the wx and y1 loci, it would appear that the standard wx allele is more resistant to the action of Mu than the standard y1 allele.
Another way to measure the "reversion" under the influence of Mu is by determining the frequency of Wx pollen grains in wx wx Mu plants using the pollen staining technique. Waxy Mu lines were produced by crossing Mu stocks as a male to yellow waxy plants. The F1 mutator plants were again crossed to a yellow waxy. Most of the waxy seeds that resulted were wx wx Mu plants. A waxy line that has been used in our program for many years served as the source of the standard wx allele. Twelve standard waxy stocks with Mu and six standard waxy stocks without Mu (controls) were studied.
The pollen staining and counting techniques used were those described by Nelson (Genetics 60:507-524, 1968), and Yu and Peterson (Theor. Appl. Genet. 43:121-133, 1973) with minor modifications.
The results are summarized in Tables 2 and 3. The "reversion" frequency of the standard waxy stock with Mu (Table 2) was higher than that of the control (Table 3). The control pollen "reversion" frequency was higher than the control of the seed experiment (Table 1). A great deal of variability in the "reversion" frequency existed among the plants of the families in Table 2. In some, no "reversions" were observed. Since Mu is lost in about 10% of Mu outcrosses, these plants with no "revertants" could be those that had lost Mu. The outcrosses of all plants will be tested for the presence of mutator activity to determine if Mu loss can account for the difference in "reversion" frequencies. The waxy Mu stock (Table 2) has a higher pollen "reversion" frequency than the same genotype studied at the seed level (Table 1). The reason for the discrepancy between these two frequencies (i.e. pollen and seed) is not known. The wx wx Mu stocks in the two studies were in different backgrounds, and thus there may be background differences. The standard control stocks, however, were the same in the seed and pollen tests, and thus the higher pollen "reversions" frequency in the pollen controls than in the seed control cannot be explained by background differences.
The evidence from both seed and pollen studies suggests that Mu may be responsible for phenotypic "reversions". In our studies, "reversion" frequencies in pollen were higher than those observed in seeds. Perhaps "reversion" in many instances is associated with a pollen lethal condition.
Donald S. Robertson and Solomon K. Sackitey
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