In last year's News Letter we presented the first results of a study to determine if somatic instability in Mu-induced aleurone mutants could be used as an indicator for Mu germinal activity (MNL 60:8-9, 1986). Last year's data suggested that lines having predominantly seeds with a high level of mutability sometimes did and sometimes did not have germinal activity, while lines that had predominantly medium or low levels of mutability had no germinal activity.
This year we have partial results to report (at this writing we still have more crosses to analyze) from an additional generation of testing (Table 1). Seeds from lines previously outcrossed to a1 sh2 tester stocks, which produced ears with predominantly low (= Class 2), medium (= Class 3) or high (= Class 4) seeds, were again outcrossed to an a1 sh2 tester and ears selected that gave again the type of ear of the parental a1-Mum stock. In some crosses, ears that differed from the parental stock were observed (e.g., ears that switched from high to stable, had a range of mutable classes instead of predominantly one class, etc.). Some of these latter types of ears also have been tested.
The 1985-1986 winter nursery had a very poor stand, thus the numbers in each test are not large. If one, however, sums all the plants in the high and medium crosses, the total population is 84. This also is not a large population but in the vast majority of Mu outcross populations of this size some mutants would be expected. As in previous tests most of the lows have no germinal Mu activity. The 1986 families 3512 and 3513 are of special interest. Two generations ago in the ancestry of these lines, two independent high mutable plants were crossed to a1 sh2. In this outcross generation, which was grown in 1985, some plants from each line had ears with predominantly stable (Class 1) seeds in both selfs and outcrosses to a1 sh2. The seeds from two of these a1 sh2 outcrosses, in which only stable (a1-Mum-stable) seeds were observed, were planted in families 86-3512 and 86-3513. These families unexpectedly showed a high germinal mutation frequency. In all previous tests of stable derivatives, they have never shown germinal mutator activity. Thus, it had been assumed prior to this observation that the loss of somatic and germinal Mu activity occurred concomitantly. These observations suggest, however, that germinal activity can remain in the absence of somatic activity. In the ancestry of 3512, the parent two generations ago was a1 sh2/al-Mum Sh2. Plants of this genotype can rarely be expected to give a stable a1 (standard allele) Sh2 seed as the result of crossing over between a1 and sh2. However, since mutable seeds were planted from this heterozygous ear, a homozygous stable plant would not be expected unless heterofertilization also occurred, involving a pollen grain with the crossover product and one with the a1-Mum allele, an extremely unlikely event. Even this explanation is not possible for 3513 because two generations ago the genotype was purple aleurone/a1-Mum2 and thus there is no standard a1 allele available and the stable must be derived from a1-Mum2.
In 1986, additional crosses of the same material planted in the winter nursery were made. Only one of these crosses had germinal mutator activity (1206, medium seed mutability). Three lines with medium mutable seeds and one with high mutable seeds did not show any germinal activity.
The evidence is accumulating that somatic mutability is not a reliable predictor of germinal activity. It neither predicts the loss or the retention of the ability of Mu stocks to induce germinal mutation. Crosses described in other work from our laboratory and presented in the report by Brad Roth and me provide further confirmation of this observation. The fact that these two different aspects of Mu activity can be disassociated suggests that the two phenomena are dependent on different aspects of the Mu system.
Table 1. Result of tests of germinal mutator activity for mutable Mu-induced a1 mutants.
Donald S. Robertson
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