P-ELEMENTS ARE OLD COMPONENTS OF THE SCAPTOMYZA-PALLIDA GENOME

We report the cloning and analysis of a sample representative of all P-elements from Scaptomyza pallida. We have compared four independent stocks of this species, using Southern blot and in situ hybridization experiments to examine the number, structure, and distribution of P-elements. All four stocks give similar results: they contain about 15 P-elements including three to four full-length elements as well as shorter, deleted elements. All elements are divergent from one another and most of them appear to be immobile since they are located at identical positions in the genomes of independent stocks. These data indicate that P-elements are old components of the S. pallida genome. Moreover, the presence of P-sequences in species closely related to S. pallida suggests that they have had a long evolutionary history in the Scaptomyza genus. We have also found that most P-elements of S. pallida are located in the pericentromeric heterochromatin. This corroborates other studies which show that in the course of their evolution, transposable elements tend to accumulate into pericentromeric heterochromatin, where they become immobile and noncoding.