Materials used.

The Drosophila corneal lens is an apical extracellular matrix structure with a biconvex shape that enables it to focus light onto the underlying photoreceptors. Here, we investigated how this shape is influenced by the source of one of its major components, the polysaccharide chitin. Knocking down the chitin synthase Krotzkopf verkehrt strongly reduced the thickness and curvature of the corneal lens. Conversely, enhancing chitin export by overexpressing Rebuf expanded and distorted the corneal lens. We found that the cone and primary pigment cells in the center of each ommatidium produce most of the chitin, and preventing chitin synthesis by these central cells reduced corneal lens curvature. Increasing chitin export from central cells increased the thickness of the central corneal lens, while increasing export from peripheral lattice cells made the edges thicker. The wild-type biconvex shape thus results from high levels of chitin production by central cells relative to peripheral cells, indicating that localized chitin secretion is critical for normal corneal lens curvature.