ABERRANT TRANSPOSITIONS OF MAIZE DOUBLE DS-LIKE ELEMENTS USUALLY INVOLVE DS ENDS ON SISTER CHROMATIDS

McClintock's analysis of chromosome-breaking Dissociation (Ds) element s in maize demonstrated that sister chromatids fuse at the position of Ds, forming a dicentric chromosome and an acentric fragment. In tobac co, Ds left and right ends in direct orientation (that is, half a doub le Ds) are sufficient to promote Activator-dependent marker gene loss. We present here a detailed analysis of germinally inherited rearrange ments promoted by ''half double Ds'' elements and a characterization o f rearrangements that involve inversion of the seg ment between the Ds ends and/or deletion of a segment adjacent to the Ds construct. The r esults support a model in which chromosome breakage promoted by these elements, and presumably by double Ds elements, involves Ds ends on si ster chromatids.