Broken chromosomal ends can be elongated by conversion in Drosophila melanogaster.

The fate of the termini of X chromosomes broken in the regulatory region of the yellow gene was followed in heterozygotes with X chromosomes carrying a point mutation inactivating the yellow gene. Each generation had a loss of about 70 terminal base pairs from the broken chromosome. However, gene conversion restoring the correct sequence at the chromosomal terminus took place with a frequency of about 1 x 10(-2) per generation. The average length of the conversion track was 2.7 kb. No recombination events occurred. In addition, we found that the normal functioning of the yellow body and wing enhancers located at the tip of the chromosome required about 4 kb of additional upstream sequence.