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for eclosion (
Kingan & Adams, 2000
). EH and ETH stimulate each other's
production in a positive feedback loop. ETH appears to be primarily
involved in stimulating pre-ecdysis behaviors, while EH stimulates secre-
tion of CCAP, which triggers ecdysis behaviors. These hormonal cascades
were originally characterized in
Bombyx
, however, similar events have
been observed during
Drosophila
eclosion (
Mesce & Fahrbach, 2002
).
Clearly, given the complexity of the neuro-endocrine network that con-
trols eclosion, more work is required to identify the exact events that
determine the timing by which the onset of eclosion is regulated. In
the next section, we will discuss the CCAP neurons in more detail and
in particular, we will consider the temporally tuned differentiation of a
sub-group of the CCAP neurons as a model for temporally controlled
neuronal differentiation during development.
9. THE TIMING OF CCAP NEURONAL DIFFERENTIATION
IS ECDYSONE-CONTROLLED
CCAP neurons are critical for proper larval and pupal ecdysis, as well
as maintaining eclosion rhythms in
Drosophila
and
Manduca
(
Ewer &
Truman, 1996; Gammie & Truman, 1997; Jackson et al., 2001; Kim,
Zitnan, Cho, & Adams, 2006; McNabb et al., 1997; Mesce & Fahrbach,
2002; Park et al., 2003
). The ablation of CCAP neurons prolongs larval
ecdysis and results in a failure to complete pupal ecdysis, thus causing lethal-
ity. CCAP neurons express the neuropeptides CCAP and the subunits of the
bursicon peptide hormone, bursicon-
a
(Burs-
a
) and its partner bursicon-
b
(Burs-
b
)(
Kim et al., 2006; Veverytsa & Allan, 2011
).
CCAP neurons are located in the ventral nerve cord, and are comprised
of a single CCAP interneuron (CCAP-IN) per hemi-segment T1-A7, as
well as a single CCAP efferent (CCAP-EN) per hemisegment T3-A4
(
Fig. 1.4
A) (
Martinez-Padron & Ferrus, 1997; Prokop, 2006; Zhao et al.,
2008
). Earlier work had demonstrated that CCAP neurons undergo mor-
phological remodeling after pupal ecdysis (
Zhao et al., 2008
). In a recent
study, Veverytsa and Allan closely examined CCAP neuron organization
during
Drosophila
prepupal development and found that the CCAP network
is also remodeled just before pupal ecdysis. In particular, they identified
12 late emerging CCAP neurons that are spatially restricted to the segments
A5-A9, and they go on to show that this subset of CCAP neurons is suffi-
cient to ensure normal pupal ecdysis (
Veverytsa & Allan, 2012
).
The late-emerging CCAP neurons appear during early pupariation
before pupal ecdysis, and can be subdivided into two groups each
