Figure 1. Structure of the P transposable element. The P element contains 31 bp ITRs (arrowheads) that flank a transposase gene (pink box) that mediates that transposition of the P element.

Figure 2. The Rubin-Spradling paradigm of germline transformation. A donor and a helper plasmid are coinjected into pre-blastoderm embryos of D. melanogaster. The surviving injected individuals (*) are then backcrossed to the parental strain and the offspring are screened for the expression of the marker gene contained in the donor plasmid. The marker used here is an eye pigmentation gene, and while the parental strain has white eye colour, the transformed individuals show red eye pigmentation. Donor plasmid: Contains a transposable element (grey arrowheads) into which a marker gene is inserted (here the wild-type gene for eye pigmentation; indicated by the blue and green bars). Helper plasmid: Contains the corresponding transposase (purple bar) under the control of an inducible promoter, in this case the heat shock 70 promoter (hsp70) of D. melanogaster (yellow rectangle).