P element excision in Drosophila melanogaster and related drosophilids

The frequency of P element excision and the structure of the resulting excision products were determined in three drosophilid species. Drosophila melanogaster, D. virilis, and Chymomyza procnemis. A transient P element mobility assay was conducted in the cells of developing insect embryos, but unlike previous assays, this mobility assay permitted the recovery of excision products from plasmids regardless of whether the excision event was precise or imprecise. Both quantitative and qualitative differences between the products of excision in the various species studied were observed. The frequency with which P element excision products were recovered from D. melanogaster was 10-fold greater than from D. virilis and C. procnemis; however, the proportion of all excision events resulting in the reversion of a P-induced mutant phenotype was the same. Virtually all excision products recovered, including those resulting in a reversion of the mutant phenotype, did not result in the exact restoration of the original target sequence. Sequence analysis suggested that duplex cleavage at the 3' and 5' termini of the P element, or their subsequent modification, occurred asymmetrically and interdependently. P element-encoded transposase was not absolutely required for P element excision.