Biology Reference
In-Depth Information
2.2.1 Fourth (penultimate) instar stadium
JH synthesis by a CC-CA complex during the fourth instar stadium fluctu-
ates in
Bombyx
(
Kinjoh et al., 2007
;
Fig. 3.1
B). When the ecdysteroid titer in
the hemolymph begins to increase on the second day of the instar, JH syn-
thesis increases dramatically, but then declines at the time of the peak of
ecdysteroid titer on day 3. In addition, mRNA levels of JH synthetic
enzymes belonging to the early steps and
JHAMT
show very similar patterns
to JH synthesis itself (
Kinjoh et al., 2007
;
Fig. 3.1
B). The close correspon-
dence between these patterns suggests that JH synthesis is controlled by
ecdysteroid through the regulation of the JH biosynthetic enzymes.
The concentration of ecdysteroid in the hemolymph increases in larvae
fed on a diet containing 20E (
Kamimura, Shimura, &Kiuchi, 2003; Kaneko,
Kinjoh, et al., 2011
); therefore, it is an ideal system to study ecdysteroid
action
in vivo
by changing an ecdysteroid titer artificially. When fourth instar
larvae are fed on diet containing 400 ppm 20E, the ecdysteroid titer in the
hemolymph begins to increase shortly after the start of the feeding. JH syn-
thetic activity by CC-CA also increases, but its activity then declines when
the ecdysteroid titer exceeds 250 ng/ml (
Kaneko, Kinjoh, et al., 2011
).
These relationships between ecdysteroid concentration and JH synthesis
are very similar to that seen in larvae fed on normal diet without 20E
(
Fig. 3.1
B), indicating that in both cases a low level of ecdysteroid stimulates
JH synthesis but at high concentration ecdysteroid suppresses JH synthesis
(
Fig. 3.3
A). The stimulation and suppression of JH synthesis are due to a
change in the expression of a subset of JH biosynthetic enzymes; activation
of JH synthesis
in vivo
is caused by increased expression of
FPPS1
and
JHAMT
, while suppression
in vivo
is due to inhibition of
IPPI
,
FPPS1
,
and
JHAMT
expression (
Kaneko, Kinjoh, et al., 2011
). Similar regulation
of the JH biosynthetic enzymes by 20E has also been observed
in vitro
.
The inhibitory action of high concentrations of ecdysteroid on
JHAMT
expression is mediated by the action of a transcription factor E75, expression
of which was stimulated by ecdysteroid, as RNAi suppression of E75 expres-
sion by the addition of dsE75, abolished ecdysteroid-induced inhibition of
JHAMT
expression
in vitro
(T. Kinjoh & K. Hiruma, unpublished data).
After the decline of JH synthesis induced by high ecdysteroid titers, JH
synthesis increases again around the time of the last larval ecdysis (
Fig. 3.1
B).
This is partly due to the falling ecdysteroid titer at this time, but additional
factors also seem to be involved in this increase (
Kaneko, Kinjoh, et al.,
2011
) (see
Section 2.4.5
).
