The gypsy element of Drosophila differs from most LTR retrotransposons in containing a third open reading frame that resembles retroviral env genes. The protein encoded by ORF3 is glycosylated and processed, like all retroviral envelope proteins. The protein is expressed at high levels in fly strains in which gypsy elements are active. In these strains the protein is found primarily in viral particles. When larvae of fly strains in which gypsy is normally inactive are exposed to sucrose gradient fractions containing these particles, a high level of gypsy insertion activity is observed in their progeny. Thus, gypsy has the expected properties of an insect retrovirus. [Key Words: Drosophila; mutagenesis; transposable element; insect retrovirus] Received May 6, 1994; revised version accepted July 11, 1994.The gypsy element of Drosophila melanogaster has been classified traditionally as a long terminal repeat (LTR) retrotransposon; however, it is one of a small group of LTR retrotransposons from insects that are unusual in that they contain three open reading frames (ORFs). In these retroelements, the first two ORFs correspond to retroviral gag and poi, whereas ORF3 is of unknown function but corresponds in size and genomic location to retroviral env (Fig. 1). All elements with three ORFs described so far are from insects, either Drosophila or the lepidopteran Trichoplusia ni. Recent results have shown that in two elements encoding an ORF3, gypsy and tom, a subgenomic mRNA similar in structure to retroviral env mRNAs can be found (P61isson et al. 1994;Tanda et al. 1994). In the case of gypsy, this transcript is observed only in certain strains in which gypsy transpositional activity is high (P41isson et al. 1994). The primary sequences of the encoded ORF3 proteins of these elements are quite variable and show no obvious similarity to retroviral Env proteins. However, retroviral Env proteins are themselves very variable in primary sequence. Like retroviral Env proteins, the proteins encoded by "retrotransposon" ORF3s contain a putative transmembrane domain near their carboxyl terminus, multiple putative N-glycosylation sites, and putative protease cleavage sites (resembling the cleavage sites in a variety of retroviral Env proteins) at conserved positions (Fig. 1 }. These features of gypsy and the other ORF3-containing insect retrotransposons have prompted the suggestion that these elements may represent endogenous insect retro- viruses (Boeke 1988;Boeke and Corces 1989;Coffin 1993).To address the question of whether gypsy represents a virus or a transposon more directly, we have turned to the use of Drosophila strains that show genetic instabilities associated with high-frequency insertion by gypsy elements. These strains are characterized by a large number of full-length gypsy elements in the euchromatin, appearance of gypsy insertion mutations at high frequency, and the presence of both large amounts of gypsy full-length RNA and spliced ORF3 mRNA (P61isson et al. 1994; N. Prud'homme, M. Gans, M. Masson, C. Ter...

Gypsy displays striking similarities to vertebrate retroviruses, including the presence of a yet uncharacterized additional open reading frame (ORF3) and the recent evidence for infectivity. It is mobilized with high frequency in the germline of the progeny of females homozygous for the flamenco permissive mutation. We report the characterization of a gypsy subgenomic ORF3 RNA encoding typical retroviral envelope proteins. In females, env expression is strongly repressed by one copy of the non‐permissive allele of flamenco. A less dramatic reduction in the accumulation of other transcripts and retrotranscripts is also observed. These effects correlate well with the inhibition of gypsy transposition in the progeny of these females, and are therefore likely to be responsible for this phenomenon. The effects of flamenco on gypsy expression are apparently restricted to the somatic follicle cells that surround the maternal germline. Moreover, permissive follicle cells display a typically polarized distribution of gypsy RNAs and envelope proteins, both being mainly accumulated at the apical pole, close to the oocyte. We propose a model suggesting that gypsy germinal transposition might occur only in individuals that have maternally inherited enveloped gypsy particles due to infection of the maternal germline by the soma.

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