Double Ds elements are involved in specific chromosome breakage.

The structure of the unstable Ds-induced sh-m5933 allele of the maize sucrose synthase gene was analysed and a double Ds structure found in opposite orientation on both sides of a 30 kb insert interrupting the sucrose synthase gene. The double Ds structures bordering the insert are identical over a distance of approximately 3 kb. These double Ds structures and the DNA segments beyond them are in opposite orientation and identical over a distance of approx. 5.3 kb. A hypothesis for how such a symmetrical structure could be formed is proposed. When one complete Ds element was excised from one of the double Ds structures a half Ds element was left behind. This half Ds element was found in one revertant strain which displayed an altered pattern of chromosome breakage compared to revertant strains which had not undergone Ds excision. Nine new maize strains which showed a similarly altered chromosome breakage pattern were isolated. In all nine cases we observed an indistinguishable deletion in the genomic DNA. These excisions are likely to be the result of similar excision events to that described above. We conclude that double Ds structures are responsible for Ds-induced chromosome breakage.