Diapause: Delaying the Developmental Clock in Response to a Changing Environment - Current Topics in Developmental Biology - page 217

3.2. Control of DH expression
The Bombyx DH gene (BmDH) exhibits a different expression profile
depending on the developmental trajectory of the larva. In “diapause-
conditioned” animals (those that will eventually lay diapausing eggs), large
peaks of BmDH expression occur during both the mid-late larval instars
(4 /5 ) and the pupal stage ( Morita, Niimi, & Yamashita, 2004; Xu,
Sato, Ikeda, &Yamashita, 1995a ). One key question in the field is how envi-
ronmental signals affect DH expression. The product of the Ptix1 gene, a
bicoid-like homeobox transcription factor binds cis-regulatory elements
in the promoter region of BmDH , as do members of the POU transcription
factor family ( Shiomi et al., 2007; Xu, Sato, Ikeda, & Yamashita, 1995b;
Zhang, Kang, Zhang, &Xu, 2004 ). It is not clear however, what inputs con-
trol Ptix1 expression. One possibility is dopamine (DA). Bombyx moths lay-
ing diapausing embryos produce a large amount of DA and they upregulate
the BmDdc gene (encoding DA decarboxylase, a key enzyme in DA synthe-
sis) during the late larval and early pupal life. Consistent with a causal role of
DA in triggering diapause, “nondiapause” larvae fed with L -DOPA (a DA
precursor) or injected with DA, develop as moths that express high levels of
DH and lay embryos primed for diapause. This suggests that DA acts
upstream of DH signaling in inducing diapause trajectory ( Noguchi &
Hayakawa, 2001 ). However, it is not clear whether DA signaling directly
results in the upregulation of Ptix1 nor is it yet clear whether DA producing
neurons respond directly to environmental cues.
4. DEVELOPMENTAL CONTROL OF LARVAL DIAPAUSE
Unlike the embryonic diapause described above, in some species, the
primary point of diapause control is within the larval stages. Last instar larvae
of the pyralid Diatraea grandiosella trigger diapause by molting from the nor-
mal “spotted” morph (developing) to an “immaculate” (diapausing) one
( Fig. 8.2 ). Some of these diapausing larvae undergo extra “stationary” molts
where approximately 50% undergo one extra molt while 14% undergo two
extra molts. Immaculate larvae remain unchanged in size through each molt,
but they modify completely their physiology: feeding stops, respiration
decreases, and fat accumulation, dehydratation, cold hardiness and fat storage
are enhanced ( Chippendale, 1977, 1984 ).
In this species as well as several others that undergo larval diapause, devel-
opmental arrest is induced by juvenile hormone (JH) which constrains ECD