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glands (Inka cells) that are located in the epitracheal endocrine system in each
spiracle ( Zitnan & Adams, 2005; Zitnan, Kingan, Hermesman, & Adams,
1996 ). When the ecdysteroid titer declines during the last larval molt,
corazonin is released from the brain, which stimulates ETH release from
the epitracheal glands. The ETH then acts on ventromedial cells in the brain
to release EH which acts to stimulate CCAP cells in the central nervous
system ( Zitnan & Adams, 2005 ).
There are two isoforms of the ETH receptor in Bombyx , both of which
are expressed in the CA shortly before the last larval ecdysis ( Yamanaka et al.,
2008 ), which suggests that ETH might modulate JH synthesis at this time.
Detailed analyses of ETH receptor expression in the CA showed that the
maximum expression is observed 5 h before the last larval ecdysis, at the time
when JH biosynthesis ceases to decline and increases again. ETH receptor is
not expressed at the time just before pupation when the CA are not synthe-
sizing JH ( Hiruma et al., 2010 ). At the time of adult ecdysis, the female CA
(but not males) that synthesize JH ( Kinjoh et al., 2007 ) also express the ETH
receptor (Zitnan, personal communication). When CA from larvae 5-6 h
before the final larval ecdysis are cultured with ETH, JH synthesis by the
CA is stimulated. ETH does not have this effect at other stages, indicating
that ETH is responsible for the increase in JH synthesis that occurs at the
time of the final larval ecdysis along with the decline of ecdysteroid titer
(see above). Since the decline of ecdysteroid titer induced JH biosynthesis
only under in vivo conditions, and not in vitro ( Kaneko, Kinjoh, et al.,
2011 ), the induction of JH synthesis by declining ecdysteroid might be
through the induction of ETH release. The detailed action of ETH, such
as the mode of action and the mechanisms of the stage specificity, needs
to be studied. The increase in JH titer that occurs at the time of the final
larval ecdysis is important to determine the length of feeding period
( Hiruma, 1980; Hiruma, Shinoda, Malone, & Riddiford, 1999; Sakurai,
1984 ) before spinning and gut purge, a factor that in turn affects pupal
weight.
2.5. Regulation by nutrition
JH synthesis is also influenced by nutrition ( Bhaskaran, Jones, & Jones, 1980;
Cymborowski, Bogus, Beckage, Williams, & Riddiford, 1982; Lee &
Horodyski, 2006; Maestro, Cobo, & BellĀ“s, 2009; Noriega, 2004 ). In
Manduca , starvation after the final larval ecdysis stimulates JH biosynthesis
and also suppresses the degradation of JH in the hemolymph, so that JH titer
in the hemolymph increases
( Cymborowski et
al., 1982; Lee &
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