Proviral amplification of the Gypsy endogenous retrovirus of Drosophila melanogaster involves env-independent invasion of the female germline

Gypsy is an infectious endogenous retrovirus of Drosophila melanogaster, Th e gypsy proviruses replicate very efficiently in the genome of the progeny of females homozygous for permissive alleles of the flamenco gene. This rep licative transposition is correlated with derepression of gypsy expression, specifically in the somatic cells of the ovaries of the permissive mothers . The determinism of this amplification was studied further by making chime ric mothers containing different permissive/restrictive and somatic/germina l lineages. We show here that the derepression of active proviruses in the permissive soma is necessary and sufficient to induce proviral insertions i n the progeny, even if the Fl flies derive from restrictive germ cells devo id of active proviruses, Therefore, gypsy endogenous multiplication results from the transfer of some gypsy-encoded genetic material from the soma tow ards the germen of the mother and its subsequent insertion into the chromos omes of the progeny. This transfer, however, is not likely to result from r etroviral infection of the germline, Indeed, we also show here that the ins ertion of a tagged gypsy element, mutant for the env gene, occurs at high f requency, independently of the production of gypsy Env proteins by any tran scomplementing helper. The possible role of the env gene for horizontal tra nsfer to new hosts is discussed.