boi controls FSC proliferation. (A) Schematic of early oogenesis. GSCs (gray) and ESCs (light blue) contact a cellular niche composed of terminal filament and cap cells (“apical cells,” green). FSCs (red) reside 3–5 cell diameters posterior to apical cells. These stem cells generate daughter cells that coordinate to produce follicles (egg chambers) composed of a 16-cell germline cyst (gray) surrounded by a single follicular epithelial layer (yellow). Egg chambers develop over 7 d to produce mature eggs. (B–D) Germaria in which germ cells (blue, anti-Vasa) and differentiating follicle cells (red, anti-Fas3) are labeled. Stalks between the germarium and the first budded egg chamber average 9 cells in wild type (WT; B) and 18 cells in boi^e mutants (C). Cell numbers for WT (B), and boi^e (C and D) are indicated (brackets). Stalks with excess follicle cells are observed at later stages (B, arrow). (D) boi mutants exhibit defects in cyst packaging (indicated by side-by-side cysts in a single plane (white arrow) or two cysts surrounded by a single epithelium (green arrow), and polarization defects (round cells, changes in Fas3 staining; arrowheads). (E) Mean cell number in the first stalk for each genotype is shown. *, significant differences relative to WT (P < 0.00000006). (F) Mean numbers of dividing FSCs (PH3⁺) per germarium are shown. *, significant differences relative to WT (P < 0.002). Error bars represent SEM.