Biology Reference
In-Depth Information
depends on the presence or absence of JH, where larval to larval molts
require high levels of JH, but larval-pupal transitions necessitate low JH
titers (
Doane, 1973; Mizoguchi, 2001; Riddiford, 1970a, 1970b;
Truman, Riddiford, & Safranek, 1974
).
An apparently complex network of signaling events regulates the pro-
duction of ecdysone (
Caceres et al., 2011; Fellner et al., 2005; Gibbens,
Warren, Gilbert, & O'Connor, 2011; Ou, Magico, & King-Jones, 2011;
Rewitz, Yamanaka, Gilbert, & O'Connor, 2009; Song & Gilbert, 1995,
1997
) and JH (
Huang et al., 2011; Kinjoh et al., 2007; Wigglesworth,
1936
). The downstream effects of these hormones result in a highly
regulated succession of developmental gene programs, and we refer the
reader to the following references for more detail (
Ou & King-Jones,
2013; Rewitz, Yamanaka, & O'Connor, 2013; Song & Gilbert, 1998;
Thummel, 1995, 2001; Yamanaka, Rewitz, & O'Connor, 2013
). Despite
the progress in dissecting the regulatory pathways that drive insect develop-
ment, the mechanisms controlling the timing of developmental transitions
are still largely unknown.
4. CIRCADIAN CONTROL AND DEVELOPMENTAL
TRANSITIONS
Although the timing of developmental transitions in insects is not
directly determined by circadian clocks, interesting links do exist between
molting, eclosion, and circadian circuitries. For instance, under standard lab-
oratory conditions, the major ecdysone pulses in
Drosophila
show a clear
daily rhythm, where the embryonic and first two larval pulses occur
24 h apart, followed by a third instar pulse 48 h later (
Fig. 1.1
A). Similar
rhythms have been described in other insects as well (
Cymborowski,
Smietanko, & Delbecque, 1989; Mizoguchi, Dedos, Fugo, & Kataoka,
2002; Mizoguchi, Ohashi, Hosoda, Ishibashi, & Kataoka, 2001; Richter,
2001; Schwartz & Truman, 1983
). At lower temperatures, however, insect
development slows down, whereas circadian rhythms are temperature-
compensated (not affected by temperature fluctuations), suggesting that
the link between circadian clocks and molting is not based on a simple linear
relationship. Rather, it appears that circadian clocks provide a frame of
reference—or gate—during which a developmental transition might occur,
thus linking the timing of molting and eclosion to daily cycles.
Four decades ago, James Truman demonstrated that in
Manduca sexta
,
the secretion of PTTH is regulated by a photosensitive circadian clock
