Abstract
Several results suggest that P elements have recently invaded natural populations of Drosophila melanogaster after a horizontal transfer from another species. The donor species is thought to come from the willistoni group, which contains P elements very homologous to those of D. melanogaster. However, more divergent P elements are present in many other Drosophilidae species. We have analyzed such elements from Scaptomyza pallida, a species phylogenetically distant to D. melanogaster. We report here the isolation of two coding P elements from S. pallida (PS2 and PS18) that are 4% divergent from one another. At least one of these elements (PS18) is active since it is able to transpose in D. melanogaster and to mobilize a D. melanogaster defective P element, even though its nucleotide sequence is 24% divergent from the canonical P element of D. melanogaster. To our knowledge, a P element that is active and strongly divergent from the D. melanogaster P element has not been reported previously. Sequence comparison between the complete P elements of D. melanogaster and S. pallida reveals that the structural characteristics are maintained: PS2 and PS18 contain terminal inverted repeats and internal repeats very similar to those of the D. melanogaster P element. In addition, the noncoding regions cis necessary for the transposition are more conserved than the coding sequences. Two domains found in the D. melanogaster P transposase (helix-turn-helix and leucine zipper) are well conserved in the putative proteins encoded by PS2 and PS18. This study provides insights into which parts of P elements are functionally important and correlates with functional studies of the P element in D. melanogaster.
