Transposition and the Stability of Modulator in Maize

Abstract

HE variegated pericarp allele (P"") in maize has been pictured as a dual T structure comprising Prr, a gene for self-red pericarp and cob, and Modulator (Mp,) a separable element which when present at the P locus in conjunction with Prr, suppresses the pigment-producing action of the latter. Puu mutates frequently in somatic tissue to stable self-color, giving the disorderly pattern of red sectors on a colorless background characteristic of the variegated pericarp ear.The mutation of Pv to Prr involves transposition of Modulator from the P locus to one or another, usually unknown, position elsewhere in the genome. An effect of such a transposed Modulator (tr-Mp) in a variegated plant is to reduce the number of P" to P r mutations by somewhat more than one half, thus changing the collective phenotype of the ear from "medium" to "light" variegated (BRINK and NILAN 1952;BRINK 1954).The strong modifying action of transposed Modulator on P"" to Prr mutations was used as a criterion in the present study to determine whether Modulator elements derived by independent transpositions from a common Pvu allele, on a given genetic background, were alike. MATERIALS A N D METHODIt was desired to produce, on a uniform genetic background, plants carrying a single Pvv allele from a common source and hemizygous for a series of transposed Modulators derived by independent mutations of P"" (separate transpositions of M p from the P locus to other chromosome sites) within a single stock. These plants would be of the composition Puv/Pww (or PLor) + tr-MpJ-, and hence would be light variegated. The frequency of mutation to Prr of the Pvv allele common to these light variegateds would be expected to reflect in each instance the modifying action on the Pvv to Prr event of the particular transposed Modulator present.Plants of the desired composition were produced in two steps from inbred foundation lines already at hand. First, colorless pericarp individuals hemizygous for transposed Modulator were identified in families in which they would be expected to occur rather frequently hy means of the Ds test (MCCLINTOCK 1951 ;BARCLAY and BRINK 1954). Secondly, all these plants were pollinated with a single com-Paper No.