Mutator-homologous sequences in normal lines and in somaclonal variants

Genetically stable maize lines and varieties have been shown to have sequences homologous to Robertson's Mutator transposons, including apparently intact transposon structures (Chandler et al., Genetics 114:1007, 1986). We have found three kinds of Mu-homologous sequences in normal maize: (1) Mu1 terminal sequences not associated with internal sequences, (2) "endogenous elements" structures very similar to Mu1 and Mu2 (also known as Mu1.7) are found in many, but not all normal maize lines we have examined, and (3) "endogenous sequences"-a sequence similar to an internal sequence of Mu2 is found in every maize line we have checked. We have also found it in teosinte, Zea diploperennis and Zea mexicana samples. Both endogenous elements and sequences appear to be very stable in normal genetic backgrounds. We have found no differences in genomic restriction patterns among individuals of the line W22 (which carries a Mu2-like element as well as the endogenous sequence) or between W23 and the Golden Glow population from which it was derived.

The "endogenous sequence" from W23 has been cloned and sequenced. It is highly homologous to Mu2, but it has no Mu termini and is missing 401 bp of internal sequence. It does not have a transposable element structure. In all maize lines and relatives examined, the endogenous sequence is flanked by the same non-Mutator sequence. The endogenous sequence has been mapped to chromosome 2 (T. Helentjaris, pers. comm.).

We are interested in the possibility that cryptic transposons may become activated during growth in tissue culture or during regeneration and lead to the production of new phenotypes or somaclonal variants. To investigate this possibility, we have examined the Mu-homologous sequences in the inbred line W182BN and in eleven somaclonal variants that were derived from it. Both the inbred and the somaclones were given to us by Elizabeth Earle.

DNA from these sources was cut with a variety of restriction enzymes, electrophoresed in adjacent lanes of agarose gels and Southern blotted. The blots were probed with clones from internal and terminal portions of Mu1 (see Chandler et al., 1986) and with an internal sequence unique to Mu2. The latter clone was given to us by Loverine Taylor. We found that W182BN carries a sequence with a structure like Mu2 and two different endogenous sequences. These two sequences differ in their degree of homology to Mu2, but they are each flanked on one side by the same non-Mutator sequence.

When the DNA samples are cut by enzymes that cleave inside the Mu-homologous regions of the three sequences, we see no difference between the inbred lines and the somaclonal derivatives, indicating that tissue culture and regeneration has not changed the structure or the DNA modification pattern of these sequences. However, when cut with restriction enzymes that cut outside Mu-homologous regions, one of the somaclonal lines shows the loss of a restriction band and the gain of a new band. One of the endogenous sequences rather than the intact Mu2-like sequence appears to be involved in this rearrangement. We are continuing to map this change to determine what kind of rearrangement has occurred in this somaclone and what role the Mu-homologous sequence may have played in creating it.

Carol Rivin, Chee Harn, Vicki Chandler1 and Luther Talbert1

1University of Oregon


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