THE ROLE OF SEGMENT POLARITY GENES DURING EARLY OOGENESIS IN DROSOPHILA

In the Drosophila ovary, hedgehog (hh) signaling from cells near the a pical tip of the germarium stimulates the proliferation and specificat ion of somatic cells in region 2 of the germarium, 2-5 cells away from the hh-expressing cells (A. J. Forbes, H. Lin, P. Ingham and A. Sprad ling (1996) Development 122, 1125-1135). This report examines the role during early oogenesis of several genes that are known to function in hh-mediated signaling during embryonic and larval development (P. Ing ham (1995) Current Opin. Genetics Dev. 5, 528-534), As in imaginal dis cs, engrailed (en) is co-expressed with hh in the germarium, while pat ched (ptc) and cubitus interruptus (ci) are expressed in somatic cells throughout the germarium and in developing egg chambers, with ptc exp ression being elevated within 10 cell diameters of the source of the h h signal, Moreover, the somatic cell overproliferation caused by ectop ic hh expression is accompanied by elevated levels of ptc and is pheno copied in ptc(-) somatic clones, These analyses suggest that ptc and c i are components of the hh signaling pathway in the germarium, However , unlike embryos and imaginal discs, neither wingless (wg) nor decapen taplegic (dpp) appear to mediate the ovarian hh signal, wg is expresse d in 'cap cells,' a subset of hh-expressing cells located adjacent to germ-line stem cells, but is unaffected by ectopic hh expression, Nor does the ectopic expression of wg or dpp mimic the effect of ectopic h h expression, We propose that Hh diffuses from apical cells, including cap cells, and regulates the proliferation of nearby ovarian somatic cells by antagonizing the negative effects of ptc on ci activity in th ese cells, thereby allowing the transcription of ci-dependent genes, i ncluding ptc itself.