EVOLUTIONARY STEPS AND TRANSPOSABLE ELEMENTS IN DROSOPHILA MELANOGASTER: THE MISSING RP TYPE OBTAINED BY GENETIC TRANSFORMATION.

The I-R and P-M hybrid dysgenesis systems in Drosophila melanogaster have been interpreted as due to recent invasions of the genome by the I and P mobile genetic elements. Temporal and geographical surveys have never shown individuals harboring P sequences but devoid of active I elements. We describe here the successful genetic transformation by autonomous P elements of embryos initially devoid of active I elements and any P sequences. The results demonstrate that P elements may invade the genome of Drosophila melanogaster in the absence of active I elements. Using gel blotting, in situ hybridization techniques, and genetic experiments, we have monitored the behavior of newly introduced P elements in several transformed lines over 30 generations. The switch of cytotype from M to P occurred very slowly and the number of P copies simultaneously increased to about 25. These RP lines possess the properties required to induce P-M hybrid dysgenesis but totally retain the R cellular state. Consequently, this new mobile element combination presents a strong reciprocal post-zygotic isolation with IM strains due to both P-M and I-R hybrid dysgenesis systems. This genomic incompatibility could be considered as a first step toward speciation in Drosophila populations.