The steroid hormone 20-hydroxyecdysone (20E) initiates metamorphosis in insects by signaling through the ecdysone receptor

Hormones, working through their receptor proteins, activate complex signaling pathways that have profound effects on development, physiology and behavior. In Drosophila the steroid hormone, 20-hydroxyecdysone (20E), initiates metamorphosis. Early metamorphic events such as chromosome puffing in salivary glands (Ashburner et al., 1974) and the elongation and eversion of the imaginal discs (Fristrom and Fristrom, 1993) have been shown to depend on the presence of 20E. Studying the puffing patterns of the salivary gland chromosomes, Ashburner et al. (1974) proposed a model in which a small set of genes is directly activated by the steroid hormone. The proteins encoded by these ‘early’ genes act to both activate a second set of genes, the so called ‘late’ genes, and repress the early genes themselves. Much research effort has focussed on how the hormone activates this transcriptional cascade (the Ashburner cascade) and on the interactions of these hormone-responsive genes (Thummel, 1996). More recently it has become evident that different concentrations of 20E can induce different sets of early genes. Karim and Thummel (1992) showed that one set is expressed at low levels of 20E (2×10−8 M), whereas others require 2×10−7 M for a 50% induction. Similarly, in the moth Manduca sexta, Champlin and Truman (1998) demonstrated that different levels of steroid can mediate qualitatively different types of developmental responses. For example, in the developing eye anlage of Manduca, low to moderate titers of 20E support prematurational events such as the movement of the morphogenetic furrow and specification of cell types in the forming retina. High titers of 20E are needed for final differentiation of the eye (cellular maturation), such as the formation of rhabdomeres, the crystalline cone, the lens, and the screening pigments. Many studies in Drosophila (Yao et al., 1992; Koelle et al., 1992; Thomas et al., 1993; Yao et al., 1993) have demonstrated that the functional receptor complex for ecdysteroids is a heterodimer of two nuclear receptors, the ecdysone receptor (EcR) and ultraspiracle (USP). Evidence for this conclusion comes from DNA gel shift assays as well as cell transfection assays (Koelle, 1992; Yao et al., 1992; Thomas et al., 1993; Yao et al., 1993). Several observations indicate that USP functions together with EcR in hormone-dependent processes in vivo as well. EcR and USP have been shown to bind to 20E regulated puffs on the salivary gland chromosomes (Yao et al., 1993). Also, usp mutants die at the time of the first larval molt (Perrimon et al., 1985; Oro et al., 1992). Recently Hall and Thummel (1998) have looked at usp mutant larvae which bypassed the early lethal phase by ectopic expression of a heat shock driven usp+ gene during the first larval instar. These animals are arrested at the end of the third larval instar but fail to initiate metamorphosis. Many of the early genes show no or 1151 Development 127, 1151-1159 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 DEV8684