Biology Reference
In-Depth Information
Yoshiyama-Yanagawa et al., 2011
). Once released into circulation, ecdysone
is taken up by several peripheral tissues including the gut, fat body and
Malphigian tubules where it is converted to the active hormone
20-hydroxyecdysone by the P450 enzyme product of the
shade
gene
(
Petryk et al., 2003
). Circulating 20-hydroxyecdysone then induces a systemic
genetic response in multiple tissues by binding to a complex of the ecdysone
receptor (EcR) and ultraspiracle, both members of the nuclear receptor family
(
King-Jones & Thummel, 2005
). Peripheral tissues interpret the ecdysone
signal in a cell-type specificmanner, determined by the combination of specific
EcR isoforms and cofactors expressed in the target tissue (
Yamanaka, Rewitz, &
O'Connor, 2013
). The response to ecdysone includes the destruction of
obsolete larval tissues by induction of cell death activators and the morpho-
genesis and differentiation of the imaginal disk tissues to adult structures
(
Baehrecke, 2000; D'Avino & Thummel, 2000; Siaussat, Porcheron, &
Debernard, 2009
).
The mechanisms timing the production and release of ecdysone are key to
understanding developmental timing of metamorphosis. These are intimately
linked to size cues because growth beyond a characteristic size triggers
ecdysone release (
Edgar, 2006; Mirth & Riddiford, 2007; Mirth &
Shingleton, 2012
). Here, we discuss progress from
Drosophila
studies that
examine how the rates of ecdysone synthesis and inactivation/removal are reg-
ulated and illustrate how these studies provide insight into the mechanisms that
time ecdysone pulses and their duration (
Caceres et al., 2011; Caldwell et al.,
2005; Colombani et al., 2005; Gibbens, Warren, Gilbert, &O'Connor, 2011;
Layalle et al., 2008; McBrayer et al., 2007; Mirth et al., 2005; Ou et al., 2011;
Rewitz, Larsen, et al., 2009; Rewitz, Yamanaka, Gilbert, &O'Connor, 2009;
Rewitz, Yamanaka, & O'Connor, 2010
).
2. CHECKPOINT CONTROLS FOR PROGRESSION OF
DEVELOPMENT
2.1. Increasing complexity for control of ecdysone
production and release
Early studies in Lepidoptera led to the development of a key concept known as
“critical weight” that appears to switch larvae between a nutrition-dependent
growth phase and a nutrition-independent timing program (
Beadle, Tatum, &
Glancy, 1938; Nijhout & Williams, 1974a,1974b
). Starvation before larvae
reach critical weight suspends growth and delays developmental progression
