How Clocks and Hormones Act in Concert to Control the Timing of Insect Development - Current Topics in Developmental Biology - page 8

located in the brain, thus introducing the concept of circadian “gating” dur-
ing development ( Truman & Riddiford, 1974 ). The photoperiodic gating
mechanism ensures that PTTH can be released every day only during a spe-
cific 8-h window ( Truman, 1972; Truman & Riddiford, 1974 ). During this
photoperiodic gate, larvae determine whether they have achieved critical
weight, a checkpoint that ensures that larvae have sufficient resources to sur-
vive metamorphosis. Larvae that have not achieved critical weight during
this time window continue feeding until the next photoperiodic gate is
reached the following day.
More direct evidence linking circadian clocks to molting comes from the
observation that the PG of the blood-sucking insect Rhodnius harbors an
endogenous circadian clock. In particular, the circadian core proteins
TIMELESS and PERIOD (see Section 5 ) have been shown to co-localize
and oscillate in PG cells of Rhodnius and Drosophila , suggesting that circadian
control over ecdysone biosynthesis may also directly occur in the PG
( Steel & Vafopoulou, 2006 ).
In Drosophila , the circadian clock neurons (aka LN or lateral neurons)
innervate the dendrites of the PTTH producing neurons ( McBrayer
et al., 2007; Siegmund & Korge, 2001 ). Interestingly, PTTH transcript
levels oscillate, but they do so with an ultradian rather than circadian rhythm.
Specifically, PTTH mRNA displays an 8-h rhythmicity in Drosophila ,at
least when determined via semi-quantitative PCR based on total RNA
( McBrayer et al., 2007 ). Also, it is unknown whether this oscillatory tran-
script behavior corresponds to similar cycles of secreted PTTH peptides.
However, there is evidence that ecdysone titers underlie daily rhythms in
Bombyx and Rhodnius , raising the possibility that Drosophila ecdysone levels
fluctuate in accordance with the transcriptional PTTH oscillations
( Ampleford & Steel, 1985; Satake, Kaya, & Sakurai, 1998 ). Drosophila larvae
with ablated pigment dispersing factor (PDF) neurons display both an
increase in PTTH periodicity and higher PTTH transcript levels.
Additional supporting evidence for circadian clocks controlling develop-
mental transitions comes from studies on eclosion in Drosophila . Eclosion
within a Drosophila population typically follows a rhythm, where most
animals eclose during dawn. Interestingly, Drosophila pupae harbor a func-
tional clock in the PG at the time of eclosion, and disrupting this clock
by overexpressing timeless in this tissue results in a disruption of normal eclo-
sion timing ( Myers, Yu, & Sehgal, 2003 ). For the PG clock to function
properly, circadian LNs must be present in order to maintain accurate
eclosion rhythms in the population, indicating that the PG acts as a periph-
eral clock.