The three yolk protein genes (yps) of Drosophila melanogaster are expressed in the ovary and fat body of the adult female. Their levels of expression in the fat body depend upon both juvenile hormone (JH) and 20-hydroxyecdysone (20E). Using transformed lines of flies with various flanking sequences from the yp genes and lacZ, Adh, or native yp genes as reporters, the regulation of the three yp genes by 20E and the JH analogue ZR515 (methoprene) was investigated. For 20E, induction of reporter gene expression in males was assayed and, for JH, upregulation of the genes in nutritionally deprived females, which express yolk proteins (YPs) at very low levels, was followed. We were able to map 20E inducible sites upstream of yp3 and sites located 3' and within the coding sequence or introns of yp3 which can interact to respond to 20E. There are also sites in the intergenic spacer between yp1 and yp2. Evidence for repressors was also found upstream of the yp genes, suggesting downstream 20E inducible elements may be important in vivo. There appears to be a difference in the response to 20E in the fat body of the thorax and abdomen between different constructs in males. It is not clear whether those sequences which respond to 20E are genuine ecdysone response elements (i.e., binding sites for the ecdysone receptor) or if the effect is indirect. Methoprene upregulation of YPs, however, was only ever observed using native yp genes as reporters, suggesting that this hormone may act on intron sequences or yp coding sequences, or perhaps by influencing stability of the yp mRNA.

The yolk proteins of Drosophila melanogaster comprise a family of three related yolk polypeptides each encoded by a single-copy gene. We show by genetic crosses that each gene makes an equivalent contribution to the fecundity and fertility of the female and they do not individually provide unique functions to the embryo. We show that the number of eggs laid by a female depends upon the number of genes encoding yolk polypeptides present in the genome and furthermore that the probability of an egg hatching into an adult also critically depends upon the number of yolk protein genes present in the mother. This suggests that the three yolk protein-encoding genes in Drosophila melanogaster may have arisen by duplication, then been maintained for quantitative reasons because they increased egg production and fertility, rather than each protein evolving a different function as is the case with most small gene families, such as tubulins and collagen genes.

The yolk protein genes (yps) of Drosophila melanogaster are only expressed in the ovary and fat body of female flies if they are supplied with proteinaceous food. This nutritional response is specific to the yp genes. We have used transgenic flies transformed with a series of constructs bearing deletions in the upstream region of the yp1 and yp2 genes attached to a reporter gene to search for DNA sequences responsible for the nutritional induction specific for yp1 and yp2 genes. Several regions were shown independently of each other to confer nutritional regulation on the expression of the yp1 and yp2 genes. This regulation can be induced both on the yp promoter and the heterologous Drosophila heat-shock 70 (hsp 70) promoter. The redundancy of sequences conferring a nutritional response on the yp genes is similar to that observed for the female specificity of these genes and suggest that several DNA binding proteins interact to provide the correct regulation of these genes. These results suggest that nutrition acts to modify the level of a trans-acting factor in the fat body. Northern blot analysis showed that the transcript levels from the dsx gene are not affected by nutrition, indicating that the response is not mediated via the dsx gene.

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