The Drosophila dorsoventral determinant pipe contains ten copies of a variable domain homologous to mammalian heparan sulfate 2-sulfotransferase

In Drosophila, the gene pipe is expressed in follicle cells, the somatic ce lls that surround the forming egg during maturation, specifically on one si de of the egg chamber. This asymmetry establishes the dorsoventral axis of the future embryo. Through the action of pipe, the ligand spatzle, that is located in the perivitelline fluid of the embryo, is activated ventrally. T his signal activates Toll, a membrane-bound receptor. According to present knowledge, pipe encodes two different transcripts, one of which restored ve ntral pattern elements to embryos when introduced into mutant pipe females. Here we show that pipe is far more complex than previously reported. It en codes not two, but at least ten different transcripts, two of which are loc alized to ventral follicle cells, The transcripts contain one of ten copies of a variable domain, all homologous to heparan sulfate a-sulfotransferase , an enzyme known to modify heparan sulfate proteoglycans, which are molecu les that can kind 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 provi de the ventralizing pipe function. 3' of pipe we identified an enhancer ele ment that drives a lacZ reporter gene specifically in ventral follicle cell s. Since pipe transcripts are found in salivary glands, and since expressio n of salivary gland genes is dependent on signaling molecules, we speculate that pipe became localized to ventral follicle cells by a preexisting cont rol system after acquiring a follicle cell enhancer. (C) 2001 Wiley-Liss, I nc.