Impact of multiple insertions of two retroelements, ZAM and Idefix at an euchromatic locus

Transposable elements represent a large fraction of eukaryotic genomes and they are thought to play an important role in chromatin structure. ZAM and Idefix are two LTR-retrotransposons from Drosophila melanogaster very simil ar in structure to vertebrate retroviruses. In all the strains where their distribution has been studied, ZAM appears to be present exclusively in the intercalary heterochromatin while Idefix copies are mainly found in the ce ntromeric heterochromatin with very few copies in euchromatin. Their distri bution varies in a specific strain called RevI in which the mobilization of ZAM and Idefix is highly induced. In this strain, 15 copies of ZAM and 30 copies of Idefix are found on the chromosomal arms in addition to their usu al distribution. Amongst the loci where new copies are detected, a hotspot for their insertion has been detected at the white locus where up to four e lements occurred within a 3-kb fragment at the 5' end of this gene. This pr operty of ZAM and Idefix to accumulate at a defined site provides an intere sting paradigm to bring insight into the effect exerted by multiple inserti ons of transposable elements at an euchromatic locus.