The cloning of the Bar region and the B breakpoint in Drosophila melanogaster: evidence for a transposon-induced rearrangement.

We have cloned the B breakpoint in Drosophila melanogaster using DNA from a P-M-induced revertant of B, which has a P element inserted at the B breakpoint. The analysis of the B DNA reveals that there is a transposable element, B104, right at the breakpoint. This suggests that this element may have been involved in the generation of the B breakpoint and the associated tandem duplication. One possible mechanism to generate the B duplication is a recombination event between two B104 elements, one at 16A1 and the other at 16A7. DNA sequencing data of the junctions of the B104 element support this model. Four partial revertants of B are the result of insertions of transposable elements very close to the B breakpoint. This supports the hypothesis that the breakpoint is the cause of the B mutation. The clones from B were used to isolate wild-type clones from 16A1, the location of the Bar gene. Four rearrangement breakpoints associated with various Bar mutations map within a 37-kb region, suggesting that the Bar gene is very large.