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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