P-ELEMENT-INDUCED INTERALLELIC GENE CONVERSION OF INSERTIONS AND DELETIONS IN DROSOPHILA-MELANOGASTER

We studied the process hy WhiCh w(hd), a P-element insertion allele of the Drosophila melanogaster white locus, is replaced by its homolog i n the presence of transposase. These events are interpreted as the res ult of double-strand gap repair following excision of the P transposon in W(hd). We used a series of alleles derived from w(hd) through P-el ement mobility as templates for this repair. One group of alleles, ref erred to collectively as w(hd-F), carried fragments of the P element t hat had lost some of the sequences needed in cis for mobility. The oth er group, W(hd-D), had lost all of the P insert and had some of the fl anking DNA from white deleted. The average replacement frequencies wer e 43% for w(hd-F) alleles and 7% for the w(hd-D) alleles. Some of the former were converted at frequencies exceeding 50%. Our data suggest t hat the high conversion frequencies for the w(hd-F) templates can be a ttributed at least in part to an elevated efficiency of repair of unex panded gaps that is possibly caused by the closer match between w(hd-F ) sequences and the unexpanded gap endpoints. In addition, we found th at the gene substitutions were almost exclusively-in the direction of w(hd) being replaced by the w(hd-F) or w(hd-D) allele rather than the reverse. The template alleles were usually unaltered in the process. T his asymmetry implies that the conversion process is unidirectional an d that the P fragments are not good substrates for P-element transposa se. Our results help elucidate a highly efficient double-strand gap re pair mechanism in D. melanogaster that can also be used for gene repla cement procedures involving insertions and deletions. They also help e xplain the rapid spread of P elements in populations.