Activation of Ac and the coincident origin of a new P-RR allele

From cross 1 performed under controlled/isolated open pollination, an exceptional bronze shrunken kernel exhibiting bz-m4 mutability was isolated. Since neither parent had Ac, this mutability demonstrates Ac activation. Testcrosses confirmed the following: the mutability of bz-m4 is heritable, the new Ac is capable of triggering Type I events in C-I Ds, and the exceptional kernel was P-RR/-, which was surprising since both parents had carried P-WR or P-WW alleles. Contamination can be ruled out; a contaminant pollen grain would necessarily have to have been C sh bz, Ac, and P-RR, a combination not present in our nursery (which was in any event quite distant), and not present in commercial stocks or ornamental corn in home gardens.

Cross 1: C sh bz-rcy/C- bz-m4, Cy x C sh bz/C sh bz

The hypothesis: a P-WR or a P-WW allele in one of the parents consisted of P-RR with an insert of an inactive Ac. Transposition of the Ac resulted in its activation and release of the P locus, resulting in a change of P-WR or P-WW to P-RR. A linkage of the new Ac to P-RR would further support this interpretation.

Patrick S. Schnable and Peter A. Peterson

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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