Drosophila is a powerful model organism in which we can elucidate the complex mechanisms controlling the development of the oocyte and egg. There are two key cell types involved, the germ line cells and somatic follicle cells. A germ line stem cell produces a cluster of 16 cells which remain together throughout oogenesis. One of them is determined to become an oocyte and the other 15 supply the oocyte with materials and positional information needed for oocyte and embryo polarity and development. Each cluster of cells is surrounded by somatic follicle cells. These undergo complex morphogenetic movement and supply components of the egg, secrete the egg membrane and eggshell and respond to signals from the oocyte, instructing the development of the egg chamber and oocyte. Thus the development of a mature egg requires constant interactions between these two cell types. The cell-cell signalling pathways involved are used at many stages in development. Thus mutations in the genes responsible for controlling these process are often lethal. We used an "enhancer trap" method that enabled us to clone genes expressed in subsets of follicle cells in Drosophila which are activated in response to a TGF? -like signal from the oocyte. We have identified transcription factors (eg, the Broad-Complex), secreted signalling molecules (eg, fringe) and genes involved in cell morphology (eg, an unconventional myosin VI encoded by jar). Since mutations in most of these genes are lethal, we determined their function in specific cells at defined stages of oogenesis using in vivo induced antisense RNA to "silence" the genes. The position of these genes in the genetic hierarchies controlling morphogenesis and patterning in the egg chamber will also be described.

Index terms: cell signalling, morphogenesis, eggshell, follicle cells