Fig. 1. Moesin and Crumbs define complementary subdomains of the photoreceptor apical surface. (A) Distinct apical membrane domains are evident in a transmission electron micrograph cross-section of an adult wild-type ommatidium: rhabdomeres, closely-packed photosensitive microvilli and the stalk (S), collaring the rhabdomere and connecting to the adherens junctions (AJ). Both stalk and rhabdomere face the inter-rhabdomeral space (IRS). Stalks of photoreceptors R1-R6 are typically folded, often with coated pits in the pocket (inset). (B) Confocal micrograph shows activated Moesin (p-Moesin, green) localized to the rhabdomere base, and Crumbs (blue) localized to the stalk. Rhodamine-phalloidin (red) stains axial actin filaments of rhabdomere microvilli and the rhabdomere terminal web (RTW). (C) Rhabdomere and stalk primordial domains differentiate by 50% pupal development (pd). Rhabdomere primordia are covered with short finger-like microvilli, and the smooth membrane of the future stalk displays a cytoplasmic density typical of membranes heavily invested with cytoskeleton. (D) 50% pd, p-Moesin (green) has resolved to the rhabdomere base and Crumbs (blue) to the stalk. Crumbs localization parallels stalk morphology. Crumbs staining is not present in R1, R3 and R6 at this stage. Anterior is to the right in all figures. Scale bars: 1 µm in A,C; 10 µm in B,D.