Suppressible expression of ij-Mu*1 plants --Chang-deok Han and Edward H. Coe, Jr. The new ij mutant from Robertson's Mu background that we described in this issue shows unusual phenotypic expression during plant development. The 'ij-like' stripes progressively disappear as the ij-Mu*1 plants grow. Since there is precedent for such epigenetic switch of a Mu element in hcf106 (Martienssen et al., EMBO J. 8:1633-1639, 1989), the gradual loss of the phenotypic expression of the ij-Mu*1 mutant might be due to 'modifiable status' (e.g., methylation or interaction with modifier) of the resident Mu element.

A most interesting observation is that the stripes almost disappear in the 6th or 7th leaf in almost every mutant that manages to grow up to mature plants in the greenhouse. Taking into account the fact that the embryo of mature seeds already contains 5-6 leaf primordia, if, indeed, gradual loss of the phenotypic expression of the mutant were due to the epigenetic switch of the resident element, the epigenetic modification might be subject to the developmental processes of the plant. A relationship between developmental processes and such reversible genetic change in transposable elements already has been implied in the early work of McClintock on Spm elements and recently has been characterized at the molecular level (for review see Fedoroff et al., BioEssays 10:139-144). Further characterization on the ij-Mu*1 mutant should be followed up to verify the hypothesis.


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