Steroid Hormones in Drosophila: How Ecdysone Coordinates Developmental Signalling with Cell Growth and Division

Abstract

1.1 The ecdysone pathway directs Drosophila development Ecdysone is the major steroid hormone in all holometabolous insects responsible for driving the metamorphosis of larval tissues into adult structures. During metamorphosis, ecdysone is essential for upregulating the genes required to control apoptosis and differentiation, essential processes for removal of larval structures which have become obsolete and for tissue remodelling. In addition, ecdysone directs cell growth and division in many tissues throughout the larval to pupal transition. This chapter will discuss the many diverse mechanisms reported for connecting the ecdysone pulse to the developmentally regulated cell growth and cycle progression required for tissue growth and for insects to reach their target body size. Like all other holometabolous insects, the size of Drosophila adult flies is set by the size of the larvae prior to metamorphosis, at the time of pupariation when feeding has ceased and growth can no longer occur. The major developmental hormone in Drosophila, the steroid hormone 20-hydroxyecdysone (20E), commonly known as ecdysone, is required for all the developmental transitions needed for metamorphosis (Figure 1-3; (Thummel 1995, 1996, 2001)). Ecdysone is produced in and released by the prothoracic gland (PG), a component of the ring gland, which also contains the corpora allata (CA) and corpora cardiaca (CC) (Figure 1; (Zitnan et al. 2007; McBrayer et al. 2007)). Ecdysone release is controlled by a complex combination of upstream factors, including peptide hormones and neuropeptide signals (see section 2.2). For example, Prothoracicotropic hormone (PTTH) from the central nervous system (CNS) is required to regulate the synthesis and release of ecdysone from the PG (McBrayer et al. 2007). Ecdysone pulses from the PG are required for all aspects of morphogenesis, starting with the formation of the body plan during late embryogenesis, hatching and development of the first larval instar, and for cuticle moulting at the end of the first and second instars. A large