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to pupariation (
Mirth et al., 2005
). One possible interpretation is that the PG
of these larvae is prematurely estimating that critical weight has been reached
because of the enhanced insulin signaling and the constant light condition
then reduces the PTTH delay period.
Future studies should examine the mechanism(s) regulating PG neuron
activity including the importance of transcriptional control, as well as
processing and release of PTTH. Presently, the mechanism controlling PTTH
release is only known for some insects in the group Hemiptera, where acti-
vation of abdominal stretch receptors in response to the attainment of a certain
size results in PTTH release (
Nijhout, 1981, 2003
). Although distension of
stretch receptors is not sufficient for PTTH release and initiation of molting
in insects outside this order, peripheral sensory neurons tiling the larval body
wall have been linked with changes in feeding behavior associated with the
progression of development from the feeding to wandering stage in
Drosophila
(
Ainsley, Kim, Wegman, Pettus, & Johnson, 2008; Wegman, Ainsley, &
Johnson, 2010
).
As discussed above, timing of pupariation involves checkpoints that
ensure coordination of growth between larval and adult precursor tissues.
However, it is less clear how each of these signals contributes to each of
the discrete pulses of ecdysone seen during the third instar (
Warren et al.,
2006
). One possibility is that clearance of all checkpoints is required to pro-
duce the first low-level, critical weight associated ecdysone peak and the
successive subsequent peaks are generated autonomously by feedback cir-
cuits. However, the ecdysone peaks do not occur with a regular ultradian
rhythm; the time between peaks being 8, 12, or 16 h (
Warren et al.,
2006
). Another, and perhaps more likely, explanation is that the endocrine
system monitors growth, energy status, and photoperiod checkpoints suc-
cessively, each of which results in a low-level ecdysone peak that drives a
subsequent part of the developmental program, ultimately resulting in
pupariation and the initiation of metamorphosis.
3. SIGNALS CONVERGING ON THE PG
3.1. PTTH and insulin/TOR coordinate PG cell growth and
ecdysone production
Like in peripheral tissues, the insulin/TOR pathway promotes growth of
PG cells and therefore can have an indirect effect on total levels of ecdysone
simply by controlling the PG cell size. However, several studies have shown
