GENOMIC DISTRIBUTION OF THE RETROVIRUS-LIKE ELEMENT ZAM IN DROSOPHILA

The mobile element ZAM, recently identified in Drosophila melanogaster , is similar in structure and coding potential to vertebrate retroviru ses. In this paper, we analyze the insertional and structural polymorp hism of this element and show that members of this family appear to ha ve a long evolutionary history in the genome of Drosophila. It is pres ent in all the species of the D. melanogaster subgroup and in more dis tantly related species like D. takahashii, D. ananassae, or D. virilis but in a lower copy number or with a lower homology. Two categories o f strains have been previously identified in D. melanogaster: strains with a high copy number of ZAM and strains with a low copy number. Her e, we show that ZAM is at least in a low copy number in each tested st rain of the species analyzed. The study of ZAM's genomic distribution by FISH mapping analysis to salivary gland polytene chromosomes or on mitotic chromosomes indicates that most of the insertion sites of ZAM elements are associated with the constitutive heterochromatin regardle ss of the ZAM copy number. In addition, our results suggest that multi ple ZAM elements are present at the insertion sites visualized by in s itu experiments.