**3. Cell intrinsic roles for ecdysone, EcR and USP in cell growth and division**

The *Drosophila* imaginal discs (see also Introduction 1.1), which form the adult head structures (eyes and antenna), appendages (wings and legs) and genitalia, have provided an excellent model for studying developmental signals controlling cell proliferation. The imaginal disc precursor cells arise early in embryonic development from invaginations of the embryonic epithelium (Alberts 2002). By the early larval stage each disc consists of a ball

Steroid Hormones in *Drosophila*:

How Ecdysone Coordinates Developmental Signalling with Cell Growth and Division 151

**A B**  Fig. 4. **A - Eye imaginal disc differentiation occurs in a wave that moves from posterior (P) to anterior (A)**. The margin between the asynchronously dividing anterior cells and the differentiated posterior cells is marked by the morphogenetic furrow (MF), where cells are delayed in G1. Mitotic division cycles become synchronized in the "Second Mitotic Wave" (SMW), which is composed of a tight band of DNA synthesis (Marked by BrdU in red) and mitosis (marked by PH3 in green). **B - The Hedgehog (Hh) and Dpp pathways control cell division in the larval eye.** *Drosophila* eye development is dependent on *hedgehog* (*hh*) expression posterior to the MF and *decapentaplegic* (*dpp*) expression within the MF. Hh and Dpp regulate key cell cycle genes to coordinate cell cycle and differentiation. Dpp and Hh act redundantly to ensure G1 arrest, thus cells unable to respond to Dpp will arrest later in response to Hh. Dpp and Hh inhibit Cyclin E and dE2F1 in the cells comprising the MF. In the anterior of the MF, Hh acts to promote cell division in the SMW by upregulating Cyclin

and Dpp, suggests there might be many connections between ecdysone, developmental pathways and cell cycle regulation during metamorphosis in *Drosophila* (Li and White 2003). The first evidence for these connections in the *Drosophila* larval eye imaginal disc came from studies implicating the ecdysone pathway in regulation of MF progression via effects on Hh and Dpp (Brennan, Ashburner, and Moses 1998; Brennan et al. 2001). In *Drosophila*, eye development is dependent on *hedgehog* (*hh*) expression posterior to the MF (Heberlein,

D to promote cell growth and Cyclin E to drive S-phase entry.

of around 10-50 undifferentiated stem cells, which undergo extensive growth and proliferation to comprise up to 100,000 cells by the end of the third larval instar. The imaginal discs start differentiation at the end of third instar and complete the process by the end of pupariation, when all adult structures such as the wings, legs and eyes have developed (Fristrom and Fristrom 1993). The third instar larval stage is a critical stage of *Drosophila* development, containing the major growth and proliferation of all tissues required to form the adult fly (Church and Robertson 1966). Indeed the size of the adult fly is determined at the time when the pupal case is formed, as after this the animal cannot feed again until eclosion. Here we will discuss the developmental signals (including Wingless, Dpp, Hedgehog, Notch) controlling growth of the eye and wing imaginal discs, and how ecdysone impacts on these signalling pathways to control cell division.
