Three mutable a1 mutants have been induced by the Mu system (a1-Mum1, a1-Mum2, and a1-Mum3). The frequency of somatic mutability is quite variable for all of these mutants. The timing of the somatic reversions is quite late for all of them and has not been seen to vary significantly for the several generations that they have been studied. No true germinal reversions have been found as yet but stable mutant phenotypes are quite common. Two stable mutant derivatives have been studied (a1-Mum1-stable and a1-Mum3-stable). Stable seeds were crossed to a purple aleurone stock and 9 of the F1 progeny were selfed or pollinated by a1 sh2. These F1 plants were at the same time crossed to a purple aleurone Mu stock (Pl Mu) and a non-Mu purple aleurone line (Pl aleur). The Pl Mu parent was outcrossed to a non-Mu purple aleurone line. None of the selfs or outcrosses to a1 sh2 of the Pl aleur/a1-Muml-stable crosses segregated for mutable seeds. Most of the selfs or outcrosses to a1 sh2 of the Pl aleur/ a1-Mum3-stable gave nothing but stable a1 seeds, but two ears had one medium mutable seed each. From each stable family, the outcrosses from two plants that had not segregated for any mutable seeds on the selfed or outcrossed ears were grown in the winter of 1983-84. Fifty seeds of each cross were planted and the selfed ears scored for mutability and the segregation of new mutants. The occurrence of new mutants would indicate Mu activity in one or both of the parents. A 5-class-scale measure of mutability was used in classifying somatic mutability: 1 stable, 2 low mutability, 3 medium mutability, 4 high mutability and 5 self color (purple). The results are found in Table 1. In column 4 we have a measure of the somatic mutability after the stable derivatives were again outcrossed to Pl aleur and selfed. The mutability value is the average for all a1 seeds produced on all the ears of the outcross progeny. The numbers in parentheses indicate the total numbers of seeds scored. It is obvious that the stable phenotype is retained through these outcrosses. The results in column 5 reveal whether or not the stable a1 parent had Mutator activity. Since in none of the selfed progeny of these crosses were any segregating mutants found, there is no evidence that these stable lines have retained Mutator activity. Column 6 measures the mutability observed in a1 seeds of the Pl Mu/a1-Mum-stable crosses; in three of the crosses significant somatic mutability was restored. Thus, it appears that in these three instances something is still present at the a1 locus that can respond to the Mu1 elements contributed by the Pl Mu parent. In one instance (8197-4) there does not appear to be a response. Perhaps this stable represents a modified insertion at the a1 locus that now only transposes infrequently. In column 7 the presence of new mutants segregating in the selfs of a cross indicates that one of the parents has Mutator activity. Since the stable a1 parent has already been shown not to have Mutator activity (column 5), the Mutator activity must have been due to the Pl Mu parent of the first two crosses. The results from column 8 confirm that the Pl Mu parent indeed had Mutator in the case of the first cross (8186-7). Unfortunately, there was not a separate test of the Pl Mu parent for the second cross (8186-10), but since the a1 stable parent did not have Mutator activity, the activity found in column 7 could only have come from the Pl Mu parent. The results from cross number 3 (8197-3) are quite interesting. The Pl Mu parent, when tested against Pl aleur for Mutator activity, is shown not to have any. That it had truly lost Mutator activity is indicated by the results in column 7, for there was no evidence of Mutator activity in this cross either. Such results are expected since neither the a1 stable or Pl Mu tested positive for Mutator activity. However, somatic mutability is restored in the Pl Mu/a1-Mum3-stable in spite of the fact that there is no evidence for Mu1 transposition (germinal or somatic) in either parent.
These results are in agreement with the evidence that transposition of Mu1 ceases if the copy number drops below a minimal number. Walbot (MGCNL 58:188-189, 1984) has evidence for this with regard to somatic transpositions at the bz2 locus. Bennetzen, Morris and Hagenson (unpublished) have evidence that if the Mu1 copy number falls below 10, germinal transpositions no longer occur. The stables in this study may have less than 10 copies of Mu1, thus eliminating somatic and germinal transpositions. When crossed to an active Pl Mu stock this parent could contribute sufficient copies of Mu1 to restore transposition and, if a potentially active Mu1 element is still at the locus, somatic mutability might be restored. The results of the third cross (8197-3) are of particular significance since, if the above interpretation is correct, both the a1 stable and Pl Mu parent evidently had a Mu1 copy number below the critical number required for transposition. However, by pooling their Mu1 elements in the Pl Mu/a1-Mum3-stable cross there resulted sufficient Mu1 copies to initiate transpositions and hence somatic mutability was restored. Whether or not germinal mutability was restored will have to be seen in the results of the outcross of Pl Mu/a1-Mum3-stable plants. That outcross was made this summer and the progeny are in our winter nursery.
Medium mutable seeds (class 2) were selected from one of the selfed ears of the Pl Mu/a1-Mum1-stable cross (8186-7) and planted. The resulting plants were self pollinated (Table 2). The eight ears are almost uniformly class 2 (low). At most, only one stable (class 1) seed was found per ear and that only for 3 ears. A few seeds higher than class 2 were found on two ears. These results give no indication that a dual factor controlling-element-like system is involved in the restoration of mutability to a1-Mum stables, similar to Ac-Ds. The results suggest that sufficient copies ofthe Mu1 element are again present in this one stable line to maintain a discernible pattern of mutability.
Table 1. Tests of a1-Mum1 and a1-Mum3 stable alleles.
Table 2. Classification for somatic a1 Mum1 mutability in the F2 progenv of the cross, Pl Mu/a1Mum1 stable. The F1 seeds planted were medium mutables (class 3).
Donald S. Robertson
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