In Drosophila, the gene pipe is expressed in follicle cells, the somatic cells that surround the forming egg during maturation, specifically on one side of the egg chamber. This asymmetry establishes the dorsoventral axis of the future embryo. Through the action of pipe, the ligand spa ¨tzle, that is located in the perivitelline fluid of the embryo, is activated ventrally. This signal activates Toll, a membrane-bound receptor. According to present knowledge, pipe encodes two different transcripts, one of which restored ventral pattern elements to embryos when introduced into mutant pipe females. Here we show that pipe is far more complex than previously reported. It encodes not two, but at least ten different transcripts, two of which are localized to ventral follicle cells. The transcripts contain one of ten copies of a variable domain, all homologous to heparan sulfate 2-sulfotransferase, an enzyme known to modify heparan sulfate proteoglycans, which are molecules that can bind ligands. The complex gene structure of pipe thus evolved by duplications of one exon, a strategy used by genes of the immunoglobulin superfamily to generate molecular diversity. We show that pipe transcripts can be eliminated by RNAi, although in this method double-stranded RNA is injected in embryos, while pipe transcripts appear in the adult ovary. Our data suggest that at least two different pipe transcripts redundantly provide the ventralizing pipe function. 3 of pipe we identified an enhancer element that drives a lacZ reporter gene specifically in ventral follicle cells. Since pipe transcripts are found in salivary glands, and since expression of salivary gland genes is dependent on signaling molecules, we speculate that pipe became localized to ventral follicle cells by a preexisting control system after acquiring a follicle cell enhancer.
In Drosophila, the sex of XX germ cells is determined by somatic signals. Whether sex-specific genes, however, are activated or repressed by somatic signals is not known. We have used mgm1, a germline-marker which is specifically expressed in male germ cells to analyze sex-specific gene expression of embryonic germ cells. We found that XX and XY germ cells that do not contact gonadal tissue can express mgm1. In contrast, XX germ cells that were associated with female somatic gonadal cells never expressed mgm1. Our results suggest that XX germ cells express male-specific genes, unless these genes are repressed by feminizing short range signals produced by the somatic cells of the prospective ovary.
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