During Drosophila development, the salivary primordia are internalized to form the salivary gland tubes. By analyzing immuno-stained histological sections and scanning electron micrographs of multiple stages of salivary gland development, we show that internalization occurs in a defined series of steps, involves coordinated cell shape changes and begins with the dorsal-posterior cells of the primordia. The ordered pattern of internalization is critical for the final shape of the salivary gland. In embryos mutant for huckebein (hkb), which encodes a transcription factor, or faint sausage (fas), which encodes a cell adhesion molecule, internalization begins in the center of the primordia, and completely aberrant tubes are formed. The sequential expression of hkb in selected cells of the primordia presages the sequence of cell movements. We propose that hkb dictates the initial site of internalization, the order in which invagination progresses and, consequently, the final shape of the organ. We propose that fas is required for hkb-dependent signaling events that coordinate internalization.
