Fixed and unstable I-related transposable elements in heterochromatin of Drosophila melanogaster.

Transposable elements are disproportionately abundant in the heterochromatin of Drosophila melanogaster. Among the forces contributing to this bias in genomic distribution, fixation due to positive selection has been put forward. We have studied I-related elements which are located in pericentromeric heterochromatin and are believed to have a role in the control of active I elements. Flies straight from the wild have been studied where fixed elements are expected to emerge clearly over the highly polymorphic background in the genomic distribution of transposable elements. The results show that some restriction fragments due to I-related elements are conserved in size and are present in all individuals tested, consistent with a selective pressure for a role. Other fragments are polymorphic in presence/absence and intensity in individuals from the wild but appear homogeneous in laboratory stocks. Although the significance of this type of instability is unclear, the finding that these polymorphic bands are recurrent in populations from distant geographical locations is also suggestive of a selective pressure for a role.