One of the most striking properties of transposable elements is their ability to undergo reversible changes of state. A change of state is defined as an alteration in the somatic reversion frequency or in the timing of reversion (or both) of a transposable element-induced mutant allele. Such a change can occur in either the autonomous or nonautonomous member of a two-element system, or in the single member of a one-element system. There is no evidence that Robertson's Mu is a two-element system.
I have recently identified two cases of changes of state in Robertson's Mu-induced mutants. One case involves a mutant of the Bz1 locus which was recovered last winter from a Mutator stock; the progenitor derives from a full color line from M. G. Neuffer. This allele, bz1-N1032, is very unstable in the aleurone. Reversion events are frequent and occur late in development. A change of state was demonstrated by a single kernel of the cross: bz1/bz1 X bzl/bz1-N1032 where both have a Mutator background. See schematic in Figure 1. Bz revertant sectors
in the aleurone of this kernel suggest the following sequence of events: 1) At the two cell stage in the development of the endosperm, bz1-N1032 changed to a stable state in one of the two cells. 2) Subsequently, the stable form of the allele changed back to the unstable state in three separate cells. These cells, each genetically unstable bz1-N1032, produced lineages which were surrounded by stable bz1-N1032 tissue. Two changes of state are indicated. The stable to unstable transition appears to have restored the original state of bz1-N1032. All four sectors of unstable bz1-N1032 are the same with respect to rate and timing of reversion. In each sector revertant tissue is approximately one tenth as frequent as non-revertant tissue. The timing of reversion results in single cell revertant sectors about twice as often as two to five cell revertant sectors.
A Robertson's Mu-induced mutant Adh1 allele, Adh1-S3034v, gave a similar result for two kernels whose aleurone had been stained for ADH enzyme activity.
Assuming that individual Mu elements are autonomous requires that the mutation encoding a change of state resides within the Mu element at Bz1 and Adh1, or in the genes themselves. However, trans-acting modifiers have not been excluded. Change of state may be a general property of maize transposable elements.
Figure 1. Crude schematic of kernel of the genotype bz1/bz1-N1032. Dotted portions represent tissue in which bz1-N1032 is in an unstable state. Clear portions represent a stable state of the allele.
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