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1996, Shionoya et al., 2003
). Interestingly, the implantation of a “cold-
activated” brain into “brainless” diapausing pupae of the Hyalophora
giant-silkworm causes the termination of diapause (
Williams, 1946, 1947,
1952
). This effect is consistent with a model in which the pupal brain, when
chilled, becomes competent to release PTTH which, in turn, restores the
ECD pulse needed to antagonize pupal diapause (
Denlinger et al., 2012
).
PTTH release from the brain, however, does not appear to be the only
way to break dormancy. In
Pieris rapae
,
Pieris brassicae
, and
Antheraea polyphe-
mus
removal of the brain only induces a “permanent diapause” if it occurs
within the first month of diapause. After this time, diapausing pupae of these
lepidopteran species, as well as several others, can break dormancy and com-
plete metamorphic development even in the absence of their brains (
Judy,
1972; Kind, 1978
;
Maslennikova, 1970; McDaniel & Berry, 1967; Wilson &
Larsen, 1974
). Similarly, diapausing pupae of the
Helicoverpa zea
are indepen-
dent from the brain and they have all the potential to resume metamorphosis
after the chilling period as long as they are de-brained within 24 h of
pupariation (
Denlinger et al., 2012; Meola & Adkisson, 1977
).
The capacity to reactivate development in the absence of a brain might
depend on an autonomous process in the PG that become independent of
neural secretions in an age and cold sensitive manner (
Denlinger et al., 2012;
Saunders et al., 2002
). Several pieces of evidence support this model. The
PG of de-brained diapausing pupae of
Papilio xuthus
is directly activated
by cold (
Ozeki, 1954
) while the PG of
Samia cynthia
larvae retains a high
degree of independence from the brain (
Mizoguchi & Ishizaki, 1982
). In
Manduca sexta
, the PG of diapausing pupae becomes refractory to PTTH sig-
naling from the day of the larva-to-pupa molt, perhaps as result of inductive
events occurring in the late larval stages, and it becomes autonomously
responsive to environmental stimuli in breaking dormancy (
Bowen et al.,
1984, 1985
). A similar refractoriness of PG to PTTH signaling also occurs
in
Mamestra brassicae
(
Agui, 1975
) and
Pieris brassicae
(
Calvez, 1976
).
At present, the mechanism behind PTTH independent reactivation is
unknown. One possibility is that an unknown inhibitor might block
PTTH/ECD signaling in the PG and it might be gradually removed by
the cold so that developmental competence is resumed. DA might be this
putative inhibitor since it promotes diapause in several Lepidoptera. For
example, in
Mamestra brassicae
or
Pieris brassicae
, diapausing pupae exhibit
higher levels of hemolymphatic DA than those set for direct development
and last instar larvae set for direct development develop into diapausing
pupae when they are fed with
L
-DOPA (
Isabel, Gourdoux, & Moreau,
