Diapause: Delaying the Developmental Clock in Response to a Changing Environment - Current Topics in Developmental Biology - page 215

32 C). The cold tolerance is enhanced by the production of
glycerol and sorbitol that act as cryoprotecting agents ( Horie et al., 2000;
Lee & Denlinger, 1991 ). Interestingly, sorbitol also appears to play a regula-
tory function since when exogenously applied to “nondiapausing embryos,”
it actively inhibits developmental growth and stimulates the embryos to enter
dormancy. Conversely, dormant embryos break diapause when sorbitol is
removed from the embryos themselves ( Horie et al., 2000 ). Similar to the
“hatch-ready” case, mid-embryonic-arrested Bombyx embryos also require
a defined period of time in the cold (2-3 months at 5 C) before they can
resume normal development ( Horie et al., 2000 ). It appears that one function
of the cold period is to reduce sorbitol levels below a threshold that then
enables the embryos to exit fromdiapause and resume developmental growth
( Horie et al., 2000 ). Part of the exit process involves ERK signaling which
promotes the conversion of the remaining sorbitol into glycogen (restoring
the energy resources needed for developmental growth) and the induction
of the required mid-embryonic ECD pulse. These two processes rely on
ERK dependant activation of two key enzymes: sorbitol dehydrogenase-2
and ecdysteroid-phosphate phosphatase (ECDPPase). The later enzyme is
crucial during the postdiapause exit stage, since embryos require an ECD
pulse at this stage of development and they depend entirely on the ability
of ECDPPase to covert maternal stores of inactive ecdysteroid-phosphates
into ECD to reactivate normal developmental growth and differentiation
( Fujiwara, Shindome, Takeda, & Shiomi, 2006; Fujiwara & Shiomi, 2006;
Fujiwara, Tanaka, et al., 2006; Horie et al., 2000; Iwata et al., 2005 ).

to tolerate

3.1. Production of diapause hormone in pupa determines
the developmental fate of the future egg
While sorbitol appears to play a role in the induction of Bombyx embryonic
diapause, it is downstreamof the actual initiating event, which is thought to be
the environmentally controlled production of diapause hormone (DH), a
24-amino acid peptide, that acts inside the pupa of future mothers ( Sato,
Ikeda, & Yamashita, 1994; Sato et al., 1993; Sato, Shiomi, Saito, Imai, &
Yamashita, 1998; Shiomi et al., 2007; Suwan, Isobe, Yamashita,
Minakata, & Imai, 1994; Yamashita, 1996; Yamashita, Shiomi, Ishida,
Katagiri, & Niimi, 2001 ). When the future mother moth grows as a larva
in the summer months (long days andwarm temperatures), the emerged adult
female lays “autumnal” embryos that are programmed for entering diapause
through the production of high levels of DHin themother ( Fig. 8.1 )( Fukuda,
1951; Hasegawa, 1951; Nakagaki, Takei, Nagashima, & Yaginuma, 1991;