--Prasanna Athma and Thomas Peterson
As mentioned above, the P-OVOV allele carries an Ac element in the inverted orientation with respect to P-VV. The P-OVOV allele shows both somatic instability (pericarp sectoring; see above) and germinal instability, as evidenced by the progeny of the cross: P-OVOV/P-OVOV x P-WW/P-WW (both directions).
Among 10,820 progeny ears, 697 (6.4%) had red pericarp and cob (P-RR), and 89 (0.8%) had white pericarp and cob (P-WW). These frequencies can be compared to those previously reported by Brink (Genetics 43:435): among 4575 offspring of the mating of P-VV/P-VV x P-WW/P-WW, 125 (2.7%) had red pericarp and cob and 8 (.17%) had white pericarp and cob. Thus, both P-OVOV and P-VV mutate to P-WW at low but detectable frequencies. It is not known whether the different frequencies arise from background effects, direction of the cross, or actual differences in mutation frequency of P-VV and P-OVOV.
We have investigated the molecular basis of seven P-WW alleles derived from P-OVOV. Each allele was obtained independently from kernels with mutant pericarp sectors from separate orange variegated ears. Southern analysis indicates that six of the seven P-WW mutants have a large deletion at the P locus; the seventh mutant has a more complex structure and will not be considered further here. In order to map the deletion end points, restriction fragments to the right and left of the Ac insertion site in P-OVOV were used as probes. Southern analysis with probes spanning a 3kb region to the right of Ac showed that this region was deleted in the mutants. Similarly the probes to the left of Ac representing a 9.5kb region were also deleted.
Molecular analysis shows that the P locus contains two direct repeats of 5.8kb, separated by 6.6kb (Lechelt et al., in preparation). The Ac element in the P-OVOV allele is situated in the 6.6kb of DNA between the two 5.8kb repeats. In the P-WW mutants the deletion end points lie within the two 5.8kb homologous direct repeats, on either side of Ac. We suspect that the deletions may have occurred by homologous recombination between the two direct repeats such that the 17kb of intervening DNA, including Ac and part or all of the P gene, is deleted.
The possible involvement of the Ac element in the occurrence of deletions is suggested by the apparent stability of the P-RR allele. Although P-RR contains the 5.8kb direct repeats, we do not know of any reports of P-WW mutants arising from P-RR. On the other hand, the P-WW-1112 allele, obtained directly from P-VV, has a deletion of the same type as the six P-WW mutants derived from P-OVOV. We do not know whether the deletions are somehow induced by the presence of an active Ac element, or the increased length of DNA between the direct repeats.
We thank Rob Fincher and Ruth Meier of Pioneer Hi-Bred for overseeing
the maize crosses and isolation fields.
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