Living and Adapting to Its Own Habitat - Building the Most Complex Structure on Earth - page 219

SOG

Diapausing egg

DH

Diapause

5°C for 2 months

ECD

Development

Embryo
yolk

Erk

Figure 4.11 Maternal control of embryonic diapauses in the silkworm B. mori . DH is
produced by the SOG of the mother that developed under the long photoperiod and high
temperature of summer. DH signals in the ovaries where it induces the development of
diapause-fated eggs. In these eggs, embryonic diapause occurs after the development of the
cephalic lobe and the following mesoderm segmentation. The embryos require 2-3 months
at low temperatures (about 5°C) before they can arrest diapause and resume development at
high temperature. In this phase, the ERK pathway is activated in the yolk cells and it promotes
the production of 20-OH ECD. ECD signals to the embryo inducing the removal of the block
imposed to development. Abbreviations : DH, diapause hormone; ECD, ecdysone; ERK,
extra-cellular signal-regulated kinase; SOG, suboesophageal ganglion.
Source : From Schiesari et al. (2011) .

The flesh fly, Sarcophaga bullata , offers another interesting example of the maternal
neural control of diapause in insects. Diapausing mothers of S. bullata , which as pupae
are reared under short photoperiod (SP), prevent the “normal” appearance of diapause in
the offspring, even when the offspring are exposed to diapause-inducing SPs. The inhi-
bition of diapause in the offspring is determined by the mother's photoperiodic experi-
ence ( Henrich and Denlinger, 1982 ). Since the primary environmental cue that induces
diapause is the length of the day and because this is perceived in the insect's brain, one
can expect that either the mother's brain must be responsible for inducing/suppressing
the diapause or it is the site where the causal chain of events leading to induction or
inhibition of diapause begins. This theoretical expectation is verified empirically.
Transplantation of ovaries between long-day history and short-day history larvae
does not influence the diapause fate of the progeny, implying that the information
for diapause is still retained within the brain at the end of larval stage ( Rockey et al.,
1991 ). Diapause can be suppressed by brain extracts of mothers reared during an SP
and by administration of the neurotransmitter gamma aminobutyric acid (GABA). It
is noteworthy that the GABAergic circuit is responsible for inhibiting DH synthesis
in the silkworm, B. mori ( Shimizu et al., 1989 ). The neurotransmitter also drastically