Biology Reference
In-Depth Information
instar larva. The requirement for a high ECD titer to induce and maintain
diapause in this species can be further demonstrated by the observation that
“chilled” diapausing embryos remain dormant, even when exposed to opti-
mal conditions, if they are simultaneously treated with exogenous ECD.
Conversely, embryos fail to induce diapause when injected with KK-42,
an ECD antagonist, but this effect can be reversed by topical applications
of ECD. Interestingly, “nondiapausing” Lymantria mutants that have genet-
ically lost the ability to induce diapause can enter dormancy when injected
with ECD, revealing that a latent, and inducible, diapause response is still
genetically encoded in these mutants (
Lee & Denlinger, 1996, 1997; Lee,
Horodyski, Valaitis, & Denlinger, 2002; Lee, Valaitis, & Denlinger, 1997;
Suzuki, Nakamura, Yanbe, Kurihara, & Kuwano, 1993
).
A similar hormonal mechanism is thought to control diapause entry in the
hesperiid
Thymelicus lineola
(
McNeil & Fields, 1985
) and the saturniid
Anthe-
raea yamamai
(
Suzuki et al., 1990
). In the latter species, prolonged pulses of
ECD induce diapause (as pharate first instar larva) and the inhibition of
ECD signaling in diapausing embryos breaks dormancy. The main source
of ECD in this, as well as other insects, is the embryonic/larval prothoracic
gland (PG) which maintains high levels of endocrine activity until diapause
termination, at which time the ECD titer drops and the embryo hatches
(
Denlinger et al., 2012; Lee & Denlinger, 1997; Suzuki et al., 1990,
1993
). Consistent with the role of the PG in regulating diapause, excision
of the thorax/head complex (containing the PG) from diapausing embryos
induces the isolated abdomens to resume growth (
Suzuki et al., 1990
). From
these studies one theme that emerges is that in certain insects, a prolonged
and intense ECD pulse can induce diapause by overriding the normal devel-
opmental program. Diapause is then broken after a predetermined cool
period and the restoration of a warmer environment and longer photoperiod.
3. HORMONAL CONTROL OF INSECT DIAPAUSE:
THE MID-EMBRYONIC ARREST EXAMPLE
Although prolonged ECD levels can induce the formation of “hatch
ready,” pre-eclosion first instar larva in some insect species, in other species it
is the failure of inducing the mid-embryonic ECD pulse that triggers dia-
pause. In these cases, however, the embryos arrest at a much earlier stage
of embryonic development. For example, in autumn, embryos of the silk
moth,
B. mori
(Bombycidae), enter diapause just after embryonic mesoderm
segmentation, at which point they become extremely resistant to cold (able
