Chromosome breakage at the Ds-induced sh-m6233 allele --Christian Korfhage and Hans-Peter Doring
 
  Chromosome breakage at the sh-m6233 allele was scored in the sporophyte and in the endosperm. Plants heterozygous for Yg2 C sh-m6233, Ac/yg2 c sh were examined for breakage events. Chromosome breakage at the sh-m6233 allele would lead to the loss of the Yg2 allele and thus to the appearance of yg2 sectors. Most of the chromosome breaks occurred late in the development of the plant. Normal size yg2 sectors were 1-2 mm wide and several mm long. Only rarely we observed sectors which comprise 1/8 or more of one leaf or of two or more successive leaves.

The sh-m6233 allele is caused by the insertion of a double Ds structure consisting of two 2044 bp Ds elements in the first intron of the sh gene. We concluded from previous experiments (Doring et al., Mol. Gen. Genet. 219:299-305, 1989) that the double Ds structure is causing chromosome breakage. If the chromosome breaks at the position of the double Ds structure, it is expected that the DNA sequences located distally to Ds are lost whereas the sequences located proximally are still present.

We analysed the DNA of 17 independent yg2 sectors. In 4 cases the hybridization to the 5' region of the sh-m6233 allele was missing and the hybridization to the 3' region was still present. This finding strongly supports the notion that chromosomes can indeed break at the double Ds structure. This finding also determines the orientation of the Sh locus with respect to the centromere. The 5' end of the Sh gene points towards the telomere.

In the DNA of 8 other sectors we did not observe any change in the hybridization compared to non-sector DNA. Apparently, these sectors were caused by a break whose position is distal to the sh-m6233 allele. In 3 sector DNAs we did not find any hybridization to the sh-m6233 allele. Probably, these sectors were caused by a break which occurred proximal to the sh-m6233 allele. In the remaining 2 cases we observed a distal chromosome break plus rearrangements of undetermined nature at the sh-m6233 allele. It is suggestive that the distal and proximal chromosome breaks are the result of aberrant transposition attempts of the double Ds structure.

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