Biology Reference
In-Depth Information
ring glands. These results suggest that the TGF
b
/Activin pathway plays a
critical role in ecdysteroidogenesis through controlling the competence of
the prothoracic gland to respond to metamorphic factors, in particular,
PTTH and insulin-like peptides (ILPs) (
Wu & Brown, 2006
).
There are several questions that would be worth addressing in future stu-
dies to further dissect roles of the TGF
b
/Activin signaling in the
Drosophila
prothoracic gland. First, it is unclear in which manner the TGF
b
/Activin
pathway is required for the expression of
torso
and
InR
. For instance, is the
requirement dynamic and therefore only needed at specific times in develop-
ment or is the requirement more general to keep
InR
and
torso
levels up at all
times? Second, are
torso
and
InR
direct targets of the Smad2/Med complex? In
vertebrates, SMAD-binding sites have been elucidated (SMAD-binding
element or SBDs) (
Zawel et al., 1998
), however, corresponding data for
Drosophila
Smad2/Med complexes are unavailable.
11.5. Per/Tim (period/timeless)
The release of ecdysone from the prothoracic glands is under circadian con-
trol in many insect species (
Giebultowicz, 1999; Schwartz & Truman,
1983
). Studies in
Rhodnius
have shown that the last larval instar displays daily
fluctuations of ecdysone in the hemolymph, which was attributed to the
rhythmic release of PTTH being controlled by the central clock in the brain.
In particular, four neurons in the dorsal brain form the central pacemaker
that releases a peptide hormone, PDF (pigmentdispersing factor), in a circa-
dian manner. Interestingly, not only do these PDF neurons innervate the
prothoracic gland but also does a mutation in the PDF gene affect the pe-
riodicity of
ptth
transcript levels, strongly suggesting that ecdysone synthesis
is under circadian control. However, further experiments are needed to ex-
amine whether PTTH synthesis and/or release of the peptide are affected in
PDF mutants (
McBrayer et al., 2007; Renn, Park, Rosbash, Hall, &
Taghert, 1999
). Interestingly, cultured prothoracic glands are able to prop-
erly time the release of ecdysone, suggesting that the gland itself contains an
independent clock (
Ampleford & Steel, 1985; Vafopoulou & Steel, 1996a,
1996b
). The circadian genes
period
and
timeless
encode transcription factors
that regulate the expression of clock-controlled genes (
Bae, Lee, Sidote,
Chuang, & Edery, 1998
). A previous report showed that
period
is rhythmi-
cally expressed in the prothoracic gland of
Drosophila
pupae, supporting the
notion of circadian clock components in the tissue, at least at the stage that
was examined (
Emery et al., 1997
). A more recent study demonstrated that a
