To determine the influence of Mu on the frequencies of forward mutations at the y1 and wx loci, a large isolation plot was set up in which the seeds of the female rows consisted of Y1 Y1 Wx Wx Mu plants. Tests had established that the parents of these plants had mutator activity. Y1 Y1 Wx Wx control rows consisted of the two standard stocks used in the propagation of Mu lines. The pollen rows were y1 y1 wx wx gl8 gl8. The ears of the female rows were screened for white starchy and yellow waxy seeds. Using graph paper, ear maps were made of each ear with one or more mutant seeds. The map arbitrarily started with the paired rows to the left of the paired row with a mutant seed. The butt was the bottom of the map and the tip the top. The total number of seeds was estimated by shelling and weighing all shelled seeds. Numerous samples of about 1 lb were taken and the seeds counted to obtain the number of seeds per lb. These values were then used to calculate the total number of seeds. The results of these tests are shown in Table 1.
The most conservative estimate of the frequency for y1 mutants in Mutator stocks is 1.7 x 10-4. This is more than a 50-fold increase in the mutant frequency over that observed in the control. The wx mutation frequency in the presence of Mu was much lower (4.0 x 10-6). This is low enough to be a spontaneous mutation frequency for many loci. However, it is higher than the zero value found in a very large control population. It is obvious that these two loci have different sensitivities to Mu.
For both the y1 and wx loci there were ears in which more than one mutant seed occurred in close proximity on the ear (i.e., a sector). These are undoubtedly due to late somatic mutations. The size distributions of these sectors are shown in Table 2.
For the y1 locus the vast majority of events were singular. When sectors did occur most were small in size. This confirms earlier observations that Mu acts in late mitotic stages, or meiosis, or later. The one sizable sector (14 seeds) may be a spontaneous mutant. Most of the wx mutants, on the other hand, are found in sectors. Only four single waxy mutants were found. Could the large wx sectors be the result of spontaneous mutations not induced by Mu? This seems unlikely since large sectors would also be expected in the controls, and yet none were found. It is possible that loci not only differ with respect to Mu sensitivity, but they may also differ with respect to when in ontogeny a locus is sensitive to Mu.
One hundred three of the Y1 mutator-induced mutants were grown last summer, self-pollinated and outcrossed to a yellow endosperm standard. Seeds from the selfed ears were grown at 95 F. Under these conditions the pastel alleles of y1 (y1-pas8549, y1-mut and pb1) are pale green. These alleles, when grown at 70 F, are green. Fourteen of the y1 mutants segregated pastel seedlings at 95 F and only green seedlings at 70 F. One rather strange feature of the Y1 mutants was the absence of any mutable y1 seeds. In a random sample of Mu-induced seedling mutants, about 40% will be mutable. Yet in spite of screening every Y1 mutant under magnification, no mutable mutants were found. However, some of the pastel mutants were mutable (similar to y1-mut). Thus, there are indeed Mu-induced y1 mutables. Why were they not obvious in the endosperm, as is the case with y1-mut, which does have a mutable endosperm? The mutable endosperm phenotype of y1-mut has rather large sectors of yellow. On the other hand, Mu-induced mutable aleurone mutants, at several loci, have very small round spots. Such a pattern may not be readily recognizable at the y1 locus if the reversions are occurring primarily in the interior of the endosperm and at a low frequency. (As expected, all the ears segregated for glossy seedlings.)
Also, no obvious mutable waxy mutants were found. However, none of the waxy mutants were stained with iodine to check for mutability. This will not be done until the mutants have been increased. However, in the next report we present evidence that at least one of the waxy mutants might be mutable (i.e., unstable).
Table 1. Putative Mu induced y1 and wx mutants.
Table 2. Distribution of sector sizes for y1 and wx mutants.
Donald S. Robertson and Philip Stinard
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