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environmental conditions. These checkpoints utilize a number of signaling pathways to
modulate ecdysone production in the prothoracic gland. Release of ecdysone activates
an autonomous cascade of both feedforward and feedback signals that determine the
duration of the ecdysone pulse at each developmental transitions. Conservation of the
genetic mechanisms that coordinate the juvenile-adult transition suggests that insights
from the fruit fly Drosophila will provide a framework for future investigation of devel-
opmental timing in metazoans.
1. INTRODUCTION
The timing of insect metamorphosis is not a simple function of chro-
nological age, but instead adjusts itself depending on environmental condi-
tions. Holometabolous insects develop through a series of larval stages called
instars, each punctuated by a molt in which the old cuticle is shed and
replaced by a new larger one. After a series of molts, the number of which
is species-specific, a terminal larval phase is reached during which extensive
growth occurs until a predetermined target size is reached. At this point,
the larva enters the pupal stage and undergoes metamorphosis to form the
reproductively mature adult ( Mirth & Riddiford, 2007 ). The processes of
metamorphosis has fascinated classical entomologist for almost a century
in part because it touches upon a fundamental biological question: how
are developmental transitions controlled? Answering this question has impli-
cations beyond just understanding developmental timing mechanisms in
insects. It also will help us understand how growth and maturation processes
are coordinated such that reproductively mature organisms are reliably
produced despite potentially different environmental conditions encoun-
tered by each individual during development.
In insects growth and maturation are distinct processes with growth being
restricted primarily to the larval stages (although some growth occurs during
metamorphosis: Okamoto et al., 2009; Slaidina, Delanoue, Gronke,
Partridge, & Leopold, 2009 ), whichmeans that the size of the larva at the onset
of metamorphosis largely determines the final size of the adult ( Edgar, 2006;
Mirth & Riddiford, 2007 ). Final body size is determined by the rate of mass
accumulation and the duration of growth. Interestingly, recent studies show
that members of the evolutionarily conserved insulin-like growth factor fam-
ily coordinate both growth rate and duration by impinging on the endocrine
system that controls onset of metamorphosis ( Caldwell, Walkiewicz, & Stern,
2005; Colombani, Andersen, & Leopold, 2012; Colombani et al., 2005;
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