TRANSPOSABLE ELEMENTS ARE STABLE STRUCTURAL COMPONENTS OF DROSOPHILA-MELANOGASTER HETEROCHROMATIN

We determined the distribution of 11 different transposable elements o n Drosophila melanogaster mitotic chromosomes by using high-resolution fluorescent in situ hybridization (FISH) coupled with charge-coupled device camera analysis. Nine of these transposable elements (copia, gy psy, mdg-1, blood, Doc, I, F, G, and Bari-1) are preferentially cluste red into one or more discrete heterochromatic regions in chromosomes o f the Oregon-R laboratory stock. Moreover, FISH analysis of geographic ally distant strains revealed that the locations of these heterochroma tic transposable element clusters are highly conserved. The P and hobo elements, which are likely to have invaded the D. melanogaster genome at the beginning of this century, are absent from Oregon-R heterochro matin but clearly exhibit heterochromatic clusters in certain natural populations. Together these data indicate that transposable elements a re major structural components of Drosophila heterochromatin, and they change the current views on the role of transposable elements in host genome evolution.