Living and Adapting to Its Own Habitat - Building the Most Complex Structure on Earth - page 218

gland synthesizes ecdysteroids throughout diapause, but its synthesis drops by the
end of the diapause period to allow it to continue with its development. In Chilo
stemborers, and other insects, diapause is controlled by JH; stimulated by the secre-
tion of brain allatotropins, corpora allata (CA) synthesizes JH during diapause and
JH levels drop by the end of diapause. In other insects, application of JH leads to
termination of diapause ( Denlinger et al., 2011 ).
In Bombyx mori larvae, the diapause is regulated by brain hormones DH and
PBAN. Although DH does not induce diapause in other insects, a DH-like neuro-
peptide is secreted with the neuropeptide PBAN from a single mRNA in the brains
and especially the subesophageal ganglion (SOG) of insects, such as Heliothis
virescens (Fabricius) ( Xu and Denlinger, 2003 ) and Helicoverpa armigera (Hübner)
( Zhang et al., 2004 ). But, contrary to its well-known diapausing effect in B. mori , in
H. virescens , secretion of DH and PBAN stimulates development and their suppres-
sion induces diapause.
Parents Transmit Epigenetic Instructions for Switching Their Offspring
to Alternative Life Histories
Epigenetic information determines gene expression, cell differentiation, and intra-
generational phenotypic changes in physiology, morphology, and behavior. All the
examples of epigenetically induced phenotypic changes presented so far are not her-
itable (i.e., they are not inherited) in the offspring in the absence of the stimuli that
induced their original expression. The following two examples demonstrate that par-
ents can transmit to their offspring new traits that the parents did not possess and
how they generate and pass these instructions down to the next generation.
The silkworm is a facultative diapausing insect, but normally it does not deter-
mine itself whether it will enter diapause or not. Whether it will enter diapause or
not depends on the secretion of DH, an oocyte-targeting neuropeptide. Under condi-
tions of the summer-long photoperiod and high temperature, the photoperiodic clock
in the insect's brain induces an increased firing activity of the labial secretory (Lb)
neurons in the SOG ( Ichikawa, 2003 ). These neurons are exclusive producers of DH
( Denlinger et al., 2011 ). Epigenetically determined differences in the firing activity
of the Lb neurons in diapausing and nondiapausing pupae are correlated with differ-
ences in their DH content. A dopaminergic circuit ( Noguchi and Hayakawa, 2001 )
and the product of the gene Pitx ( Shiomi et al., 2007 ) also stimulate these neurons to
synthesize DH from the transcript of the diapause hormone-pheromone biosynthe-
sis activating neuropeptide ( DH-PBAN ) gene, which, via hemolymph, targets ovaries
( Kamei et al., 2011; Kitagawa et al., 2005; Shiomi et al., 2007 ). The diapause-
inducing action of the DH is mediated by its receptor that is expressed in the oocyte
( Homma et al., 2006 ) ( Figure 4.11 ).
In the ovary of the silkmoths that produce diapause-producing eggs, DH increases
trehalase activity and the amount of glucose in the oocyte as a result of the uptake
from the hemolymph ( Kamei et al., 2011 ). At the beginning of diapause, glycogen
is converted into sorbitol and glycerol, which prevent the freezing of eggs in cold
weather ( Kihara et al., 2009 ).