THE FLAMENCO GENE OF DROSOPHILA - HOW TO RESIST A RETROVIRUS

For a long time, retroviruses, that are responsible for serious diseas es, have been thought to be restricted to vertebrates. They are actual ly more widely dispersed. Recent results indicate that the gypsy eleme nt of Drosophila melanogaster, usually considered a transposable eleme nt of the class of retrotransposons, has infective properties and is t herefore the first retrovirus identified in invertebrates. The genome of insects contain other retroelements which, like gypsy, are striking ly similar to vertebrates proviruses of retroviruses. It is likely tha t they are also endogenous retroviruses. Gypsy is controlled by a Dros ophila gene called flamenco. Many strains contain non permissive allel es of this gene that maintain the retrovirus in a repressed state. Whe n the flamenco gene is mutated gypsy transposes at high frequency and produces infectious particles. Many results show that gypsy invaded th e Drosophila melanogaster species, or an ancestor of this species, a l ong time ago, indicating that the species at that time was permissive for propagation of the retrovirus and presumably contained permissive alleles of the flamenco gene. The simultaneous presence in flies of pe rmissive alleles of the gene and of functional gipsy elements results in the occurrence of many abnormalities, and one can imagine that the species survived the retroviral invasion because non permissive allele s of flamenco were selected. The characterization of a retrovirus in D rosophila, one of the most advanced model organisms for molecular gene tics, provides us with an exceptional clue to study how a species can resist a retroviral invasion. Similar investigations are presently not possible in vertebrates. Analysis of the phylogenetic relationships b etween retroviruses and retrotransposons indicate that gypsy might be a member of an ancestral class of retroviruses evolved. Therefore, the identification of a retrovirus in Drosophila provides a very powerful experimental system to investigate both the genetic relationships bet ween retroviruses and their hosts and the evolutionary relationship be tween retroviruses and retrotransposons. In addition, this endogenous retrovirus might be used to construct retroviral vectors. Therefore th e characterization of a retrovirus in Drosophila might be extremely us eful to develop tools which could be used also to study insects of med ical and agricultural interest.