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.