AbstractThe high frequency of mutations in Mutator stocks of maize is the result of transposition ofMuelements. Nine differentMuelements that share the 220 bpMuterminal inverted repeats have been described.Mulelements have been found inserted into most of the molecularly characterized mutant alleles isolated from Mutator stocks, and most Mutator stocks contain a high number ofMulelements (10‐60). However, it is clear that additionalMuelements, which share theMultermini but have unrelated internal sequences, can also transpose in Mutator stocks. We were interested in comparing the mutation frequency and type of elements that inserted into a particular locus when Mutator stocks with differing numbers ofMulelements were utilized. Furthermore, previous studies with Mu‐induced mutations have demonstrated that the element that inserted most frequently wasMul.Therefore, to try to obtainMuelements different fromMulwe utilized a stock that had a low number (3‐6) ofMulelements as well as a Mutator stock with a more typical number ofMulelements (20‐60).Utilizing both stocks, we isolated numerous mutants at one gene, Bronze1(Bz1), and compared the type of elements inserted. In this paper we report that both the high and lowMu1stocks producedbz1mutants at frequencies characteristic of Mutator stocks, 6.6 and 4.3 ± 10−5, respectively. We describe the isolation of 20 bz1 mutations, and the initial molecular characterization of eight unstable mutations: two from the highMu1stock and six from the lowMu1stock. The six alleles isolated from the lowMu1stock appear to contain deletedMu1elements, and the two alleles isolated from the highMu1stock contain elements very similar toMu1.When the mutants from the lowMu1stocks were examined, it was found that theMu1‐related elements increased from 3‐6 copies to 9‐20 copies in one generation. The high number ofMu1‐related elements was maintained in subsequent out‐crosses. This spontaneous activation and amplification ofMu‐related elements occurred in at least
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