A change in state of the Ac-induced P-VV allele is associated with inversion of Ac

The maize P locus controls pigmentation of the pericarp and glumes of the cob. The P-VV allele, which gives variegated pericarp and cob, comprises the transposable element Ac situated at the P locus. I have isolated an allele termed P-OVOV (orange variegated pericarp and cob) derived from P-VV. The P-OVOV allele was recovered from a sector of about 30 kernels with orange variegated pericarp on an ear with otherwise ordinary variegated pericarp, from a plant of P-VV/P-WR genotype. P-VV specifies colorless pericarp with red sectors, whereas P-OVOV specifies orange pericarp with many dark red sectors, and some colorless sectors. Thus, P-OVOV represents a change in state of the P-VV allele.

Two hypotheses have been proposed to explain the molecular basis for changes in state. The composition hypothesis attributes changes in state to changes within the transposable element, such as internal deletions or DNA modification. The position hypothesis proposes that the element is unchanged, but that it has moved to a new position within the locus. The P-OVOV allele was studied to determine the basis of this change in state.

The genetic properties of the P-OVOV allele were investigated in progeny of the cross P-OVOV/P-WR x P-WW. For each progeny plant, ear phenotype was scored by inspection, and the presence or absence of Ac was determined in testcrosses. The results are in the following table:
 
 
Ear phenotype
 
  WR OV0V RR Other
Ac + 2 109 5 2
Ac - 124 0 5 1
Totals: 126   122  

Three points can be drawn from the data. First, the number of P-WR ears (126) is approximately equal to the combined number of P-OVOV, P-RR, and variant ears (122). This shows that P-OVOV segregates as an allele of P-WR; the P-OVOV phenotype does not depend on unlinked modifying factors. Second, Ac activity is tightly linked to P-OVOV. 109 plants producing P-OVOV ears carried Ac; there were no examples of plants producing P-OVOV ears which did not carry Ac. Third, the P-OVOV allele is itself unstable, as evidenced by the P-RR and "other" ears among the progeny. These variants are presumed to arise from mutation of P-OVOV, because the P-WR allele used in this cross is stable.

Has the Ac element at the P locus undergone a compositional change in the mutation of P-VV to P-OVOV? This possibility was tested by comparing the abilities of the Ac elements associated with the P-VV and P-OVOV alleles to trans-activate a Ds element. No differences in the Ds-response were seen, suggesting that the transacting functions of Ac are unchanged. These genetic results are similar to those reported by F.A. Valentine (Orange variegated, a mutant originating from variegated pericarp in maize. Unpublished Ph.D. thesis, Univ. Wisconsin Library, Madison, 1957).

Southern analysis of DNA from the progenitor P-VV allele and the P-OVOV allele shows that the Ac element associated with P-OVOV has remained within an 850bp genomic DNA fragment at the P locus. Interestingly, the Ac element is in an inverted orientation in P-OVOV compared to P-VV.

The genetic and molecular data are consistent with the following explanation for the origin and behavior of the P-OVOV allele. The progenitor P-VV allele comprises the Ac element inserted in a particular orientation at the P locus. The P-OVOV allele may have arisen by an event in which Ac has inverted, or flipped, from its orientation in P-VV to the opposite orientation. Inversion might have occurred by 1) short range transposition and reinsertion in an inverted orientation, 2) a specialized inversion mechanism of Ac, or 3) inversion mediated by sequences just outside Ac. In the inverted orientation, the Ac insertion may partially suppress P expression, resulting in a lowered level of P function, less pigment formation, and thereby an orange pericarp color. Ac can excise from P-OVOV to restore a P-RR gene, producing the red sectors on P-OVOV kernels and P-RR germinal revertants. The colorless sectors may result from other mutagenic Ac activities, such as imprecise excision, formation of deletions, or transposition of Ac to a new site within the P locus, where it may partially or completely suppress P expression.

These results suggest that the change in state of P-VV to P-OVOV may be due to inversion of Ac. A definitive explanation of how this inversion alters P expression awaits further information on the structure of the P gene.

Thomas Peterson


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