Molecular evidence for Mu-element transposition in plants regenerated from an active Mutator callus line --Martha James and Joan Stadler Mutator activity was assessed in plants regenerated from an embryogenic callus line derived from the F1 of an A188/Mu2 cross (88-646-8C). This line was determined to have an active Mutator system based on the generation of new mutants in outcross progeny of the Mu2 parent, and by the lack of Mu-element modification in callus DNA. Regenerant progeny from this line displayed Mutator activity as well. Genetic studies showed that new mutations continued to arise in R3 and R4 plants and molecular analyses confirmed these findings. HindIII hybridization profiles of DNA from eight R2 plants revealed Mu-homologous restriction fragments in tissues of six plants which were not present in callus DNA. These novel fragments undoubtedly represent transpositions of Mu elements to new sites in the genomes of the regenerant progeny. Thus, molecular evidence substantiates the genetic findings that Mutator activity can be retained through a tissue culture phase.

The maintenance of Mutator activity during and after tissue culture may have interesting applications in the generation of useful variants. Linkage of mutagenesis with somaclonal variation has previously been explored with the incorporation of mutagenic agents in the culture medium. The high mutation rate seen in the Mutator system (30-50X the spontaneous level) and the ability of Mutator to generate mutants by insertion at virtually any maize locus could make this system a desirable tool for increasing culture-induced variation.

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