--Carol Rivin and Douglas Underwood

Normal lines contain several types of sequences with homology to Robertson's Mutator transposons. We are interested in the possibility that Mu-homologous DNA may play a role in generating genomic diversity in non-Mutator plants. To investigate this question, we have looked for polymorphisms involving Mu-homologous DNA sequences in the genomes of somaclonal variants. We are working with the inbred W182BN and regenerated stocks given to us by Elizabeth Earle. As we reported here last year, some variation is observed when DNA from these plants is probed with internal sequences of Mu1 and Mu2. We now find that probing at lower stringency with the Mu1 terminal repeat sequence reveals a very high level of restriction band polymorphism.

Nineteen bands hybridizing to the terminus probe can be distinguished on an EcoR1-HindIII digest of W182BN DNA. The banding pattern and the relative band intensities are the same among individuals of this line. Among the somaclones, however, variation was observed for 16 of the 19 bands. Eight somaclonal lines, each having lost one to six of the Mu terminus bands, were also found to have new bands and large increases in the intensity of individual bands. In contrast, only a small amount of heterogeneity was observed using Mu internal sequences or genic sequences as probes.

Many of the Mu-homologous sequences of maize consist of inverted terminal repeats surrounding DNA unrelated to Mutator transposons. Several examples have been cloned and sequenced (Mu4, Mu5, Mu6, Mu7, Talbert et al., MNL 1988, and submitted), but have not been shown to transpose. Other sequences with this general structure have been found to transpose in Mutator lines (Mu3, Chen et al., Genetics 116:469, 1987; and an element from wx-mum5 isolated by Sue Wessler). We are testing whether these types of elements are associated with the rearrangements observed in the non-Mutator somaclones.

To date we have compared the inbred and somaclone banding patterns for the internal sequences of Mu3, Mu4, Mu5 and Mu6. Clones were provided by Karen Oishi and Vicki Chandler. No polymorphisms were observed with Mu5. Some band loss, but no new bands were found with the Mu3 probe. Mu6 showed some new bands and a greatly intensified band. Mu4 was the most variable. Seven of the eight polymorphic lines showed evidence of band loss, new bands and band intensification with this probe.

We do not know the molecular nature of the polymorphisms we observe in these lines. Experiments are in progress examining the DNA flanking the novel Mu elements to look for evidence of transposition, deletion, amplification, recombination between dispersed repeats or other rearrangements. We are also asking if other transposable element families show a similar level of variability.

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