Fig. 4. The levels of Dl from follicle cells determine stalk size. (A,B) Precursor-stage polar cells (green, marked by A101-GAL4/UAS-mCD8::GFP) and stalk cells (red, marked by the how^93F reporter) initially align as broad neighboring bands in region 3 of the germarium. The polar cell precursors are always positioned posteriorly. Nuclei are stained with DAPI (blue). (C,D) In situ hybridization using a fluorescent Kul RNA probe. In surface (C) and cross-section (D) views, Kul RNA (red) is detected in all follicle cells at early stages, including the stalk (arrow), but hardly in the germ line itself (arrowhead). (E,F) Notch activation during early oogenesis, monitored by X-Gal staining of the Su(H)_m8-lacZ reporter. Arrows point to the germarium and arrowheads to stage 2 polar cells. Notch activation in A101-GAL4/UAS-dskul-ovarioles (F) is elevated as compared with wild type (E). (G) Expression of UAS-dskul in polar cells by A101-GAL4 (green) leads to an increase in stalk-cell number. The stalk is marked by the 93F lacZ reporter (red). (H) Quantification of stalk size in various genetic backgrounds. Stalk-cell numbers are increased following expression of dskul in either polar (dark red columns) or stalk cells (orange columns), and reduced in Dl heterozygous ovaries (green columns). (I,I') Expression of Notch dsRNA in the stalk cells (green) using the 24B-GAL4 driver leads to loss of the stalk marker Bib (red). (J) Expression of p35 in both polar and stalk cells using the 7025-GAL4 driver results in an abnormal stalk containing an excess of cells (arrows).