The three Mu-induced a1 mutable mutants (a1-Muml, a1-Mum2, a1-Mum3) have the same very late somatic reversion pattern. This pattern is independent of the doses of a1-Mum in the aleurone, being the same whether the mutable allele is present in one, two or three doses. The frequency of reversions, however, is quite variable for all alleles from cross to cross and frequently even among seeds upon the same ear.
Reversion is probably the result of transposition of the Mu insert away from the a1 locus. Evidence is accumulating that suggests that transpositions are dependent upon the number of Mu1 inserts in the genome. Both too high a number of inserts and too few (below 10) seem to result in the loss of transposition in the germ line (unpublished) and in the aleurone (Walbot, MGCNL 58:188-189,1984). Whether or not variation in the number of Mul inserts between these two extreme conditions influences the frequency of transpositions is not known. Since the number of Mu1 elements varies by a factor of two on average in reciprocal crosses of a1-Mum to a1 testers, it may be possible to get an estimate of the effect of copy number upon somatic transposition. The fact that the a1-Mum allele also varies by a factor of two complicates the interpretation of the results of such a test. Data from such reciprocal crosses are given in Table 1. The level of mutability (i.e., frequency of transposition) was measured on a 10 point scale with 1 being stable mutant and 10 being full color (purple).
Although there is considerable variation from cross to cross it is obvious that crosses of a1-Mum plants as males consistently result in a lower level of mutability than the reciprocal crosses. The female frequency is 1.42 times greater than the male on average, and does not seem to vary in a consistent manner relative to the level of mutability found in these crosses.
Although the female crosses have twice as many a1-Mum alleles as the male crosses and probably have on average twice as many Mu1 sequences, the female reversion frequency is not twice that of the male. Thus, there does not seem to be an additive relationship between the number of copies of the mutated locus or of the Mu1 inserts. In the studies of the mutator activity of Mu1o.c. and Mu2o.c. (Robertson, Mol. Gen. Genet., 191:86-90, 1983), where the number of copies of Mu1 insert differs by a putative value of two, the Mu2o.c. has only 1.5 fold higher mutation frequency than Mu1o.c. Again there is not an additive relationship between the putative Mu1 dosage and transpositions.
In the case of the a1-Mum alleles the nonadditive effect of Mu1 copy number is seen even when the a1-Mum parent is crossed as a female, and twice as many a1-Mum alleles are available at which transpositions restoring function to this locus can occur. It may be that the depressive effect on the frequency of transpositions resulting from an increase in copy number of Mu1 in the aleurone more than offsets the increased opportunity for transposition that results from doubling the number of a1-Mum alleles.
Table 1. Intensity of somatic Mutator activity in reciprocal crosses.
Donald S. Robertson
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