The origin and behavior of mutable loci in maize

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Abstract
In maize, variegation is affected by a large no. of mutable loci, some of which are well-known genetic units such as c, yg2, wx, a2, y, and pyd, while others involve unknown units. The new mutable loci arose in progeny of plants which had undergone structural modifications of the short arm of chromosome 9 during a breakage-fusion-bridge cycle. In addition, some plants had other modifications involving the heterochromatic knobs and centromeres. About 40 different mutable loci were recognized comprising 2 major classes: (1) those requiring a separate activator factor for their expression, and (2) those which were autonomous. Each class could be subdivided with respect to the stability and expression of the mutation. A Ds (Dissociation) locus in the short arm of chromosome 9 gave breakage events equal in time and frequency to those of some mutable loci. Such loci are thought to arise when a chromatin element independent of the genic material is inserted adjacent to genes and undergoes mutational events leading to a changed expression of the genes. It is assumed from observable events that the inserted chromatin of the Ds locus has a stickiness at a precise time in development, which leads to alterations resembling the effects of irradiation or chemicals. In some cases transposition of the Ds locus is associated with a gross chromosomal rearrangement. In one instance, a Ds locus was transposed in chromosome 9 from a known position to a new position adjacent to the C locus. Events typical of the Ds locus occurred at the new position and the C action disappeared, giving a c phenotype (c-m1). Removal of the Ds chromatin restored the action of C. A different mutable c locus (c-m2) gave a range of quantitative expression and a mutable wx locus associated with it behaved similarly. This suggests that the difference in phenotypic expression between the c loci may be due to differences in inserted chromatin. Newly arising mutable loci, such as Ds, c-m1, c-m2, and wx-m1, require an activator, Ac, for mutable and chromosome breakage events to occur. Ac behaves like Ds in its transposition and change-of-state properties. The dosage effect of Ac is expressed in diploid plant or triploid endosperm tissue, the higher dosage delaying the time of occurrence of mutations at the Ac-controlled mutable loci. Ac is believed to produce a stickiness of the inhibiting materials adjacent to all mutable loci at the same time in the same cell. The changes in dosage or state of Ac account for the varied patterns of variegation. Mutable loci not requiring an activator show similar expressions of variegation to those needing an activator, and could be produced by Ac or Ac-like loci transposed to an adjacent position. It is believed that Ac material is heterochromatin, both from homologies of variegation in maize and Drosophila and from an experiment concerned with the action of the Dt (Dotted) locus on the a1 locus. The Dt action is strikingly similar to the action of Ac on mutable loci, and the same effect can be produced in the absence of Dt by a breakage-fusion-bridge cycle involving the knob in chromosome 9. Heterochromatin in the normal nucleus seems to be associated with exchange of materials between nucleus and cytoplasm, so that any changes in the heterochromatin may affect the exchange and possibly chromosome organization and genic action. The analysis of Dt indicates that a heterochromatic element may be responsible for the origin and behavior of variegation in maize. Future papers will present a full account of this phenomenon.

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