The Mirror transcription factor links signalling pathways in Drosophila oogenesis

Many genetic cascades are conserved in evolution, yet they trigger differen t responses and hence determine different cell fates at specific times and positions in development. At stage 10 of oogenesis, mirror is expressed in anterior-dorsal follicle cells, and we show that this is dependent upon the Gurken signal from the oocyte. The fringe gene is expressed in a complemen tary pattern in posterior-ventral follicle cells at the same stage. Ectopic expression of mirror represses fringe expression, thus linking the epiderm al growth factor receptor (EGFR) signalling pathway to the Fringe signallin g pathway via Mirror. The EGFR pathway also triggers the cascade that leads to dorsal-ventral axis determination in the embryo. We used twist as an em bryonic marker for ventral cells. Ectopic expression of mirror in the folli cle cells during oogenesis ultimately represses twist expression in the emb ryo, and leads to similar phenotypes to the ectopic expression of the activ ated form of EGFR. Thus, mirror also controls the Toll signalling pathway, leading to Dorsal nuclear transport. In summary, we show that the Mirror ho meodomain protein provides a link that coordinates the Gurken/EGFR signalli ng pathway (initiated in the oocyte) with the Fringe/Notch/Delta pathway ti n follicle cells). This coordination is required for epithelial morphogenes is, and for producing the signal in ventral follicle cells that determines the dorsal/ventral axis of the embryo.