ABOUT THE ORIGIN OF RETROVIRUSES AND THE COEVOLUTION OF THE GYPSY RETROVIRUS WITH THE DROSOPHILA-FLAMENCO HOST GENE

The gypsy element of Drosophila melanogaster is the first retrovirus i dentified so far in invertebrates. According to phylogenetic data, gyp sy belongs to the same group as the Ty3 class of LTR-retrotransposons, which suggests that retroviruses evolved from this kind of retroeleme nts before the radiation of vertebrates. There are other invertebrate retroelements that are also likely to be endogenous retroviruses becau se they share with gypsy some structural and functional retroviral-lik e characteristics. Gypsy is controlled by a Drosophila gene called fla menco, the restrictive alleles of which maintain the retrovirus in a r epressed state. In permissive strains, functional gypsy elements trans pose at high frequency and produce infective particles. Defective gyps y proviruses located in pericentromeric heterochromatin of all strains seem to be very old components of the genome of Drosophila melanogast er, which indicates that gypsy invaded this species, or an ancestor, a long time ago. At that time, Drosophila melanogaster presumably conta ined permissive alleles of the flamenco gene. One can imagine that the species survived to the increase of genetic load caused by the retrov iral invasion because restrictive alleles of flamenco were selected. T he characterization of a retrovirus in Drosophila, one of the most adv anced model organisms for molecular genetics, provides us with an exce ptional clue to study how a species can resist a retroviral invasion.