The timing of germ line Mutator activity has been subject to considerable study. Last year, I summarized some data that bear on this question (MNL 60:12-14, 1986). One suggested time for Mutator activity was early in development. If Mu induces mutations at this time, occasional large somatic sectors containing a mutant allele will be expected. To date, no sectors of this type have been demonstrated for a known gene locus. However, last summer we made a large number of exact reciprocal crosses between Mu2 Y1 Y1 Wx Wx Gl1 Gl1 and non-Mu y1 y1 wx wx gl1 gl1 stocks. In one such set of crosses, when the Mu2 parent was used as a female, all seeds were yellow starchy (n = 65). In the reciprocal cross (i.e., Mu2 parent used as the male), however, there were observed 216 yellow starchy seeds and 106 white starchy seeds. Evidently two-thirds of the tassel shedding at the time this pollination was made consisted of a sector carrying a y1 mutant allele. Because half of the alleles in this sector are Y1, this sector must have contributed 106 Y1 alleles to the next generation. Thus, the portion of the tassel not included in this sector must have contributed the remaining Y1 alleles (110). Since there are twice as many Y1 alleles in this sector, it must be only half as large as the mutant containing sector. This mutant sector was not observed in the ear, thus, the mutation must have occurred after the cell lineages giving rise to the tassel and ear diverged but early enough in development to give rise to a significant portion of the tassel.
There is no way to determine by seed phenotype if this is a Mu-induced or a spontaneous mutant. We will test this mutant to determine if it is a mutable, temperature-sensitive, pastel allele. A positive result would suggest that this was Mu induced because most of the Mu-induced y1 mutants are of this phenotype (see another report in this issue). Final determination that this is a Mu-induced mutant will depend upon its molecular characterization.
Donald S. Robertson
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