P-element distribution in Eurasian populations of Drosophila melanogaster: A genetic and molecular analysis.

Genetic and molecular investigations were carried out with Eurasian Drosophila melanogaster populations on the P-M system of hybrid dysgenesis. In 27 strains sampled from France to Middle Asia, a clear gradient exists between Western Europe, in which most modern strains are of the Q type, and eastern areas, in which M-cytotype strains predominate. Molecular analysis on individual flies was performed with two complementary probes of the cloned 2.9-kilobase P element. The results provide evidence for a gradually decreasing frequency of P elements from west to east, but the presence of P-homologous sequences has been ascertained in all of the wild M-cytotype populations analyzed. Moreover, some active P elements with GD sterility potential were revealed in the majority of M-cytotype populations when tested with a highly sensitive reference line. The gradual change in distribution of the polymorphic P family in Eurasia is discussed in relation to the structure of the elements together with the theories of P-M evolution and is interpreted as the present invasion of Eurasian populations by these elements.