Biology Reference
In-Depth Information
E75A is also induced by JH in
Drosophila
S2 cells (
Dubrovsky,
Dubrovskaya, & Berger, 2004
). This induction of E75A is mediated by
an intercellular pathway utilizing GCE/FTZ-F1 and Met/FTZ-F1 that
forms transcriptionally active heterodimers, so that the removal of FTZ-
F1 prevents JH activation of E75A (
Bernardo & Dubrovsky, 2012;
Dubrovsky et al., 2011
). Although about 20% of E75A mutants undergo
precocious metamorphosis in
Drosophila
(
Bialecki, Shilton, Fichtenberg,
Segraves, & Thummel, 2002
), this transcription factor does not seem to
be involved in the JH signaling pathway; rather the phenotype of the
E75A mutant involves a feed-forward pathway that amplifies or maintains
the ecdysteroid level, so that there is submaximal ecdysteroid induction
of FTZ-F1 expression (
Bialecki et al., 2002
) in turn reducing the latter's abil-
ity to act as a competence factor facilitating JH (
Dubrovsky et al., 2011
) and
ecdysteroid (
Broadus, McCabe, Endrizzi, Thummel, & Woodard, 1999
)
activation of gene expression.
4. CONCLUSION
After entering the last larval stadium, lepidopteran larvae prepare for
pupal metamorphosis, which is characterized by the larval-pupal commit-
ment of various tissues such as epidermal cells (
Muramatsu et al., 2008;
Riddiford, 1985, 1996
). The indispensable event for this process is the
removal of JH from the hemolymph, and one of the most important con-
tributors to this endocrine event is the shut down of JH synthesis by the CA.
Here, we have shown that at least three factors, ecdysteroid, dopamine, and
sNPF, work together to guarantee cessation of JH synthesis; all of these fac-
tors have different mode of actions (
Figs. 3.3
A and
3.4
), so that the cessation
is definitely accomplished.
Many additional factors are involved in JH synthesis (see
Goodman &
Granger, 2005
), but the roles of these other factors on JH synthesis in asso-
ciation with insect development have not been well studied. One of the
interesting peptides among them is allatinhibin found in
Manduca
, released
from the brain shortly before the larvae empty their gut content, and inhibits
JH synthesis (
Bhaskaran et al., 1990
). Therefore, allatinhibin may also be
involved in the cessation of the JH synthesis in the last larval stage, although
we have not determined the presence of allatinhibin in
Bombyx
and other
insect species. This peptide is distinct from AST (
Bhaskaran et al., 1990
)
and apparently from sNPF as well (
Yamanaka et al., 2008
), but no other
studies have been done and we do not know much about this peptide.
