Fat2 acts through the WRC to specify the protrusive leading edge of follicle cells driving global egg rotation. (A) Confocal image from the basal surface of a fat2 mosaic egg chamber (stage 7) shows that mutant cells (negatively marked by the absence of the ubi-EGFP marker, blue) show strongly reduced actin-rich protrusions (F-actin, white). In fat2 mutant cells, endogenous Abi protein (green) is no longer enriched at the basal surface (top). However, Abi is still present at the baso-lateral region (bottom). The clonal border is marked by a yellow dotted line. The arrow shows the direction of the A-P body axis. Bars, 10 µm. (B–D) Single frames of spinning disc microscopy time-lapse movies of ex vivo–cultured ovarioles expressing LifeAct-EGFP in mosaic pattern (da-Gal4 driver). Time intervals and A-P axis direction are indicated. Bars, 5 µm. (B) Wild-type actin-rich cell protrusions are formed predominantly at tricellular junctions in the direction of the movement (arrowheads). (C) Nonmigrating follicle cells depleted for fat2 or (D) lacking the WIRS interaction form more uniform cell protrusions, pointing into opposite directions (arrowheads). (E and F) Quantification of cell protrusions emerging either perpendicular (red) or along (green) the A-P axis over 15 min as depicted in E. Wild-type (six cell borders of each in four independent experiments) or Abi-WT (six cell borders of each in five independent experiments) follicle cells preferentially form filopodia at the cell front perpendicular to the A-P axis (red, 75–85%). AbiΔWIRS (eight cell borders of each in four independent experiments) mutant and fat2-depleted cells (ten cell borders of each in five independent experiments) show a strongly reduced bias for protrusion orientation. (G) Quantification of follicle cell migration velocity of indicated genotypes (n = 5–13). Each data point represents an individual egg chamber. Bars represent mean ± SD.