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 s tocks of this species, using Southern blot and in situ hybridization e xperiments to examine the number, structure, and distribution of P-ele ments. All four stocks give similar results: they contain about 15 P-e lements including three to four full-length elements as well as shorte r, deleted elements. All elements are divergent from one another and m ost of them appear to be immobile since they are located at identical positions in the genomes of independent stocks. These data indicate th at P-elements are old components of the S. pallida genome. Moreover, t he presence of P-sequences in species closely related to S. pallida su ggests that they have had a long evolutionary history in the Scaptomyz a genus. We have also found that most P-elements of S. pallida are loc ated in the pericentromeric heterochromatin. This corroborates other s tudies which show that in the course of their evolution, transposable elements tend to accumulate into pericentromeric heterochromatin, wher e they become immobile and noncoding.