We are attempting to isolate the Rf3 gene using transposon tagging; our plan is to use an Rf3 homozygote containing a transposable element system as male parent onto cms-S rf3 rf3 plants. The resultant rare sterile or semisterile plant is the potential recipient of an Rf3 locus with an inserted transposable element. While making the stock constructions for these experiments, we noticed differences in the level of somatic instability of the mutable allele in a number of inbred backgrounds.
We used a mutable allele of B-Peru isolated by V. Chandler and V. Walbot. Although this allele, b-Peru-mu5, was derived from Robertson's Mutator material, it contains neither a Mu1 nor a Mu1.7 element at the B locus (V. Chandler, MNL, this issue). It should be noted that expression of the somatic instability of this allele is very stable in backcrosses to b r-g tester stocks in a W23/K55 hybrid background (V. Chandler).
Our first crosses were onto B37 cms-S plants; the result was that very few ears exhibited the expected spotted kernel phenotype. In subsequent seasons we performed similar crosses onto 16 other inbred lines with cms-S cytoplasm. Four to eight ears from each cross were analyzed. Three basic classes of F1 progeny resulted. In class I, all F1 ears had only rare (2-3 per ear) spotted kernels. The inbred lines that gave this result were Oh5lA, W64A, Tr, and Mo17. Class II progeny contained two populations of ears; those that had only rare spotted kernels and those with close to 100% spotted kernels. These inbred lines were 38-11, N28, K55, R138, B37 and B84. The class III result was 100% spotted kernels on all F1 ears. The inbred lines giving this result were WF9, B73, M14, Va26, SK2 (all with pale spots) and R802A and Oh545 (deeply pigmented spots). The results were unaffected by the cytoplasmic type, e.g. four different cms-S cytoplasms in the N28 background gave the same result, and B37 cms-S and N (normal) maternal parents gave the same result. It should be noted that in all of our crosses the mutable parent was the male; it would be interesting to see the results of the reciprocal crosses considering the differential expression of somatic instability reported with a bz2-mu1 allele using reciprocal crosses (V. Walbot, Genetics 114:1293, 1986).
Our results suggest that different inbred lines contain factors which influence the expression of somatic instability of the b-Peru-mu5 allele. The most puzzling is the class II result, in which two types of ears were produced in the F1. We will test the heritability of this result in future studies.
P. Bedinger, M. Albertsen, S. Gabay-Laughnan and J.R. Laughnan
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