Mechanisms of cell rearrangement and cell recruitment in Drosophila ovary morphogenesis and the requirement of bric à brac.

The Drosophila ovary consists of repeated units, the ovarioles, where oogenesis takes place. The repetitive structure of the ovary develops de novo from a mesenchymal cell mass, a process that is initiated by the formation of a two-dimensional array of cell stacks, called terminal filaments, during the third larval instar. We have studied the morphogenetic process leading to the formation of terminal filaments and find that this involves recruitment, intercalation and sorting of terminal filament cells. Two other types of cell stacks that participate in ovary morphogenesis, the basal stalks and interfollicular stalks, also form by cell rearrangement utilizing a convergence and extension mechanism. Terminal filament formation depends on the Bric à brac protein, which is expressed in the nuclei of terminal filament cells and is cell autonomously required. Disruption of terminal filament formation, together with defects of basal and interfollicular stalk development, leads to disruption of ovariole formation and female sterility in bric à brac mutants.