EXPRESSION OF THE DROSOPHILA RETROVIRUS GYPSY AS ULTRASTRUCTURALLY DETECTABLE PARTICLES IN THE OVARIES OF FLIES CARRYING A PERMISSIVE FLAMENCO ALLELE

The endogenous retrovirus gypsy is controlled by the Drosophila gene f lamenco (flam), New insertions of gypsy occur in any individual Drosop hila if its mother is homozygous for the flam(1) permissive allele and contains functional gypsy proviruses, The ovaries of flam(1) females also contain high amounts of gypsy RNAs, Unexpectedly however, gypsy d erepression does not occur in the flam(1) female germline proper but i n the somatic follicular epithelium of the ovary. Since extracts from these females are able to efficiently infect the germ-line of a strain devoid of active gypsy proviruses,;we assume that a similar kind of g erm-line infection, which would occur inside the flam(1) females thems elves, could be required for gypsy insertions to occur in their progen y, This hypothesis was confirmed by electron microscopy observations s howing that non-enveloped intracytoplasmic particles containing gypsy RNAs accumulate in the apical region of the flam(1) follicle cells, cl ose to specific membrane domains to which the gypsy envelope proteins are targeted, whereas both are absent in the flam(+) controls. Low amo unts of similar virus-like particles were also observed in flam(1) ooc ytes, but it is not yet known whether they entered passively or as a r esult of membrane fusion. This is the first report of the beginning of a retrovirus cycle in invertebrates and these observations should be taken into account when explaining the maternal effect of the flamenco gene on the multiplication of gypsy proviruses.