Biology Reference
In-Depth Information
Duman, 1988; Lee & Denlinger, 1991
). Similarly, diapausing embryos of
B. mori
can resist freezing down to
32
C for several months using only
supercooling (
Lee & Denlinger, 1991; Suzuki et al., 1983
).
In this chapter, we will focus primarily on a discussion of diapause in
Lepidoptera species largely because of their rich diversity in the types of dia-
pause and polyphenism responses they produce in the face of similar or
unique environmental challenges. This plasticity, together with their long
history of study by entomologists and naturalists, make them an ideal model
system to determine at the molecular genetic level, how adaptive develop-
mental pathways evolve. Although understanding the mechanisms behind
lepidopteran diapause induction has been a long standing issue, the use of
modern genomic and physiological methods now offers new opportunities
to answer important outstanding issues about adaptive change, a process of
increasing relevance in light of recent rapid global climatic changes.
In the first section of this chapter, we will describe the examples of lep-
idopteran species that arrest development at several different stages, and then
discuss what is presently know about the molecular mechanisms behind dia-
pause induction. Lastly, we will briefly describe one vertebrate example in
which diapause is induced not by seasonal temperature and photoperiod
changes, but instead responds to seasonal changes in hydration levels of
the local environment.
2. HORMONAL CONTROL OF EMBRYONIC DIAPAUSE
IN INSECTS: THE
“
HATCH-READY
”
EXAMPLE
Diapause can be triggered in embryos by blocking developmental
growth. Some species undergo diapause as early embryos, while others enter
diapause as pharate first instar larvae (“hatch-ready” larvae) within the egg-
shell. Embryonic diapauses can be very different in regulation, although they
are all under the control of the steroid hormone ecdysone (ECD). However,
the role of ECD as a diapause inhibitor or promoter depends on the diapaus-
ing stage (
Denlinger et al., 2012; Saunders et al., 2002
). For example, con-
sider the gypsy moth,
Lymantria dispar
. It diapauses in autumn as a pharate
first instar larvae and does not hatch until it has spent a genetically established
period in the cold. As described, the cold phase is needed to reestablish
developmental competence for growth, once more optimal conditions
are encountered. In this example, diapause is induced and sustained by
the production of high ECD titers, whereas under normal developmental
conditions the ECD titer would be low in fully developed prehatch first
