Molecular evidence that chromosome breakage by Ds elements is caused by aberrant transposition

The transposable Dissociation (Ds) element of maize was first discovered as a site of high-frequency chromosome breakage. Because both Ds-mediated breakage and transposition require the presence of the Activator (Ac) element, it has been suggested that chromosome breakage may be the outcome of an aberrant transposition event. This idea is consistent with the finding that only complex structures containing multiple Ds or Ac and Ds elements have been correlated with chromosome breakage. In this report, we describe two chromosome-breaking maize alleles that contain pairs of closely linked but separate Ds elements inserted at the Waxy locus. A polymerase chain reaction assay was utilized to isolate intermediates in the breakage process. The DNA sequence of these intermediates reveals deletions and base pair changes consistent with transposon footprints that may represent the junctions between fused sister chromatids. These results provide direct molecular evidence that chromosome breakage is the result of aberrant transposition events.