Ac transpositions reflected as twin sectors on P-ovov ears --Prasanna Athma and Thomas Peterson We have been studying a variegated P allele derived from P-vv termed P-ovov. P-ovov specifies orange variegated pericarp and cob, and resulted from excision of Ac from P-vv, transposition 161 bp towards the 5' end of the P gene, and reinsertion in the opposite orientation. In order to study new P locus mutants derived from P-ovov, we examined a large number of ears produced on plants of P-ovov/P-ww or P-ovov/P-wr genotype. Among 22,916 progeny ears, 13 ears had recognizable twin sectors on otherwise orange variegated ears. The twin sectors were of several types: red twinned with light orange variegated; red twinned with white; light orange variegated twinned with white; and light orange variegated twinned with dark orange variegated. Kernels from the twin sectors were sown and DNA was prepared from the resulting plants for Southern analysis. We used a combination of probes to detect insertions of Ac or other alterations within a 26 kb region of the P locus. We recovered progeny carrying the alleles derived from both cotwins of 12 of the 13 twin sectors. The derivative alleles were grouped into three classes based on their molecular structure:

Class I (2 cases): In these cases, a sector of light orange variegated pericarp is twinned with red in one case and with dark orange variegated in another case. In both cases the light orange variegated sector contains two copies of Ac at the P locus: one at the original site as in the progenitor P-ovov allele, and one at a different site nearby. The red or dark orange variegated cotwin contained only the single transposed Ac, in the same genomic restriction fragment as in its twin. These cases appear to have resulted by excision of Ac from its original site after replication of the donor locus and insertion into an unreplicated target site. Thus, these cases are analogous to the Type I twin sectors derived from P-vv; Type I twins contain Ac activity in the red sector (Greenblatt and Brink, Genetics 47:489, 1962; Chen, Greenblatt and Dellaporta, Genetics 117:109-116, 1987).

Class II (5 cases): In these five cases, red pericarp is twinned with either light orange variegated, light variegated, or white pericarp. In all five cases Ac was not detected within the red sector. The light orange variegated, light variegated, and white twin sectors contained Ac at the original P-ovov site, and a transposed Ac elsewhere within the region covered by our probes. These cases may have resulted from Ac excision from one chromatid after replication of the donor locus and insertion into the target site that has already replicated.

Class III (5 cases): In these cases, white pericarp is twinned with either light orange variegated or red pericarp. In this class, Ac transposition appears to be accompanied by deletions or other chromosomal rearrangements at the P locus.

Although the structures of P-ovov and P-vv are quite similar, twin sectors occur at an apparently much lower frequency with P-ovov than with P-vv. With P-vv, sectors of red pericarp are twinned with light variegated pericarp. The red pericarp derives from excision of Ac from P-vv, while the light variegated pericarp results from the dosage effect of a transposed Ac on the timing of sectoring of the P-vv allele. In P-ovov, the effect of a transposed Ac outside the P locus may also decrease the frequency of revertant red sectors, but the change is probably not noticeable against the dark orange background conditioned by P-ovov. The Class I and Class II twin sectors studied here involve Ac transpositions to other sites within the P locus. Such intralocus transpositions occur much less frequently than transpositions to sites outside the P locus.

Thanks to Susan Allan for excellent technical assistance.

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