polished rice mediates ecdysone-dependent control of Drosophila embryonic organogenesis

In many animals, progression of developmental stages is temporally controlled by steroid hormones. In Drosophila, the level of ecdysone titer oscillates and developmental stage transitions, such as larval molting and metamorphosis, are induced at each of ecdysone peaks. Ecdysone titer also peaks at the stage of mid-embryogenesis and the embryonic ecdysone is necessary for morphogenesis of several organs, although the regulatory mechanisms of embryonic organogenesis dependent on ecdysone signaling are still open questions. In this study, we find that absence or interruption of embryonic ecdysone signaling caused multiple defects in the tracheal system, including decrease in luminal protein deposition, uneven dilation of the dorsal trunk and loss of terminal branches. We also reveal that an ecdysone-inducible gene polished rice (pri) is essential for tip cell fate decision in dorsal branches. As over-expression of pri can restore the defects caused by disturbance of ecdysone biosynthesis, pri functions as one of the major mediators of embryonic ecdysone signal in tracheogenesis. These results demonstrate that ecdysone and its downstream target pri play essential roles in tracheal development by modulating cell fate decision.