Stability of European natural populations of Drosophila melanogaster with regard to the P-M system: a buffer zone made up of Q populations

Current natural populations of Drosophila melanogaster from Eurasia, Africa and Oceania were investigated with regard to the P-M system of hybrid dysg enesis, for both genetic properties (gonadal dysgenesis sterility analyses) and molecular characteristics (number of full-size elements and particular P element deletion-derivatives, the KP elements). Full-size and KP element s are, respectively, at the origin of two distinct regulation systems, the maternally transmitted P cytotype and the KP-mediated repression whose tran smission is biparental. The results show both qualitative and quantitative differences in the geographical distribution of P elements. Comparison with distributions observed in 1980-1983 reveals a great stability of natural p opulations with regard to this system. In particular, the eastward gradient of P susceptibility previously described in Europe is still observed. This stability could result from the existence of a 'buffer zone' made up of th e French and bordering Q populations (with no P activity and completely reg ulating the transposition of active P elements). Indeed, in such population s repression mechanisms are redundant, as revealed by the study of repressi on inheritance. These populations are thus potentially able to limit the pr ogression of P elements that occurs by step by step migrations. This distri bution also allows us to enrich the P element invasion model, which can be divided into three steps: (1) a decrease in the number of full-size element s which coincides with an increase in the number of ICP elements due to a r egulatory role or a high transposition capacity; (2) an equilibrium, when t he number of ICP elements reaches a maximum and in which populations still have some full-size elements; (3) ICP elements reduce in number in the abse nce of full-size elements allowing transposition the populations losing the ir repression potential.