Sexual phenotype and vitellogenin synthesis in Drosophila melanogaster

An ovary transplanted from a Drosophila melanogaster female into a male will mature and form morphologically normal yolk-filled oocytes. Since it has been supposed that the yolk polypeptides come only from the female fat body, it was hypothesized that the implanted ovary induces the fat body of the male host to synthesize and secrete yolk polypeptides (YPs). To test this hypothesis, fat body preparations from females, untreated males, and males containing transplanted ovaries were cultured in vitro with ³⁵S-methionine and the medium was examined for the presence of newly labeled YPs. Female fat body secreted newly labeled YPs, but no freshly synthesized YPs were secreted by fat bodies from untreated males or from males containing transplanted ovaries. In vitro cultured ovaries, however, both from females and from male hosts did secrete newly synthesized YPs. Therefore, the YPs in an ovary that matured in a male come mainly from endogenous synthesis by the implanted ovary. To find whether males were responsive to the hormones that stimulate YP production in isolated female abdomens, we treated males with the juvenile hormone analogue ZR-515 and with 20-hydroxyecdysone. The latter, but not the former, was able to cause synthesis and secretion of three bands migrating precisely as YPs in SDS gels. Partial peptide digests of the 20-hydroxyecdysone-stimulated polypeptides in males showed them to be identical with those stimulated by 20-hydroxyecdysone or ZR-515 in isolated female abdomens and with the three YPs found in normal female hemolymph. Finally, YP synthesis was assayed in mutants that affect the phenotypic sex of a fly. It was found that flies bearing two X chromosomes and the mutations dsx, dsx^D, ix or three sets of autosomes continued to make YPs, but tra-3-pseudomales did not. These results suggest that the process of sex determination involves steps leading to synthesis of an ecdysteroid in females, which then activates synthesis of the YPs by the fat body. A hypothesis is suggested to explain the fact that two different hormones can stimulate YP synthesis and two different organs can synthesize YPs.