CONSERVATION AND DIVERGENCE IN THE CONTROL OF YOLK PROTEIN GENES IN DIPTERAN INSECTS

We have investigated the conservation of regulatory elements for sex-a nd tissue-specific gene expression in three dipteran species, Drosophi la melanogaster Musca domestica and Calliphora erythrocephala, using t he yolk protein (yp) genes. Yolk proteins of the fruitfly, medfly, hou sefly and blowfly are very well conserved both in their sequence and t heir expression in ovarian follicle cells and in fat bodies of adult f emales. Furthermore, yp regulation by both hormonal and nutritional fa ctors shows similar features in all four species. To study conservatio n of yp regulation in dipteran insects, we tested 5' flanking regions from one Musca yp gene and one Calliphora yp gene for enhancer functio ns in D. melanogaster. Two fragments of 823 and 1046 bp isolated from Musca and Calliphora yp genes, respectively, are able to direct correc t expression of a reporter gene in the ovarian follicle cells of trans formed Drosophila at specific stages during oogenesis. Surprisingly, t hese enhancers do not confer sex-specific reporter gene expression in the fat body, as expression was found in both sexes of the transformed flies. None-the-less by in vitro DNA/protein interaction assays, a 28 4-bp DNA region from the Musca yp, enhancer was able to bind the Droso phila DOUBLESEX (DSX) protein, which in D.melanogaster confers sex-spe cific expression of yp. We speculate that the sex-determining pathway is not directly involved in yp regulation in Musca or Calliphora adult females, but depends instead on hormonal controls to achieve sex-spec ific expression of yp genes in the adult.