Molecular analysis of the loss of somatic instability in the bz2::mu1 allele of maize.

Multiple genetic and epigenetic changes were detected within one plant generation at the bz2::mu1 mutable allele in a population of 118 plants. Loss of somatic instability in bz2::mu1 was usually correlated with methylation of the Mu1 transposable element; in 6 plants, somatic instability was lost as a result of mutations in bz2::mu1. This is a surprisingly high frequency of mutation per allele (2.5%) for the Mutator family, for which germinal revertants occur at a frequency of about 10(-4) per gamete. One germinal excision event was found that contained an 8 bp deletion, frameshift mutation in Bronze-2. The three other mutants described occurred as a result of abortive transposition, in which 75-77 bp deletions were generated at the junction between Bronze-2 and Mu1. We discuss the possible mechanisms, and the role of host factors in abortive transposition in maize.