In order to be sure that a given plant has Mutator activity, the plant must be outcrossed, 50 or more of the outcross progeny selfed, and the selfs scored for the segregation of new mutants. This is a very burdensome procedure. With the advent of mutable aleurone mutants induced by Mu, somatic instability may be a possible indicator of Mu activity in the germ line. Certainly the mutable phenotype indicates Mu1 transpositions are occurring in the somatic cells. But is the somatic behavior of Mu1 correlated with its germinal behavior?
In the previous paper, evidence was presented that the loss of somatic mutability seems to be correlated with the loss of germinal transposition. In addition to the test for Mutator activity in stable derivatives of a1-Mum mutants reported in the previous paper, two additional stables also tested out to have lost Mutator activity. Thus it seems that loss of germinal mutability accompanies the loss of somatic mutability. But is the converse true? Does the presence of somatic mutability indicate the presence of germinal mutability? In Table 1 are the results of some tests made this summer. Seeds from outcrosses to a purple aleurone non-Mu stock of plants with ears having predominantly seeds with three different levels of mutability, low, medium and high, were sown and selfed. The parental mutability was scored either on selfed ears of the a1-Mum parent or on ears of the a1-Mum parent that had been pollinated by a1 sh2. The selfed-outcross progeny were scored for the presence of new mutants (Table 1).
It does not appear that there is any correlation between the level of somatic mutability per se and Mutator activity observed in the germ line. The highest Mutator activity came from the outcross of plants whose ears had been pollinated by a1 sh2 and showed a high or medium level of mutability. High, medium or low mutability on selfed ears did not appear to be a good predictor of Mutator activity in a plant. Only two out of 8 crosses showed Mutator activity, and that was relatively weak. Why this difference between mutability scored on selfs versus scored on outcrosses? Since the number of plants that were scored after outcrossing rather than selfing is small, this may just be a chance event. However, there is a basic difference between the Mu1 situation found in a self and that of an outcross. In a self both gametes carry the a1-Mum allele and other copies of Mu1 that are present elsewhere in the genome. Thus the mutable phenotype one observes results from the sum of these two contributions. Any one (male or female) gamete may not have sufficient Mu1 copies to cause transpositions, but combining the contributions from both the pollen and egg may be sufficient for somatic transpositions to occur. When the plant bearing the selfed ear is outcrossed for the Mutator tests reported here, the low copy number carried in only the male germ line may not be sufficient for transpositions to occur, and hence no mutation would be found in the outcross progeny scored in such a cross. In the case where mutability scoring at the a1-Mum locus was done on ears pollinated by a1 sh2, only the female gamete contributed the a1-Mum allele and any other Mu1 sequences that might be present. If a1-Mum mutability is a function of the number of active Mu1 segments present, then aleurone instability in this case may be a good predictor since it may more accurately represent the condition of the Mu1 state in the male germ line. More tests will be required in which the a1-Mum parent's mutability is scored by outcrossing to a tester to confirm these results.
It should be noted, however, that even in Mutator tests that showed no Mutator activity there was some somatic mutability found in the outcross progeny; sometimes a significant amount. Thus the mere presence of seeds with somatic mutability in an outcross progeny does not insure that the Mu parent of the outcross had Mutator activity.
Donald S. Robertson
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