R A Brink
1954 Genetics  
OST of the variegated offspring of maize plants carrying the unstable M allele Pvv, conditioning medium variegated pericarp and cob, are themselves medium variegated. Regularly, however, in the maize strain here dealt with, a few light variegated mutants also appear. A third phenotype, " very light," has now been isolated as a mutation from light variegated. Mediums, lights, and very lights differ in the frequency with which the Pvv allele changes to stable PRR (self-colored, i.e., red pericarp
more » ... i.e., red pericarp and cob). Medium variegated ears carry a large number of such mutations which, collectively, give a rather strongly, although irregularly pigmented phenotype. The number of mutant stripes is sharply reduced in light variegateds. Very lights, as the name indicates, show only a few scattered red stripes. These three classes of variegated ears, together with the stable colorless type, Pww, are illustrated in figure 1. It will be shown in the present report that (1 ) mediums, lights, and very lights differ in the number of doses present of a chromosomal unit termed transposed Modlulator ( t r -M p ) which controls mutability of a common Pvv allele and (2) the second tr-Mp unit present in very light variegateds probably originated by reduplication and transposition of the transposed Modulator carried by the light variegated parent rather than as a new mutation from PT'v and ( 3 ) tr-Mp in increasing doses reduces the frequency of Pvv to PRR mutations exponentially. The term " reduplication " as used in this paper is intended to imply an increase in dosage, and does not refer simply to the normal doubling in number of chromosomal elements which occurs at mitosis. BRINK and NILAN (1952) postulated that the mutable variegated pericarp allele, pvv, is a dual structure. One component is the PRR gene, unique for the locus involved, namely, P, on the short arm of chromosome 1. The other component was termed Modulator ( M p ) , a unit adventive to the locus which suppresses the pigment-producing capacity of PRR. These two elements conjoined at the P locus to form the prv allele were given the symbol PRR Mp. The mutation of Pvv to PRR was assumed to consist in the loss of M p from the P locus, thus restoring PRR to its normal activity. Studies on twin mutations (adjoining red and light variegated patches on otherwise medium variegated ears) show that this mutation at the P locus dloes not necessarily, or even usually, lead to loss of Modulator from the genome. M p may be transferred to another site in the chromosome complement where it acts as a modifier of Paper from the Department of Genetics, College of Agriculture, University of Wisconsin, No. 541. GENETICS 39: 724 September 1954. Downloaded from by guest on 25 April 2021 VERY LIGHT VARIEG.4TED PERICARP IN M.4IZE 725 FIGURE 1.-Three classes of variegated pericarp in maize in comparison with the stable colorless type. Left to right : medium variegated, light variegated, very light variegated, and colorless. the Pr'" allele, reducing the mutation frequency of the latter to the level characteristic of light variegated ears. Thus, the light variegated phenotype was interpreted as the product of a transposed Modulator ( t r -M p ) interacting with the PRR M p complex at the P locus. The newly discovered phenotype, very light variegated, is explainable in terms of these same units. It differs from light variegated in carrying two, rather than only one, transposed Modulator. Furthermore, the two Modulators in this case do not reside at homologous loci. MATERIALS AND METHODS The very light variegated phenotype was first recognized as an atypical ear in a third-generation family in a backcross series designed to substitute the P'" allele for Pwm in a highly inbred line known as 4C063. The 4Co63 stock was used recurrently as the pollen source in these matings. Variegated ears were selected for propagation in each generation, and the colorless (Prvrv) segregates were discarded, usually without counting. The seed ear in the second generation in the sub-line in which the new mutant appeared was light variegated. Its recorded off spring comprised two Downloaded from by guest on 25 April 2021
doi:10.1093/genetics/39.5.724 fatcat:2sdi4ypeuzhebepcnvendwpw4a