THE FOLLICLE CELLS ARE A MAJOR SITE OF VITELLOGENIN SYNTHESIS IN DROSOPHILA-MELANOGASTER

Pulse labeling of proteins, in vivo, followed by indirect immunoprecipitation of the vitellogenin polypeptides, has shown that not only the thoracic and abdominal fat bodies but also the ovary devote a significant percentage of their synthetic capacity to vitellogenin (VG) production. Ovarian stages 9 and 10 contribute the majority of VG synthesized by the ovary. The follicular epithelium of these stages is the specific site of VG synthesis. In situ hybridization (of a probe containing the coding regions of the 2 larger polypeptides) to sections of ovaries confirmed that the VG mRNA are abundant species in the cytoplasm of stage 9 and 10 follicle cells. Of the 3 polypeptides, 2 (VGP1 and VGP2) are produced at roughly equal levels by the follicle cells but the smallest polypeptide (VGP3) is produced at 1/4 this level by these cells. Hybridization of cloned genomic probes to RNA bound on nitrocellulose filters has shown that the ovary contributes .apprx. 35% of the total amount of the mRNA coding for VGP1 and VGP2 but only about 10% of the mRNA for VGP3. The levels of all 3 VG mRNA during follicular development closely parallel VG polypeptide synthesis. Culture of ovaries in males has shown that the mRNA levels accurately reflect the follicle cell contribution to VG synthesis.