The effect of heterologous insertions on gene conversion in mitotically dividing cells in Drosophila melanogaster.

We examined the influence that heterologous sequences of different sizes have on the frequency of double-strand-break repair by gene conversion in Drosophila melanogaster. We induced a double-strand break on one X chromosome in female flies by P-element excision. These flies contained heterologous insertions of various sizes located 238 bp from the break site in cis or in trans to the break, or both. We observed a significant decrease in double-strand-break repair with large heterologous insertions located either in cis or in trans to the break. Reestablishing the homology by including the same heterologous sequence in cis and in trans to the double-strand break restored the frequency of gene conversion to wild-type levels. In one instance, an allelic nonhomologous insertion completely abolished repair by homologous recombination. The results show that the repair of a double-strand break by gene conversion requires chromosome pairing in the local region of the double-strand break.