The regulation of yolk protein gene expression in Drosophila melanogaster

The three genes, located in the X chromosome, which code for the three yolk polypeptides (YPs) of Drosophila melanogaster are expressed in the fat bodies and ovarian follicle cells of adult females. Both juvenile hormone and ecdysone are involved in regulating their expression. The yolk protein genes (YP genes), normally not transcribed in males, become expressed when males are injected with or fed 20-hydroxyecdysone. Superimposed on this hormonal regime is a sex determination mechanism which ensures that normally YP gene expression is female-specific. There are a series of autosomal genes in D. melanogaster which ensure that individual cells follow a male or female developmental pathway. When they are mutant, flies with two X chromosomes, which would normally be female, can become intersexual in phenotype or transformed into sterile males and flies with one X and one Y chromosome can become intersexual. It has been found that the YPs are part of the set of female characteristics controlled by these sex genes. The YP genes are expressed in female and intersexual flies, regardless of the X chromosome constitution, but not in males or pseudomales. Transcript levels of yolk proteins have been measured in female and intersexual flies by hybridization to cloned YP DNA sequences. It is suggested that transcription of the YP genes is under the cell-autonomous control of the sex genes and that the sex genes do not exert their effect by modulating the levels of steroid hormones in adults.