RETROVIRUSES IN INVERTEBRATES - THE GYPSY RETROTRANSPOSON IS APPARENTLY AN INFECTIOUS RETROVIRUS OF DROSOPHILA-MELANOGASTER

Retroviruses are commonly considered to be restricted to vertebrates. However, the genome of many eukaryotes contains mobile sequences known as retrotransposons with long terminal repeats (LTR retrotransposons) or viral retrotransposons, showing similarities with integrated provi ruses of retroviruses, such as Ty elements in Saccharomyces cerevisiae , copia-like elements in Drosophila, and endogenous proviruses in vert ebrates. The gypsy element of Drosophila melanogaster has LTRs and con tains three open reading frames, one of which encodes potential produc ts similar to gag-specific protease, reverse transcriptase, and endonu clease. It is more similar to typical retroviruses than to LTR retrotr ansposons. We report here experiments showing that gypsy can be transm itted by microinjecting egg plasm from embryos of a strain containing actively transposing gypsy elements into embryos of a strain originall y devoid of transposing elements. Horizontal transfer is also observed when individuals of the ''empty'' stock are raised on medium containi ng ground pupae of the stock possessing transposing elements. These re sults suggest that gypsy is an infectious retrovirus and provide evide nce that retroviruses also occur in invertebrates.