Le gène flamenco de la drosophile, ou comment résister à un rétrovirus.

Abstract

Pendant longtemps, on a pensé que les rétrovirus étaient spécifiques des vertébrés. Or, des résultats récents montrent que l’élément gypsy de la drosophile, considéré habituellement comme un élément transposable de la classe des rétrotransposons, est un véritable rétrovirus, le premier identifié chez les invertébrés. Un gène de l’hôte, appelé flamenco, maintient le virus à l’état réprimé dans la plupart des souches de drosophile. Lorsque le gène est muté, gypsy est transposé avec des fréquences très élevées et produit des particules infectieuses. De nombreux résultats indiquent que le rétrovirus gypsy a envahi l’espèce Drosophila melanogaster dans un passé relativement ancien, et que l’espèce n’a survécu à cette invasion que grâce à la sélection d’allèles du gène flamenco permettant de bloquer la propagation du virus. L’analyse des relations phylogénétiques entre rétrotransposons et rétrovirus indique que gypsy pourrait correspondre à la forme ancestrale des rétrovirus, à partir de laquelle les rétrovirus des vertébrés sont apparus.

For a long time, retroviruses, that are responsible for serious diseases, have been thought to be restricted to vertebrates. They are actually more widely dispersed. Recent results indicate that the gypsy element of Drosophila melanogaster, usually considered as a transposable element of the class of retrotransposons, has infective properties and is therefore the first retrovirus identified in invertebrates. The genome of insects contain other retroelements which, like gypsy, are strikingly similar to vertebrates proviruses of retroviruses. It is likely that they are also endogenous retroviruses. Gypsy is controlled by a Drosophila gene called flamenco. Many strains contain non permissive alleles of this gene that maintain the retrovirus in a repressed state. When the flamenco gene is mutated gypsy transposes at high frequency and produces infectious particles. Many results show that gypsy invaded the Drosophila melanogaster species, or an ancestor of this species, a long 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 permissive alleles of the gene and of functional gypsy elements results in the occurrence of many abnormalities, and one can imagine that the species survived the retroviral invasion because non permissive alleles of flamenco were selected. The characterization of a retrovirus in Drosophila, one of the most advanced model organisms for molecular genetics, 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 between retroviruses and retrotransposons indicate that gypsy might he a member of an ancestral class of retroviruses from which vertebrate retroviruses evolved. Therefore the identification of a retrovirus in Drosophila provides a very powerful experimental system to investigate both the genetic relationships between retroviruses and their hosts and the evolutionary relationships between retroviruses and retrotransposons. In addition, this endogenous retrovirus might be used to construct retroviral vectors. Therefore the characterization of a retrovirus in Drosophila might be extremely useful to develop tools which could be used also to study insects of medical and agricultural interest.